“For studying the fundamental properties of protons and antiprotons, you need to take extremely precise measurements – as precise as you can possibly make it,” explains principal investigator Christian Smorra. “This level of precision is extremely difficult to achieve in the antimatter factory, and can only be reached when the accelerator is shut down. This is why we need to relocate the measurements – so we can get rid of these problems and measure anytime.”

The team has made considerable strides to miniaturise their apparatus. BASE-STEP is far and away the most compact design for an antiproton trap yet built, measuring just 2 metres in length, 1.58 metres in height and 0.87 metres across. Weighing in at 1 tonne, transportation is nevertheless a complex operation. On 24 October, 70 protons were introduced into the trap and lifted onto a truck using two overhead cranes. The protons made a round trip through CERN’s main site before returning home to the antimatter factory. All 70 protons were safely transported and the experiment with these particles continued seemlessly, successfully demonstrating the trap’s performance.

Antimatter needs to be handled carefully, to avoid it annihilating with the walls of the trap. This is hard to achieve in the controlled environment of a laboratory, let alone on a moving truck. Just like in the BASE laboratory, BASE–STEP uses a Penning trap with two electrode stacks inside a single solenoid. The magnetic field confines charged particles radially, and the electric fields trap them axially. The first electrode stack collects antiprotons from CERN’s antimatter factory and serves as an “airlock” by protecting antiprotons from annihilation with the molecules of external gases. The second is used for long-term storage. While in transit, non-destructive image-current detection monitors the particles and makes sure they have not hit the walls of the trap.

“We originally wanted a system that you can put in the back of your car,” says Smorra. “Next, we want to try using permanent magnets instead of a superconducting solenoid. This would make the trap even smaller and save CHF 300,000. With this technology, there will be so much more potential for future experiments at CERN and beyond.”

With or without a superconducting magnet, continuous cooling is essential to prevent heat from degrading the trap’s ultra-high vacuum. Penning traps conventionally require two separate cooling systems – one for the trap and one for the superconducting magnet. BASE-STEP combines the cooling systems into one, as the Harvard team proposed in 1993. Ultimately, the transport system will have a cryocooler that is attached to a mobile power generator with a liquid-helium buffer tank present as a backup. Should the power generator be interrupted, the back-up cooling system provides a grace period of four hours to fix it and save the precious cargo of antiprotons. But such a scenario carries no safety risk given the miniscule amount of antimatter being transported. “The worst that can happen is the antiprotons annihilate, and you have to go back to the antimatter factory to refill the trap,” explains Smorra.

With the proton trial-run a success, the team are confident they will be able to use this apparatus to successfully deliver antiprotons to precision laboratories in Europe. Next summer, BASE-STEP will load up the trap with 1000 antiprotons and hit the road. Their first stop is scheduled to be Heinrich Heine University in  Germany.

“We can use the same apparatus for the antiproton transport,” says Smorra. “All we need to do is switch the polarity of the electrodes.”

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Hypernuclei are exotic nuclei formed by a mix of protons, neutrons and hyperons, the latter being unstable particles containing one or more strange quarks. More than 70 years since their discovery in cosmic rays, hypernuclei remain a source of fascination for physicists due to their rarity in nature and the challenge of creating and studying them in the laboratory.

In heavy-ion collisions, hypernuclei are created in significant quantities, but only the lightest hypernucleus, hypertriton, and its antimatter partner, antihypertriton, have been observed. Hypertriton is composed of a proton, a neutron and a lambda hyperon containing one strange quark. Antihypertriton is made up of an antiproton, an antineutron and an antilambda.

Following hot on the heels of the observation of antihyperhydrogen-4 (a bound state of an antiproton, two antineutrons and an antilambda) earlier this year by the STAR collaboration at the Relativistic Heavy Ion Collider (RHIC), the ALICE collaboration at the LHC has now seen the first ever evidence for antihyperhelium-4, which is composed of two antiprotons, an antineutron and an antilambda. The result has a significance of 3.5 standard deviations. If confirmed, antihyper­helium-4 would be the heaviest antimatter hypernucleus yet seen at the LHC.

Hypernuclei remain a source of fascination due to their rarity in nature and the challenge of creating and studying them in the lab

The ALICE measurement is based on lead–lead collision data taken in 2018 at a centre-of-mass energy of 5.02 TeV for each colliding pair of nucleons, be they protons or neutrons. Using a machine-learning technique that outperforms conventional hypernuclei search techniques, the ALICE researchers looked at the data for signals of hyperhydrogen-4, hyperhelium-4 and their antimatter partners. Candidates for (anti)hyperhydrogen-4 were identified by looking for the (anti)helium-4 nucleus and the charged pion into which it decays, whereas candidates for (anti)hyperhelium-4 were identified via its decay into an (anti)helium-3 nucleus, an (anti)proton and a charged pion.

In addition to finding evidence of antihyperhelium-4 with a significance of 3.5 standard deviations, and evidence of antihyperhydrogen-4 with a significance of 4.5 standard deviations, the ALICE team measured the production yields and masses of both hypernuclei.

For both hypernuclei, the measured masses are compatible with the current world-average values. The measured production yields were compared with predictions from the statistical hadronisation model, which provides a good description of the formation of hadrons and nuclei in heavy-ion collisions. This comparison shows that the model’s predictions agree closely with the data if both excited hypernuclear states and ground states are included in the predictions. The results confirm that the statistical hadronisation model can also provide a good description of the production of hyper­nuclei modelled to be compact objects with sizes of around 2 femtometres.

The researchers also determined the antiparticle-to-particle yield ratios for both hypernuclei and found that they agree with unity within the experimental uncertainties. This agreement is consistent with ALICE’s observation of the equal production of matter and antimatter at LHC energies and adds to the ongoing research into the matter–antimatter imbalance in the universe.

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Magnetic moments

BASE (Baryon Antibaryon Symmetry Experiment) specialises in the study of antiprotons by measuring properties such as the magnetic moment and charge-to-mass ratio. The latter quantity has been shown to agree with that of the proton within an experimental uncertainty of 16 parts per trillion. While not nearly as precise due to much higher complexity, measurements of the antiproton’s magnetic moment provide an equally important probe of CPT symmetry.

To determine the antiproton’s magnetic moment, BASE measures the frequency of spin flips of single antiprotons – a remarkable feat that requires the particle to be cooled to less than 200 mK. BASE’s previous setup could achieve this, but only after 15 hours of cooling, explains lead author Barbara Latacz (RIKEN/CERN): “As we need to perform 1000 measurement cycles, it would have taken us three years of non-stop measurements, which would have been unrealistic. By reducing the cooling time to eight minutes, BASE can now obtain all of the 1000 measurements it needs – and thereby improve its precision – in less than a month.” By cooling antiprotons to such low energies, the collaboration has been able to detect antiproton spin transitions with an error rate (< 0.000023) more than three orders of magnitude better than in previous experiments.

Underpinning the BASE breakthrough is an improved cooling trap. BASE takes antiprotons that have been decelerated by the Antiproton Decelerator and the Extra Low Energy Antiproton ring (ELENA) and stores them in batches of around 100 in a Penning trap, which holds them in place using electric and magnetic fields. A single antiproton is then extracted into a system made up of two Penning traps: the first trap measures its temperature and, if it is too high, transfers the antiproton to a second trap to be cooled further. The particle goes back and forth between the two traps until the desired temperature is reached.

The new cooling trap has a diameter of just 3.8 mm, less than half the size of that used in previous experiments, and is equipped with innovative segmented electrodes to reduce the amplitude of one of the antiproton oscillations – the cyclotron mode – more effectively. The readout electronics have also been optimised to reduce background noise. The new system reduces the time spent by the antiproton in the cooling trap during each cycle from 10 minutes to 5 seconds, while improvements to the measurement trap have also made it possible to reduce the measurement time fourfold.

“Up to now, we have been able to compare the magnetic moments of the antiproton and the proton with a precision of one part per billion,” says BASE spokesperson Stefan Ulmer (Max Planck–RIKEN–PTB). “Our new device will allow us to reach a precision of a tenth of a billion and, on the very long-term, will even allow us to perform experiments with 10 parts-per-trillion resolution. The slightest discrepancy could help solve the mystery of the imbalance between matter and antimatter in the universe.”

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“This is a breakthrough for the antimatter community that has been awaited for almost 30 years, and which has both a broad physics and technological impact,” says AEgIS physics coordinator Benjamin Rienacker of the University of Liverpool. “Precise Ps spectroscopy experiments could reach the sensitivity to probe the gravitational interaction in a two-body system (with 50% on-shell antimatter mass and made of point-like particles) in a cleaner way than with antihydrogen. Cold ensembles of Ps could also enable Bose–Einstein condensation of an antimatter compound system that provides a path to a coherent gamma-ray source, while allowing precise measurements of the positron mass and fine structure constant, among other applications.”

Laser cooling, which was applied to antihydrogen atoms for the first time by the ALPHA experiment in 2021 (CERN Courier May/June 2021 p9), slows atoms gradually during the course of many cycles of photon absorption and emission. This is normally done using a narrowband laser, which emits light with a small frequency range. By contrast, the AEgIS team uses a pulsed alexandrite-based laser with high intensity, large bandwidth and long pulse duration to meet the cooling requirements. The system enabled the AEgIS team to decrease the temperature of the Ps atoms from 380 K to 170 K, corresponding to a decrease in the transversal component of the Ps velocity from 54 to 37 km s^–1.

The feat presents a major technical challenge since, unlike antihydrogen, Ps is unstable and annihilates with a lifetime of only 142 ns. The use of a large bandwidth laser has the advantage of cooling a large fraction of the Ps cloud while increasing the effective lifetime, resulting in a higher amount of Ps after cooling for further experimentation.

“Our results can be further improved, starting from a cryogenic Ps source, which we also know how to build in AEgIS, to reach our dream temperature of 10 K or lower,” says AEgIS spokesperson Ruggero Caravita of INFN-TIFPA. “Other ideas are to add a second cooling stage with a narrower spectral bandwidth set to a detuning level closer to resonance, or by coherent laser cooling.”

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Equivalence principle

Using a modified setup, the ALPHA collaboration recently clocked the freefall of antihydrogen, paving the way for precision studies of the magnitude of the gravitational acceleration between antiatoms and Earth. The goal is to test the weak equivalence principle of general relativity, which requires that all test masses must react identically to Earth’s gravity. While models have been built that suggest differences could exist between the freefall rates of matter and antimatter (for example due to the existence of new, long-range forces), the theoretical consensus is clear: they should fall to Earth at the same rate. In physics, however, you don’t really know something until you observe it, emphasises ALPHA spokesperson Jeffrey Hangst: “This is the first direct experiment to actually observe a gravitational effect on the motion of antimatter. It’s a milestone in the study of antimatter, which still mystifies us due to its apparent absence in the universe.”

The ALPHA collaboration creates antihydrogen by binding antiprotons produced and slowed down in the Antiproton Decelerator and ELENA rings with positrons accumulated from a sodium-22 source. It then confines the neutral, but slightly magnetic, antimatter atoms in a magnetic trap to prevent them from coming into contact with matter and annihilating. Until now, the team has concentrated on spectroscopic studies with the ALPHA-2 device. But it has also built an apparatus called ALPHA-g, which makes it possible to measure the vertical positions at which antihydrogen atoms annihilate with matter once the trap’s magnetic field is switched off, allowing the antiatoms to escape.

The ALPHA team trapped groups of about 100 antihydrogen atoms and then slowly released them over a period of 20 seconds by gradually ramping down the top and bottom magnets of the trap. Numerical simulations indicate that, for matter, this operation would result in about 20% of the atoms exiting through the top of the trap and 80% through the bottom – a difference caused by the downward force of gravity. By averaging the results of seven release trials, the ALPHA team found that the fractions of antiatoms exiting through the top and bottom were in line with simulations. Since vertical gradients in the magnetic field magnitude can mimic the effect of gravity, the team repeated the experiment several times for different values of an additional bias magnetic field, which could either enhance or counteract the force of gravity. By analysing the data from this bias scan, the team found that the local gravitational acceleration of antihydrogen is directed towards Earth and has magnitude a_g = [0.75 ± 0.13 (stat. + syst.) ± 0.16 (sim.)]g, which is consistent with the attractive gravitational force between matter and Earth.

This is the start of a new avenue of experimental exploration that pushes the development of trapping and other techniques

The next step, says Hangst, is to increase the precision of the measurements via laser-cooling of the antiatoms, which was first demonstrated in ALPHA-2 and will be implemented in ALPHA-g in 2024. Two other experiments at CERN’s Antimatter Factory, AEgIS and GBAR, are poised to measure a_g  using complementary methods. AEgIS will measure the vertical deviation of a pulsed horizontal beam of cold antihydrogen atoms in an approximately 1 m-long flight tube, while GBAR will take advantage of new ion-cooling techniques to measure ultra-slow antihydrogen atoms as they fall from a height of 20 cm. All three experiments are targeting a measurement of  a_g  at the 1% level in the coming years.

Even higher levels of precision will be needed to test models of new physics, say theorists. “The role of antimatter in the ‘weight’ of antihydrogen is very little, since practically all the mass of a nucleon or antinucleon comes from binding gluons, not antiquarks,” says Diego Blas of Institut de Física d’Altes Energies and Universitat Autònoma de Barcelona. “Any new force that couples differently to matter and antimatter would therefore need to have a huge effect in antiquarks, which makes it difficult to build models that are consistent with existing observations and where the current measurements by ALPHA-g would be different.” Things start to get interesting when the precision reaches about one part in 10 million, he says. “This is the start of a new avenue of experimental exploration that pushes the development of trapping and other techniques. If you compare the situation with the sensitivity of the first prototypes of gravitational-wave detectors 50 years ago, which had to be improved by six or seven orders of magnitude before a detection could be made, anything is possible in principle.”

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Producing and slowing down an anti­atom enough to see it in free fall is no mean feat. To achieve this, the AD’s 5.3 MeV antiprotons are decelerated and cooled in the ELENA ring and a packet of a few million 100 keV antiprotons is sent to GBAR every two minutes. A pulsed drift tube further decelerates the packet to an adjustable energy of a few keV. In parallel, a linear particle accelerator sends 9 MeV electrons onto a tungsten target, producing positrons, which are accumulated in a series of electromagnetic traps. Just before the antiproton packet arrives, the positrons are sent to a layer of nanoporous silica, from which about one in five positrons emerges as a positronium atom. When the antiproton packet crosses the resulting cloud of positronium atoms, a charge exchange can take place, with the positronium giving up its positron to the antiproton, forming antihydrogen.

At the end of 2022, during an operation that lasted several days, the GBAR collaboration detected some 20 antihydrogen atoms produced in this way, validating the “in-flight” production method for the first time. The collaboration will now improve the production of antihydrogen atoms to enable precision measurements, for example, of its spectroscopic properties.

The first antihydrogen atoms were produced at CERN’s LEAR facility in 1995, but at an energy too high for any measurement to be made. Following this early success, CERN’s Antiproton Accumulator (used for the discovery of the W and Z bosons in 1983) was repurposed as a decelerator, becoming the AD, which is unique worldwide in providing low-energy antiprotons to antimatter experiments. After the demonstration of storing antiprotons by the ATRAP and ATHENA experiments, ALPHA, a successor of ATHENA, was the first experiment to merge trapped antiprotons and positrons and to trap the resulting antihydrogen atoms. Since then, ATRAP and ASACUSA have also achieved these two milestones, and AEgIS has produced pulses of antiatoms. GBAR now joins this elite club, having produced 6 keV antihydrogen atoms in-flight.

GBAR is also not alone in its aim of testing Einstein’s equivalence principle with atomic antimatter. ALPHA and AEgIS are also working towards this goal using complementary approaches.

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Antinuclei can travel vast distances through the Milky Way without being absorbed, concludes a novel study by the ALICE collaboration. The results, published in December, indicate that the search for ³He in space is a highly promising way to probe dark matter. 

First observed in 1965 in the form of the antideuteron at CERN’s Proton Synchrotron and Brookhaven’s Alternating Gradient Synchrotron, antinuclei are exceedingly rare. Since they annihilate on contact with regular matter, no natural sources exist on Earth. However,  light antinuclei have been produced and studied at accelerator facilities, including recent precision measurements of the mass difference between deuterons and antideuterons and between ³He and ³He by ALICE, and between the hypertriton and antihypertriton by the STAR collaboration at RHIC. 

Antinuclei can in principle also be produced in space, for example in collisions between cosmic rays and the interstellar medium. However, the expected production rates are very small. A more intriguing possibility is that light antinuclei are produced by the annihilation of dark-matter particles. In such a scenario, the detection of antinuclei in cosmic rays could provide experimental evidence for the existence of dark-matter particles. Space-based experiments such as AMS-02 and PAMELA, along with the upcoming Antarctic balloon mission GAPS, are among a few experiments that are able to detect light antinuclei. But to be able to interpret future results, precise knowledge of the production and disappearance probabilities of antinuclei is vital. 

The latter is where the new ALICE study comes in. The unprecedented energies of proton–proton and lead–lead collisions at the LHC produce, on average, as many nuclei as antinuclei. By studying the change in the rate of ³He as a function of the distance to the production point, the collaboration was able to determine the inelastic cross section, or disappearance probability, of ³He nuclei for the first time. These values were then used as input for astrophysics simulations. 

Two models of the ³He flux expected near Earth after the nuclei’s journey from sources in the Milky Way were considered: one assumes that the sources are cosmic-
ray collisions with the interstellar medium, and the other annihilations of hypothetical weakly interacting massive particles (WIMPs). For each model, the Milky Way’s transparency to ³He – that is, its ability to let the nuclei through without being absorbed – was estimated. The WIMP dark-matter model led to a transparency of about 50%, whereas for the cosmic-ray model the transparency ranged from 25 to 90%, depending on the energy of the antinucleus. These values show that ³He originating from dark-matter or cosmic-ray collisions can travel distances of several kiloparsecs in the Milky Way without being absorbed, even from as far away as the galactic centre. 

“This new result illustrates the close connection between accelerator-based experiments and observations of particles produced in the cosmos,” says ALICE spokesperson Marco van Leeuwen. “In the near future, these studies will be extended to ⁴He and to the lower-momentum region with much larger datasets.”

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The BASE collaboration at the CERN Antiproton Decelerator (AD) has made the most precise comparison yet between the properties of matter and antimatter. Reporting in Nature in January, following a 1.5 year-long measurement campaign, the collaboration finds the charge-to-mass ratios of protons and antiprotons to be identical within an experimental uncertainty of just 16 parts per trillion. The result is four times more precise than the previous BASE comparison in 2015 and places strong constraints on possible violations of CPT invariance in the Standard Model.

The charge-to-mass ratio is now the most precisely measured property of the antiproton

Stefan Ulmer

Invariance under the simultaneous operations of charge conjugation, parity transformation and time reversal is a pillar of quantum field theories such as the Standard Model. Direct, high-precision tests of CPT invariance are therefore powerful probes of new physics, and of the possible mechanisms through which the universe came to be matter-dominated.

“The charge-to-mass ratio is now the most precisely measured property of the antiproton,” says BASE spokesperson Stefan Ulmer of RIKEN in Japan. “To reach this precision, we made considerable upgrades to the experiment and carried out the measurements when the antimatter factory was closed down, so that they would not be affected by disturbances from the experiment’s magnetic field.” The upgrades include a rigorous re-design of the cryostage of the experiment and the development of a multi-layer shielding-coil system, which considerably reduced magnetic-field fluctuations in the central measurement trap, explains Ulmer. “Another important ingredient is the implementation of a superconducting image-current detection system with tunable resonance frequency and ultra-high non-destructive detection efficiency, which eliminates the dominant systematic shift of the previous charge-to-mass ratio comparison.”

The BASE team confined antiprotons and negatively charged hydrogen ions in a state-of-the-art Penning trap, in which charged particles follow a cyclical trajectory with a frequency that scales with the trap’s magnetic-field strength and the particle’s charge-to-mass ratio.

By alternately feeding antiprotons and hydrogen ions one at a time into the trap, the team was able to measure their cyclotron frequencies under the same conditions. Performed over four campaigns between December 2017 and May 2019, the measurements involved more than 24,000 cyclotron-frequency comparisons, each lasting 260 seconds. Within the experimental uncertainty, the result, –(q/m)_p/(q/m)_p̄= 1.000000000003(16), demonstrates that the Standard Model respects CPT invariance at an energy scale of 1.96×10^–27 GeV at 68% confidence. It also improves knowledge of 10 coefficients in the Standard Model extension – a generalised, observer-independent effective field theory used for investigations of Lorentz violation.

Weak equivalence principle

The BASE team also used their data to test the weak equivalence principle, which states that different bodies in the same gravitational field undergo the same acceleration. Any difference between the gravitational interaction of protons and antiprotons, for example due to anomalous gravitational scalar or tensor couplings to antimatter, would result in a difference in the proton and antiproton cyclotron frequencies. Sampling the varying gravitational field of Earth as it orbits the Sun, BASE found no such difference, constraining the strength of anomalous antimatter/gravitational interactions to less than 1.8×10^–7 and enabling the first differential test of the weak equivalence principle (WEP) using antiprotons.

“From this interpretation we constrain the differential matter–antimatter WEP-violating coefficient to less than 0.03, which is comparable to the initial precision goals of other AD experiments that aim to drop antihydrogen in the Earth’s gravitational field,” explains Ulmer. “BASE did not directly drop antimatter, but our measurement of the influence of gravity on a baryonic antimatter particle is, according to our understanding, conceptually very similar, indicating no anomalous interaction between antimatter and gravity at the achieved level of uncertainty.”

The collaboration expects to reach even higher sensitivities on both the WEP test and the proton–antiproton charge-to- mass ratio comparison by increasing the experiment’s magnetic-field strength, stability and homogeneity. Further improvements are anticipated from the use of transportable antiproton traps, such as BASE-STEP, which allow precision antiproton experiments to be moved from the fluctuating accelerator environment to a calm laboratory space.

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Gato-Rivera sets out with a detailed exploration of the differences between atoms and antiatoms, as well as of matter–antimatter annihilation, motivating the reader to delve into a fairly complete introduction to particle physics: the concepts that underpin the Standard Model, and some that lie beyond. She then focuses on diverse aspects of antimatter science, beginning with the differences between antimatter, dark matter and dark energy, and the different roles they play in the universe. This touches upon the observed accelerating expansion of the universe. In particular, Gato-Rivera discusses dark-matter and dark-energy candidates, attempts to detect dark matter and its relation to the fate of the universe. She also carefully explains the distinction between primordial and secondary antimatter, and their roles in cosmology.

Next up, a historical chapter reviews the major landmarks of the discovery of antimatter particles, from elementary antiparticles to anti-hadrons, and anti-nuclei to antiatoms. In particular, the ground-breaking discovery of the first antiparticle, the positron, is described in excellent detail. In a separate appendix, Gato-Rivera passionately clears up a historical controversy about its discovery. The positron was first found in cosmic rays by Carl Anderson and later artificially produced en masse in particle accelerators. Gato-Rivera then turns to a detailed historical overview of cosmic-ray research, from balloon experiments to large-scale ground-based detectors, finally culminating in modern space-based detectors on board satellites and the ISS. The next chapter covers the production of antimatter by particle collisions in accelerators at high energies, including a brief history of the facilities at CERN.

The focus is then put on one of the most interesting and important conundrums in particle physics and astrophysics: the apparent huge asymmetry between matter and antimatter in the observed universe. This touches upon the processes of the primordial creation of matter and antimatter, and on the open question of whether anti-stars, or even anti-galaxies, could exist somewhere in the universe. 

Gato-Rivera returns to Earth to discuss current experiments in particle physics such as those at CERN’s Antimatter Factory, asking whether antiatoms really have the same properties as atoms, at least as far as their excitation spectra and gravitational pull is concerned. The author doesn’t shy away from popular questions such as whether antimatter anti-gravitates and would float up away from Earth. While the answers to these questions are firmly predicted in theory, there could be surprises, like the discovery of CP violation in the 1950s, so it is important to actually test these fundamental properties.

Sceptical words dash hopes of using antimatter as an energy source

The book finishes by exploring practical uses of antimatter in everyday life, such as the use of PET scanners to detect positrons emitted from short-lived radioactive substances administered to patients. The same principle is also used in material analysis, for example to test the mechanical integrity of turbine blades. But sceptical words dash any hopes of using antimatter as an energy source: the effort of artificially producing a single gram of antimatter would be prohibitive.

Gato-Rivera’s semi-popular text is comprehensive and well structured, with a minimum of mathematical expressions and technicalities. It will be most profitable for a scientifically educated audience with an interest in particle physics, however, experienced researchers who are interested in the history of the subject will also enjoy reading it.

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“The factor 10 reduction in temperature which has been achieved in our paper is just a first step,” says BASE deputy spokesperson Christian Smorra of the University of Mainz and RIKEN. “With optimised procedures we should be able to reach particle temperatures of order 20 mK to 50 mK, ideally in cooling times of order 10 seconds. Previous methods allowed us to reach 100 mK in 10 hours.”

The new setup consists of two Penning traps separated by 9 cm. One trap contains a single proton. The other contains a cloud of beryllium ions that are laser-cooled using conventional techniques. The proton is cooled as its kinetic energy is transferred through a superconducting resonant electric circuit into the cooler beryllium trap.

The proton and the beryllium ions can be thought of as mechanical oscillators within the magnetic and electric fields of the Penning traps, explains lead author Matthew Bohman of the Max Planck Institute for Nuclear Physics in Heidelberg and RIKEN. “The resonant electric circuit acts like a spring, coupling the oscillations — the oscillation of the proton is damped by its coupling to the conventionally cooled cloud of beryllium ions.”

The collaboration’s unique two-trap sympathetic-cooling technique was first proposed in 1990 by Daniel Heinzen and David Wineland. Wineland went on to share the 2012 Nobel prize in physics for related work in manipulating individual particles while preserving quantum information. The use of a resonant electric circuit to couple the two Penning traps is an innovation by the BASE collaboration which speeds up the rate of energy exchange relative to Heinzen and Wineland’s proposal from minutes to seconds. The technique is useful for protons, but game-changing for antiprotons.

Antiproton prospects

A two-trap setup is attractive for antimatter because a single Penning trap cannot easily accommodate particles with opposite charges, and laser-cooled ions are nearly always positively charged, with electrons stripped away. BASE previously cooled antiprotons by coupling them to a superconducting resonator at around 4 K, and painstakingly selecting the lowest energy antiprotons in the ensemble over many hours. 

Our technique shows that you can apply the laser-physics toolkit to exotic particles

Matthew Bohman

“With two-trap sympathetic cooling by laser-cooled beryllium ions, the limiting temperature rapidly approaches that of the ions, in the milli-Kelvin range,” explains Bohman. “Our technique shows that you can apply the laser-physics toolkit to exotic particles like antiprotons: a good antiproton trap looks pretty different from a good laser-cooled ion trap, but if you’re able to connect them by a wire or a coil you can get the best of both worlds.”

The BASE collaboration has already measured the magnetic moment of the antiproton with a record fractional precision of 1.5 parts per billion at CERN’s antimatter factory. When deployed there, two-trap sympathetic cooling has the potential to improve the precision of the measurement by at least a factor of 20. Any statistically significant difference relative to the magnetic moment of the proton would violate CPT symmetry and signal a dramatic break with the Standard Model.

“Our vision is to continuously improve the precision of our matter-antimatter comparisons to develop a better understanding of the cosmological matter-antimatter asymmetry,” says BASE spokesperson Stefan Ulmer of RIKEN. “The newly developed technique will become a key method in these experiments, which aim at measurements of fundamental antimatter constants at the sub-parts-per-trillion level. Further developments in progress at the BASE-logic experiment in Hanover will even allow the implementation of quantum-logic metrology methods to read-out the antiproton’s spin state.”

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After many years of research and development, the ALPHA collaboration has succeeded in laser-cooling antihydrogen – opening the door to considerably more precise measurements of antihydrogen’s internal structure and gravitational interactions. The seminal result, reported on 31 March in Nature, could also lead to the creation of antimatter molecules and the development of antiatom interferometry, explains ALPHA spokesperson Jeffrey Hangst. “This is by far the most difficult experiment we have ever done,” he says. “We’re over the moon. About a decade ago, laser cooling of antimatter was in the realm of science fiction.” 

The ALPHA collaboration synthesises antihydrogen from cryogenic plasmas of antiprotons and positrons at CERN’s Antiproton Decelerator (AD), storing the antiatoms in a magnetic trap. Lasers with particular frequencies are then used to measure the antiatoms’ spectral response. Finding any slight difference between spectral transitions in antimatter and matter would challenge charge–parity–time symmetry, and perhaps cast light on the cosmological imbalance of matter and antimatter.

Historically, researchers have struggled to laser-cool normal hydrogen, so this has been a bit of a crazy dream for us for many years.

Makoto Fujiwara

Following the first antihydrogen spectroscopy by ALPHA in 2012, in 2017 the collaboration measured the spectral structure of the antihydrogen 1S–2S transition with an outstanding precision of 2 × 10^–12 – marking a milestone in the AD’s scientific programme. The following year, the team determined antihydrogen’s 1S–2P “Lyman–alpha” transition with a precision of a few parts in a hundred million, showing that it agrees with the prediction for the equivalent transition hydrogen to a precision of 5 × 10^–8. However, to push the precision of spectroscopic measurements further, and to allow future measurements of the behaviour of antihydrogen in Earth’s gravitational field, the kinetic energy of the antiatoms must be lowered.

In their new study, the ALPHA researchers were able to laser-cool a sample of magnetically trapped antihydrogen atoms by repeatedly driving the antiatoms from the 1S to the 2P state using a pulsed laser with a frequency slightly below that of the transition between them. After illuminating the trapped antiatoms for several hours, the researchers observed a more than 10-fold decrease in their median kinetic energy, with many of the antiatoms attaining energies below 1 μeV. Subsequent spectroscopic measurements of the 1S–2S transition revealed that the cooling resulted in a spectral line about four times narrower than that observed without laser cooling – a proof-of-principle of the laser-cooling technique, with further statistics needed to improve the precision of the previous 1S–2S measurement (see figure).

“Historically, researchers have struggled to laser-cool normal hydrogen, so this has been a bit of a crazy dream for us for many years,” says Makoto Fujiwara, who proposed the use of a pulsed laser to cool trapped antihydrogen in ALPHA. “Now, we can dream of even crazier things with antimatter.”

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The AEgIS collaboration at CERN’s Antiproton Decelerator (AD) has reported a milestone in its bid to measure the gravitational free-fall of antimatter – a fundamental test of the weak equivalence principle. Using a series of techniques developed in 2018, the team demonstrated the first pulsed production of antihydrogen, which allows the time at which the antiatoms are formed to be known with high accuracy. This is a key step in determining “g” for antimatter.

“This is the first time that pulsed formation of antihydrogen has been established on timescales that open the door to simultaneous manipulation, by lasers or external fields, of the formed atoms, as well as to the possibility of applying the same method to pulsed formation of other antiprotonic atoms,” says AEgIS spokesperson Michael Doser of CERN. “Knowing the moment of antihydrogen formation is a powerful tool.”

General relativity’s weak equivalence principle holds that all particles with the same initial position and velocity should follow the same trajectories in a gravitational field. It has been verified for matter with an accuracy approaching 10^–14. Since theories beyond the Standard Model such as supersymmetry, or the existence of Lorentz-symmetry violating terms, do not necessarily lead to an equivalent force on matter and antimatter, finding even the slightest difference in g would suggest the presence of quantum effects in the gravitational arena. Indirect arguments constrain possible differences to below 10^–6g, but no direct measurement for antimatter has yet been performed due to the difficulty in producing and containing large quantities of it.

ALPHA, AEgIS and GBAR are all targeting a measurement of g at the 1% level in the coming years.

Antihydrogen’s neutrality and long lifetime make it an ideal system in which to test this and other fundamental laws, such as CPT invariance. The first production of low-energy antihydrogen, reported in 2002 by the ATHENA and ATRAP collaborations at the AD, involved a three-body recombination reaction (e⁺+e⁺+p → H+e⁺) involving clouds of antiprotons and positrons. Since then, steady progress by the AD’s ALPHA collaboration in producing, manipulating and trapping ever larger quantities of antihydrogen has enabled spectroscopic and other properties of antimatter to be determined in exquisite detail.

Whereas three-body recombination results in an almost continuous antihydrogen source, in which it is not possible to tag the time of the antiatom formation, AEgIS has employed an alternative charge-exchange process between trapped and cooled antiprotons and positronium (e⁺e^– bound system). Bursts of positrons are accelerated and then implanted into a nano-channelled silicon target above an electromagnetic trap containing cold antiprotons, where, with the aid of laser pulses, they produce a cloud of excited positronium a few millimetres across. This can lead to the formation of antihydrogen within sub-μs timescales, the moment of production being defined by the wellknown laser firing time and the transit time of positronium toward the antiproton cloud. Since the antihydrogen is not trapped in the apparatus, it drifts in all directions until it annihilates on the surrounding material, producing pions and photons that are detected by a scintillating array read out by photomultipliers. The scheme allows the time at which 90% of the atoms are produced to be determined with an uncertainty of around 100 ns.

Further steps are required before the measurement of g can begin, explains Doser. These include the formation of a pulsed beam, greater quantities of antihydrogen, and the ability to make it colder. “With only three months of beam time this year, and lots of new equipment to commission, most likely 2022 will be the year in which we establish pulsed beam formation, which is a prerequisite for us to perform a gravity measurement.”

Targeted approach

Following a proof-of-principle measurement of g for antihydrogen by the ALPHA collaboration in 2013, ALPHA, AEgIS and a third AD experiment, GBAR, are all targeting a measurement of g at the 1% level in the coming years. In contrast to AEgIS’s approach, whereby the vertical deviation of a pulsed horizontal beam of cold antihydrogen atoms will be measured in an approximately 1 m-long flight tube, GBAR will take advantage of advances in ion-cooling techniques to measure ultraslow antihydrogen atoms as they fall from a height of 20 cm. ALPHA, meanwhile, will release antihydrogen atoms from a vertical magnetic trap and measure the distribution of annihilation positions when they hit the wall – ramping the trap down slowly so that the coldest atoms, which are most sensitive to gravity, come out last. All three experiments have recently been hooked up to the AD’s ELENA synchrotron, which enables the production of very low-energy antiprotons.

Given that most of the mass of antinuclei comes from massless gluons that bind their constituent quarks, physicists think it unlikely that antimatter experiences an opposite gravitational force to matter and therefore “falls up”. Nevertheless, precise measurements of the free fall of antiatoms could reveal subtle differences that would open an important crack in current understanding.

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In 1947, US physicist Willis Lamb and his colleagues observed an incredibly small shift in the n = 2 energy levels of hydrogen in a vacuum. Under traditional physics theories of the day, namely the Dirac equation, these states should have the same energy and the Lamb shift shouldn’t exist. The discovery spurred the development of quantum electrodynamics (QED), which explains the discrepancy as being due to interactions between the atom’s constituents with vacuum-energy fluctuations, and won Lamb the Nobel Prize in Physics in 1955.

Antimatter spectroscopy

The ALPHA team creates antihydrogen atoms by binding antiprotons delivered by CERN’s Antiproton Decelerator (AD) with positrons. The antiatoms are then confined in a magnetic trap in an ultra-high vacuum, and illuminated with a laser to measure their spectral response. This technique enables the measurement of known quantum effects such as the fine structure and the Lamb shift, which have now been measured in the anti­hydrogen atom for the first time. The ALPHA team previously used this approach to measure other quantum effects in antihydrogen, the most recent being a measurement of the Lyman–alpha (1S–2P) transition in 2018.

The splitting of the n = 2 energy level of hydrogen is a separation between the 2P_3/2 and 2P_1/2 levels in the absence of a magnetic field, and is caused by the interaction between the electron’s spin and the orbital momentum. The classic Lamb shift is the splitting between the 2S_1/2 and 2P_1/2 levels, also in the absence of a magnetic field, and is the result of the effect on the electron of quantum fluctuations associated with virtual photons.

The work confirms that a key portion of QED holds up in both matter and antimatter

Jeffrey Hangst

In its new study, the ALPHA team determined the fine-structure splitting and the Lamb shift by inducing transitions between the lowest (n = 1) energy level of antihydrogen and the 2P_3/2 and 2P_1/2 levels in the presence of a 1  T magnetic field. Using the value of the frequency of a previously measured transition (1S–2S), the team was able to infer the values of the fine-structure splitting and the Lamb shift. The results were found to be consistent with theoretical predictions of the splittings in normal hydrogen, within the experimental uncertainties of 2% for the fine-structure splitting and 11% for the Lamb shift. “The work confirms that a key portion of QED holds up in both matter and antimatter, and probes aspects of antimatter interaction – such as the Lamb shift – that we have long looked forward to addressing,” says ALPHA spokesperson Jeffrey Hangst.

The seminal measurements of antihydrogen’s spectral structure that are now possible follow more than 30 years of effort by the low-energy antimatter community at CERN. The first antihydrogen atoms were observed at CERN’s LEAR facility in 1995 and, in 2002, the ATHENA and ATRAP collaborations produced cold (trappable) antihydrogen at the AD, opening the way to precision measurements of antihydrogen’s atomic spectra. In addition to spectral measurements, the charge-to-mass ratios for the proton and antiproton have been shown to agree to 69 parts per trillion by the BASE experiment, and the antiproton-to-electron mass ratio has been measured to agree with its proton counterpart to a level of 0.8 parts per billion by the ASACUSA experiment. The newly completed ELENA facility at the AD will increase the number of available antiprotons by up to two orders of magnitude.

Next for the ALPHA team is chilling large samples of antihydrogen using state-of-the-art laser cooling techniques. “These techniques will transform antimatter studies and will allow unprecedentedly high-precision comparisons between matter and antimatter,” says Hangst.

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Axions are extremely light, spinless bosons that were originally proposed in the 1970s to resolve the strong charge–parity problem of quantum chromodynamics and, later, were predicted by theories beyond the Standard Model. Being stable, axions produced during the Big Bang would still be present throughout the universe, possibly accounting for dark matter or some portion of it. In this case, Earth would experience a “wind” of gravitationally interacting dark-matter particles that would couple to matter and antimatter and periodically modulate their fundamental properties, such as their magnetic moment. However, no evidence of such an effect has so far been seen in laboratory experiments with ordinary matter, setting stringent limits on the microscopic properties of cosmic axion-like particles.

Our ALP–antiproton coupling limits are much more stringent than limits derived from astrophysical observations

Stefan Ulmer

The BASE team has now searched for the phenomenon in antimatter via measurements of the precession frequency of the antiproton’s magnetic moment, which it is able to determine with a fractional precision of 1.5×10^-9. The technique relies on single-particle spin-transition spectroscopy – comparable to performing NMR with a single antiproton – whereby individual antiprotons stored in a Penning trap are spin-flipped from one state to another (CERN Courier March 2018 p25). An observed variation in the precession frequency over time could provide evidence for the nature of dark matter and, if antiprotons have a stronger coupling to these particles than protons do, such a matter–antimatter asymmetric coupling could provide a link between dark matter and the baryon asymmetry in the universe.

“We’ve interpreted these data in the framework of the axion wind model where light axion like particles (ALP’s) oscillate through the galaxy, at frequencies defined by the ALP mass,” explains lead author and BASE co-spokesperson Christian Smorra of RIKEN in Japan. “The particles couple to the spins of Standard Model particles, which would induce frequency modulations of the Larmor precession frequency.”

Accruing around 1000 measurements over a three-month period, the team determined a time-averaged frequency of the antiproton’s precession of around 80 MHz with an uncertainty of 120 mHz. No signs of regular variations were found, producing the first laboratory constraints on the existence of an interaction between antimatter and a dark-matter candidate. The BASE data constrain the axion-antiproton interaction parameter (a factor in the matrix element inversely proportional to the postulated coupling between axions and antiprotons) to be above 0.1 GeV for an axion mass of 2×10⁻²³ and above 0.6 GeV for an axion mass of 4×10⁻¹⁷ eV, at 95% confidence. For comparison, similar experiments using matter instead of antimatter achieve limits of above 10 and 1000 TeV for the same mass range – demonstrating that a major violation of established charge-party-time symmetry would be implied by any signal given the current BASE sensitivity. The collaboration also derived limits on six combinations of previously unconstrained Lorentz- and CPT-violating coefficients of the non-minimal Standard Model extension.

“We have not observed any oscillatory signature, however, our ALP–antiproton coupling limits are much more stringent than limits derived from astrophysical observations,” says BASE spokesperson Stefan Ulmer of RIKEN, who is optimistic that BASE will be able to improve the sensitivity of its axion search. “Future studies, with a ten-fold improved frequency stability, longer experimental campaigns and broader spectral scans at higher frequency resolution, will allow us to increase the detection bandwidth.”

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Two new experiments at CERN, ALPHA-g and GBAR, have begun campaigns to check whether antimatter falls under gravity at the same rate as matter.

The gravitational behaviour of antimatter has never been directly probed, though indirect measurements have set limits on the deviation from standard gravity at the level of 10^–6 (CERN Courier January/February 2017 p39). Detecting even a slight difference between the behaviour of antimatter and matter with respect to gravity would mean that Einstein’s equivalence principle is not perfect and could have major implications for a quantum theory of gravity.

ALPHA-g, a close model of the ALPHA experiment, combines antiprotons from CERN’s Antiproton Decelerator (AD) with positrons from a sodium-22 source and traps the resulting antihydrogen atoms in a vertical magnetic trap about 2 m tall. To measure their free-fall, the field is switched off so that the atoms fall under gravity and the position where the antiatoms annihilate with normal matter allows the rate to be determined precisely.

GBAR adopts a similar approach but takes antiprotons from the new and lower-energy ELENA ring attached to the AD (CERN Courier December 2016 p16) and combines them with positrons from a small linear accelerator to make antihydrogen ions. Once a laser has stripped all but one positron, the neutral antiatoms will be released from the trap and allowed to fall from a height of 20 cm.

ALPHA-g began taking beam on 30 October, while ELENA has been delivering beam to GBAR since the summer, allowing the collaboration to perfect the beam-delivery system. Both experiments are being commissioned before CERN’s accelerators are shut down on 10 December for a two-year period. The ALPHA-g team hopes to be able to gather enough data during this short period to make a first measurement of antihydrogen in free fall, while the brand new GBAR experiment aims to make a first measurement when antiprotons are back in the machine in 2021. A third experiment at the AD hall, AEgIS, which has been in operation for several years, is also measuring the effect of gravity on antihydrogen using yet another approach, based on a beam of antihydrogen atoms. AEgIS is also hoping to produce its first antihydrogen atoms this year.

So far, most efforts at the AD have focused on looking for charge–parity–time violation by studying the spectroscopy of antihydrogen and comparing it with that of hydrogen (CERN Courier March 2018 p30). This latest round of experiments opens a new avenue in antimatter exploration.

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The ALPHA experiment at CERN’s Antiproton Decelerator (AD) has made yet another seminal measurement of the properties of antiatoms. Following its determination last year of both the ground-state hyperfine and the 1S–2S transitions in antihydrogen, the latter representing the most precise measurement of antimatter ever made (CERN Courier May 2018 p7), the collaboration has reported in Nature the first measurement of the next fundamental energy level: the Lyman-alpha transition. The result demonstrates that ALPHA is quickly and steadily paving the way for precision experiments that could uncover as yet unseen differences between the behaviour of matter and antimatter (CERN Courier March 2018 p30).

The Lyman-alpha (or 1S–2P) transition is one of several in the Lyman series that were discovered in atomic hydrogen just over a century ago. It corresponds to a wavelength of 121.6 nm and is a special transition in astronomy because it allows researchers to probe the state of the intergalactic medium. Finding any slight difference between such transitions in antimatter and matter would shake one of the foundations of quantum field theory, charge–parity–time (CPT) symmetry, and perhaps cast light on the observed cosmic imbalance of matter
and antimatter.

The ALPHA team makes antihydrogen atoms by taking antiprotons from the AD and binding them with positrons from a sodium-22 source, confining the resulting antihydrogen atoms in a magnetic trap. A laser is used to measure the antiatoms’ spectral response, requiring a range of laser frequencies and the ability to count the number of atoms that drop out of the trap as a result of interactions between the laser and the trapped atoms. Having successfully employed this technique to measure the 1S–2S transition, ALPHA has now measured the Lyman-alpha transition frequency with a precision of a few parts in a hundred million: 2,466,051.7 ± 0.12 GHz. The result agrees with the prediction for the equivalent transition hydrogen to a precision of 5 × 10^–8.

Although the precision is not as high as that achieved in hydrogen, the finding represents a pivotal technological step towards laser cooling of antihydrogen and the extension of antimatter spectroscopy to quantum states possessing orbital angular momentum. Simulations indicate that cooling to about 20 mK is possible with the current ALPHA set-up, which, combined with other planned improvements, would reduce the 1S–2S transition line width (see figure) by more than an order of magnitude. At such levels of precision, says the team, antihydrogen spectroscopy will have an impact on the determination of fundamental constants, in addition to providing elegant tests of CPT symmetry. Laser cooling will also allow precision tests of the weak equivalence principle via antihydrogen free-fall or antiatom-interferometry experiments.

“The Lyman-alpha transition is notoriously difficult to probe – even in normal hydrogen”, says ALPHA spokesperson Jeffrey Hangst. “But by exploiting our ability to trap and hold large numbers of antihydrogen atoms for several hours, and using a pulsed source of Lyman-alpha laser light, we were able to observe this transition. Next up is laser cooling, which will be a game-changer for precision spectroscopy and gravitational measurements.”

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The 13th Low Energy Antiproton Physics (LEAP) conference was held from 12–16 March at the Sorbonne University International Conference Center in Paris. A large part of the conference focused on experiments at the CERN Antiproton Decelerator (AD), in particular the outstanding results recently obtained by ALPHA and BASE.

One of the main goals of this field is to explain the lack of antimatter observed in the present universe, which demands that physicists look for any difference between matter and antimatter, apart from their quantum numbers. Specifically, experiments at the AD make ultra-precise measurements to test charge-party-time (CPT) invariance and soon, via the free-fall of antihydrogen atoms, the gravitational equivalence principle to look for any differences between matter and antimatter that would point to new physics.

The March meeting began with talks about antimatter in space. AMS-02 results, based on a sample of 3.49 × 10⁵ antiprotons detected during the past four years onboard the International Space Station, showed that antiprotons, protons and positrons have the same rigidity spectrum in the energy range 60–500 GeV. This is not expected in the case of pure secondary production and could be a hint of dark-matter interactions (CERN Courier December 2016 p31). The development of facilities at the AD, including the new ELENA facility, and at the Facility for Antiproton and Ion Research Facility (FAIR), were also described. FAIR, under construction in Darmstadt, Germany, will increase the antiproton flux by at least a factor of 10 compared to ELENA and allow new physics studies focusing, for example, on the interactions between antimatter and radioactive beams (CERN Courier July/August 2017 p41).

Talks covering experimental results and the theory of antiproton interactions with matter, and the study of the physics of antihydrogen, were complemented with discussions on other types of antimatter systems, such as purely leptonic positronium and muonium. Measurements of these systems offer tests of CPT in a different sector, but their short-lived nature could make experiments here even more challenging than those on antihydrogen.

Stefan Ulmer and Christian Smorra from the AD’s BASE experiment described how they managed to keep antiprotons in a magnetic trap for more than 400 days under an astonishingly low pressure of 5 × 10^–19 mbar. There is no gauge to measure such a value, only the lifetime of antiprotons and the probability of annihilation with residual gas in the trap. The feat allowed the team to set the best direct limit so far on the lifetime of the antiproton: 21.7 years (indirect observations from astrophysics indicate an antiproton lifetime in the megayear range). The BASE measurement of the proton-to-antiproton charge over mass ratio (CERN Courier September 2015 p7) is consistent with CPT invariance and, with a precision of 0.69 × 10^–12, it is the most stringent test of CPT with baryons. The BASE comparison of the magnetic moment of the proton and the antiproton at the level of 2 × 10^–10 is another impressive achievement and is also consistent with CPT (CERN Courier March 2017 p7).

Three new results from ALPHA, which has now achieved stable operation in the manipulation of antihydrogen atoms that has allowed spectroscopy to be performed on 15,000 antiatoms, were also presented. Tim Friesen presented the hyperfine spectrum and Takamasa Momose presented the spectroscopy of the 1S–2P transition. Chris Rasmussen presented the 1S–2S lineshape, which gives a resonant frequency consistent with that of hydrogen at a precision of 2 × 10^–12 or an energy level of 2 × 10^–20 GeV, already exceeding the precision on the mass difference between neutral kaons and antikaons. ALPHA’s rapid progress suggests hydrogen- like precision in antihydrogen is achievable, opening unprecedented tests of CPT symmetry (CERN Courier March 2018 p30).

The next edition of the LEAP conference will take place at Berkeley in the US in August 2020. Given the recent pace of research in this relatively new field of fundamental exploration, we can look forward to a wealth of new results between now and then.

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The ALPHA collaboration at CERN’s Antiproton Decelerator (AD) has reported the most precise direct measurement of antimatter ever made. The team has determined the spectral structure of the antihydrogen 1S–2S transition with a precision of 2 × 10^–12, heralding a new era of high-precision tests between matter and antimatter and marking a milestone in the AD’s scientific programme (CERN Courier March 2018 p30).

Measurements of the hydrogen atom’s spectral structure agree with theoretical predictions at the level of a few parts in 10¹⁵. Researchers have long sought to match this stunning level of precision for antihydrogen, offering unprecedented tests of CPT invariance and searches for physics beyond the Standard Model. Until recently, the difficulty in producing and trapping sufficient numbers of delicate antihydrogen atoms, and acquiring the necessary optical laser technology to interrogate their spectral characteristics, has kept serious antihydrogen spectroscopy out of reach. Following a major programme by the low-energy-antimatter community at CERN during the past two decades and more, these obstacles have now been overcome.

“This is real laser spectroscopy with antimatter, and the matter community will take notice,” says ALPHA spokesperson Jeffrey Hangst. “We are realising the whole promise of CERN’s AD facility; it’s a paradigm change.”

ALPHA confines antihydrogen atoms in a magnetic trap and then measures their response to a laser with a frequency corresponding to a specific spectral transition. In late 2016, the collaboration used this approach to measure the frequency of the 1S–2S transition (between the lowest-energy state and the first excited state) of antihydrogen with a precision of 2 × 10^–10, finding good agreement with the equivalent transition in hydrogen (CERN Courier January/February 2017 p8).

The latest result from ALPHA takes antihydrogen spectroscopy to the next level, using not just one but several detuned laser frequencies with slightly lower and higher frequencies than the 1S–2S transition frequency in hydrogen. This allowed the team to measure the spectral shape, or spread in colours, of the 1S–2S antihydrogen transition and get a more precise measurement of its frequency (see figure). The shape of the spectral line agrees very well with that expected for hydrogen, while the 1S–2S resonance frequency agrees at the level of 5 kHz out of 2.5 × 10¹⁵ Hz. This is consistent with CPT invariance at a relative precision of 2 × 10⁻¹² and corresponds to an absolute energy sensitivity of 2 × 10⁻²⁰ GeV.

Although the precision still falls short of that for ordinary hydrogen, the rapid progress made by ALPHA suggests hydrogen-like precision in antihydrogen is now within reach. The collaboration has also used its unique setup at the AD to tackle the hyperfine and other key transitions in the antihydrogen spectrum, with further seminal results expected this year. “When you look at the lineshape, you feel you have to pinch yourself – we are doing real spectroscopy with antimatter!” says Hangst.

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A project carried out at the Technische Universität (TU) Darmstadt in Germany, funded by the European Commission, aims to build a magnetic trap that allows antiprotons to be transported from one location to another. Launched in January, the ultimate goal of the PUMA (antiProton Unstable Matter Annihilation) project is to transfer antiprotons from CERN’s Antiproton Decelerator (AD) to the nearby ISOLDE facility to study exotic nuclear phenomena.

One of PUMA’s physics goals is to explore the occurrence of neutron halos and neutron skins in very neutron-rich radioactive nuclei. By measuring pions emitted after the capture of low-energy antiprotons by nuclei, researchers will be able to determine how often the antiprotons annihilate with the constituent nucleons and therefore deduce their relative densities at the surface of the nucleus. It would be the first time that such effects were investigated in medium-mass nuclei, contributing to a better understanding of the complex nature of nuclei and related astrophysical processes. In the future, PUMA might also allow the spectroscopy of single-particle states in heavy-nuclei with atomic numbers above 100, offering new insight into the unknown shell structure at the top of the nuclear landscape.

To make such studies possible, PUMA must trap antiprotons for long enough to be transported by truck for use in nuclear experiments at the ISOLDE facility, located a few hundred metres away from the AD. Keeping the antiprotons from annihilating with ordinary matter during this process is no easy task. The idea is to develop a double-zone trap inside a one-tonne superconducting solenoid magnet and keep it under an extremely high vacuum (10^–17 mbar) and at a temperature of 4 K. One region of the trap will confine the antiprotons, while a second zone will host collisions between antiprotons and radioactive nuclei that are produced at ISOLDE but decay too rapidly to be transported and studied elsewhere.

PUMA will eventually trap a record one billion antiprotons at CERN’s GBAR experiment, which is currently being hooked up to the ELENA facility at the AD (CERN Courier December 2016 p16), and keep them for several weeks to allow the measurements to be made. The team plans to build and develop the solenoid, trap and detection apparatus in the next two years, targeting 2022 for first collisions at ISOLDE.

Today, CERN is the only place in the world where low-energy antiprotons are produced, but “this project might lead to the democratisation of the use of antimatter,” says project leader Alexandre Obertelli of TU Darmstadt, who was awarded a €2.55 million five-year grant from the European Research Council. Along with researchers from RIKEN in Japan, CEA Saclay and IPN Orsay in France, Obertelli has submitted a letter of intent to CERN’s experiment committee concerning the future ELENA and ISOLDE activities. The PUMA apparatus could also, at a later stage, provide antiprotons to experiments beyond CERN. “For example, to universities or nuclear-physics laboratories where specific nuclei can be produced, such as the new SPIRAL2 facilities in Caen, France,” says Obertelli.

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Lurking in the background to all this is the baryon asymmetry problem: the mystery of what happened to all the antimatter that should have been created after the Big Bang. This mystery forces us to question whether antimatter and terrestrial matter really obey the same laws of physics. There is no guarantee that AD experiments will find any new physics, but if you can get your hands on some antimatter, it seems prudent to take a good, hard look at it.

We live in interesting times for antimatter. In addition to experiments at the AD, physicists study potential matter–antimatter asymmetries at the energy frontier at the LHCb experiment, and search for evidence of primordial antimatter streaming through space using the AMS-02 spectrometer onboard the International Space Station. Antihelium-4 nuclei were observed for the first time at Brookhaven’s Relativistic Heavy Ion Collider (RHIC) in 2011, while the LHC’s ALICE collaboration observed and studied anti-deuterons and antihelium-3 nuclei in 2015. By contrast, the experiments at the AD are low-energy affairs: we are essentially dealing with antimatter at rest.

One of the unique advantages of AD physics, therefore, is that we can address antimatter using precision techniques from modern atomic and ion-trap physics. Following three decades of development in advanced experimental techniques by the low-energy antimatter community, the ALPHA collaboration has recently achieved the major goal of examining the spectrum of antihydrogen atoms for the first time. These results herald the start of a new field of inquiry that should enable some of the most precise comparisons between matter and antimatter ever attempted.

Unprecedented precision

If you want to measure something precisely, you should probably ask an atomic physicist. For example, the measured frequency of the electronic transition between the ground state and the first excited state in hydrogen (the so-called 1S–2S transition) is 2 466 061 413 187 035 (10) Hz, corresponding to an uncertainty of 4.2 × 10^–15, and the measurement is referenced directly to a cesium time standard. Sounds impressive, but, to quote a recent article in Nature Photonics, “Atomic clocks based on optical transitions approach uncertainties of 10⁻¹⁸, where full frequency descriptions are far beyond the reach of the SI second”. In other words, the current time standard just isn’t good enough anymore, at least not for matter. For comparison, the current best value for the mass of the Higgs boson is 125.09 ± 0.24 GeV/c², representing an uncertainty of about 2 × 10^–3.

To be fair, scientists had already been observing hydrogen’s spectrum for about 200 years by the time the Higgs was discovered. Fraunhofer is credited with mapping out absorption lines, some of which are due to hydrogen, in sunlight in 1814. From there we can trace a direct path through Kirchhoff and Bunsen (1859/1860), who associated Fraunhofer lines with emission lines from distinct elements, to Rydberg, Balmer, Lyman and ultimately to Niels Bohr, who revolutionised atomic physics with his quantum theory in 1913. It is no exaggeration to say that physicists learned modern atomic physics by studying hydrogen, and we are therefore morally obligated to subject antihydrogen to all of the analytical tools at our disposal.

Anti-atomic spectra are not the only hot topic in precision physics at the AD. In 2015 the BASE collaboration determined that the charge-to-mass ratios for the proton and antiproton agree to 69 parts per trillion. The following year, the ASACUSA experiment – which has been making precision measurements on antiprotonic helium for more than a decade – reported that the antiproton-to-electron mass ratio agrees with its proton counterpart to a level of 8 × 10^–10 (CERN Courier December 2016 p19). One of the long term and most compelling goals of the AD programme has always been to compare the properties of hydrogen and antihydrogen to precisions like these.

A word of caution is in order here. In searching for deviations from existing theories, it is tempting to use dimensionless uncertainties such as Δm/m, Δf/f or Δq/q (corresponding to mass, frequency or charge) to compare the merits of different types of measurements. Yet, it is of course not obvious that a hitherto unknown mechanism that breaks CPT or Lorentz invariance, or reveals some other new physics, should create an observable effect that is proportional to the mass, frequency or charge of the state being studied. An alternative approach is to consider the absolute energy scale to which a measurement is sensitive. There is good historical precedent for this in the quantum mechanics of atoms. Roughly speaking, atomic structure, fine structure, hyperfine structure and the Lamb shift reflect different energy scales describing the physical effects that became apparent as experimental techniques became more precise in the 20th century.

At the time of the construction of the AD in the late 1990s, the gold standard for tests of CPT violation was the neutral kaon system. The oft-quoted limit for the fractional difference between the masses of the neutral kaon and anti-kaon was of the order 10^–18. Although there are many other tests of CPT using particle/antiparticle properties, this one in particular stands out for its precision. In the most recent review of the Particle Data Group, the kaon limit is presented as an absolute mass difference of less than 4 × 10^–19 GeV. Although purists of metrology will argue that nothing has actually been measured with a precision of 10^–18 here, the AD physics programme needed a potential goal that could compete, at least in principle, with this level of precision.

The holy grail

Thus the hydrogenic 1S–2S transition became a kind of “holy grail” for antihydrogen physics. The idea was that if the transition in antihydrogen could be measured to the same precision (10^–15) as in hydrogen, any difference between the two transition frequencies could be determined with a precision approaching that of the kaon system. On an absolute scale, the 1S–2S transition energy is about 10.2 eV, so a precision of 10^–15 in this value corresponds to an energy sensitivity of 10^–14 eV (10^–23 GeV). Other features in hydrogen such as the ground-state hyperfine splitting or the Lamb shift have even smaller energies, on the order of µeV. They are also of fundamental interest in antihydrogen and test different types of physical phenomena than the 1S–2S transition. The BASE antiproton experiment probes CPT invariance in the baryon sector at the atto-electron volt scale – 10^–27 GeV – and recently measured the magnetic moment of the antiproton to a precision of 1.5 parts-per-billion. Amazingly, the result was better than the most precise measurement of the proton at the time.

It is sobering to reflect on the state of antihydrogen physics when the AD started operations in 2000. The experiments at CERN’s Low Energy Antiproton Ring (LEAR) in 1996 and at the Accumulator at Fermilab in 1998 had detected nine and 66 relativistic atoms of antihydrogen, respectively, which were produced by interactions between a stored antiproton beam and a gas-jet target. These experiments proved the existence of antihydrogen, but they held no potential for precision measurements.

The pioneering TRAP experiment had already developed the techniques needed for stopping and trapping antiprotons from LEAR, and demonstrated the first capture of antiprotons way back in 1986. The PS200 collaboration succeeded in trapping up to a million antiprotons from LEAR, and TRAP compared the charge-to-mass ratio of protons and antiprotons to a relative precision of about 10^–9. However, no serious attempt had yet been made to synthesise “cold” antihydrogen by the time LEAR stopped operating in 1996.

In 2002 the ATHENA experiment won the race to produce low-energy antihydrogen and the global number of antihydrogen atoms jumped dramatically to 50,000, observed over a few weeks of data taking. This accomplishment had a dramatic effect on world awareness of the AD via the rapidly growing Internet, and it even featured on the front page of the New York Times. Today in ALPHA, which succeeded ATHENA in 2005, we can routinely produce about 50,000 antihydrogen atoms every four minutes.

The antihydrogen atoms produced by ATHENA, and subsequently by ATRAP and ASACUSA, were not confined; they would quickly encounter normal matter in the walls of the production apparatus and annihilate. It would take until 2010 for ALPHA to show that it was possible to trap antihydrogen atoms. Although antihydrogen atoms are electrically neutral, they can be confined through the interaction of their magnetic moments with an inhomogeneous magnetic field. Using superconducting magnets, we can trap antihydrogen atoms that are created with a kinetic energy of less than 43 μeV, or about 0.5 K in temperature units.

In ALPHA’s milestone 2010 experiment, we could trap on average one atom of antihydrogen every eight times we tried, with a single attempt requiring about 20 minutes. Today, in the second-generation ALPHA-2 apparatus, we trap up to 30 atoms in a procedure that takes four minutes. We have also learned how to “stack” antihydrogen atoms. In December 2017 we accumulated more than 1000 anti-atoms at once – limited only by the time available to mess about like this without measuring anything useful! It is no exaggeration to say that no one would have found this number credible in 2000 when the AD began running.

Since the first demonstration of trapped antihydrogen, we have induced quantum transitions in anti-atoms using microwaves, probed the neutrality of antihydrogen, and carried out a proof-of-principle experiment on how to study gravitation by releasing trapped antihydrogen atoms. These experiments were all performed with a trapping rate of about one atom per attempt. In 2016 we made several changes to our antihydrogen synthesis procedure that led to an increase in trapping rate of more than a factor of 10, and we also learned how to accumulate multiple shots of anti-atoms. At the same time, the laser system and internal optics necessary for exciting the 1S–2S transition were fully commissioned in the ALPHA-2 apparatus, and we were finally able to systematically search for this most sought-after spectral line in antimatter.

Antihydrogen’s colours

The ALPHA-2 apparatus for producing and trapping antihydrogen is shown in figure 1. It involves various Penning traps that utilise solenoidal magnetic fields and axial electrostatic wells to confine the charged antiprotons and positrons from which antihydrogen is synthesised. Omitting 30 years of detail, we produce cold antihydrogen by gently merging trapped clouds of antiprotons and positrons that have carefully controlled size, density and temperature. The upshot is that we can combine about 100,000 antiprotons with about two million positrons to produce 50,000 antihydrogen atoms. We trap only a small fraction of these in the superconducting atom trap, which comprises an octupole for transverse confinement and two “mirror coils” for longitudinal confinement.

Anti-atoms that are trapped can be stored for at least 1000 s, but we have yet to carefully characterise the upper limit of the storage lifetime, which depends on the quality of the vacuum. The internal components of ALPHA are cooled to 4 K by liquid helium, and antihydrogen annihilations are detected using a three-layer silicon vertex detector (SVD) surrounding the production region. The SVD senses the charged pions that result from the antiproton annihilation, and event topology is used to differentiate the latter from cosmic rays, which constitute the dominant background (figure 2).

A tough catch

Trapping antihydrogen is extremely challenging because the trapped, charged particles that are needed to synthesise it start out with energies measured in eV (in the case of positrons) or keV (antiprotons), whereas the atom can only be confined if it has sub-meV energy. The antihydrogen is trapped due to the interaction of its magnetic moment, which is dominated by the positron spin, with an inhomogeneous magnetic field. Even with very careful preparation of the trapped positron and antiproton clouds in a cryogenic trap, only a small fraction of the produced antiatoms are “cold” enough to be trapped. The good news is that once you have trapped them, the antiatoms stick around for long enough to perform experiments.

Compared to atomic physics with normal matter, one has to somehow make up for the dramatic reduction – at least 20 orders of magnitude – in particle number at the source. The key to this is twofold: the long interaction times available with trapped particles, and the single-atom detection sensitivity afforded by antimatter annihilation. The annihilation of an antihydrogen atom is a microscopically violent event, releasing almost 2 GeV of mass-energy that can be easily detected. This is perhaps the only good thing about working with antihydrogen: if you lose it, even just one atom of it, you know it. Conversely, the loss of a single atom of hydrogen in an equivalent experiment would go unnoticed and un-mourned if there are, say, 10¹² remaining (a typical number for trapped hydrogen). Thus, the two experiments recently reported by ALPHA are conceptually simple: trap some antihydrogen atoms; illuminate them with electromagnetic radiation that causes the anti-atoms to be lost from the trap when the radiation is on-resonance; sit back and watch what falls out.

Let’s consider first the “holy grail” (1S–2S) transition, which is excited by two, counter-propagating ultraviolet photons with a wavelength of 243 nm. The power from our Toptica 243 nm laser is enhanced in a Fabry–Pérot cavity formed by two mirrors inside the cryogenic, ultra-high vacuum system. (This cavity owes its existence to the paucity of atoms available; without the optical power buildup achieved, the experiment would not be currently possible.) The 1S–2S transition has a very narrow linewidth – this is what makes it interesting – so the laser frequency needs to be just right to excite it. The other side of the same coin is that the 2S state lives for a relatively long time, about one eighth of a second, so there can be time for an excited antihydrogen atom to absorb a third photon, which will ionise it. Stripped of its positron, the antiproton is no longer confined in the magnetic trap and is free to escape to the wall and annihilate. There is also a chance that an un-ionised 2S state atom will suffer a positron spin-flip in the decay to the ground state, in which case the atom is also lost.

In the actual experiment, we illuminate trapped antihydrogen atoms with a laser for about 10 minutes, then turn off the trap (in a period of 1.5 s) and use the SVD to count any remaining atoms as they escape. Also, using the SVD we can observe any antihydrogen atoms that are lost during the laser illumination. In this way, we obtain a self-consistent picture of the fate of the atoms that were initially trapped. The evidence for the laser interaction comes from comparing what happens when the laser has the “right” frequency, compared to what happens when we intentionally de-tune the laser to a frequency where no interaction is expected (for hydrogen). As a control, and to monitor the varying trapping rate, we perform the same sequence with no laser present. The whole thing can be summarised in a simple table (figure 3), which shows the results of 11 trials of each type.

A quick glance reveals that the off-resonance and no-laser numbers are consistent with each other and with “nothing going on”. In contrast, the on-resonance numbers show excess events due to atoms knocked out when the laser is on, and a dearth of events left over after the exposure. If we consider the overall inventory of antihydrogen atoms and compare the on- and off-resonance data only, we see that about 138 atoms (79–27)/0.376 have been knocked out, and 134 atoms (159–67)/0.688 are missing from the left-over sample, so our interpretation is self-consistent within the uncertainties.

This initial “go/no-go” experiment demonstrates that the transition is where we expect it to be for hydrogen and localises it to a frequency of about 400 kHz (the laser detuning for the off-resonance trials) out of 2.5 × 10¹⁵ Hz. That’s a relative precision of about 2 × 10^–10, or 2 × 10^–18 GeV in absolute energy units, just for showing up, and this was achieved by employing a total of just 650 or so trapped atoms. The next step is obviously to measure more frequencies around the resonance to study the shape of the spectral line, which will allow more precise determination of the resonance frequency. Note that CPT invariance requires that the shape must be identical to that expected for hydrogen in the same environment. Determination of this lineshape was the main priority for ALPHA’s 2017 experimental campaign, so stay tuned.

To hyperfine splitting and beyond

A similar strategy can be used to study other transitions in antihydrogen, in particular its hyperfine splitting. With ALPHA we can drive transitions between different spin states of antihydrogen in the magnetic trap. In a magnetic field, the 1S ground state splits into four states that correspond, at high fields, to the possible alignments of the positron and antiproton spins with the field (figure 4). The upper two states can be trapped in ALPHA’s magnetic trap and, using microwaves at a frequency of about 30 GHz, it is possible to resonantly drive transitions from these two states to the lower energy states, which are not trappable and are thus expelled from the trap.

We concentrate on the two transitions |d〉 →|a〉 and |c〉 →|b〉, which in the ALPHA trapping field (minimum 1 T) correspond to positron spin flips. We had previously demonstrated that these transitions are observable, but in 2016 we took the next step and actually characterised the spectral shapes of the two discrete transitions in our trap. We are now able to accumulate antihydrogen atoms, scan the microwave frequency over the range corresponding to the two transitions, and watch what happens using the SVD. The result, which may be considered to be the first true antihydrogen spectrum, is shown in figure 5.

The difference between the onset frequencies of the two spectral lines gives us the famous ground-state hyperfine splitting (in hydrogen, the ground-state hyperfine transition is the well known “21 cm line”, so beloved of radioastronomers and those searching for signs of extraterrestrial life). From figure 5 we extract a value for this splitting of 1420.4 ± 0.5 MHz, for a relative precision of 3.5 × 10^–4; the energy sensitivity is 2 × 10^–18 GeV. In normal hydrogen this number has been measured to be 1420.405751768 (2) MHz – that’s 1.2 × 10^–12 relative precision or a shockingly small 10^–26 GeV. ALPHA is busily improving the precision of the antihydrogen hyperfine measurement, and the ASACUSA collaboration at the AD hopes to measure the same quantity to the ppm level using a challenging antihydrogen-beam technique; an analogous experiment on hydrogen was recently reported (CERN Courier December 2017 p23).

The antihydrogen atom still holds many structural secrets to be explored. Near-term perspectives in ALPHA include the Lyman-alpha (1S–2P) transition, with its notoriously difficult-to-produce 121.5 nm wavelength in the vacuum ultraviolet. We are currently attempting to address this with a pulsed laser, with the ultimate goal to laser-cool antihydrogen for studies in gravitation and for improved resolution in spectroscopy. To give a flavour of the pace of activities, a recent daily run meeting saw ALPHA collaborators actually debate which of the three antihydrogen transitions we should study that day, which was somewhat surreal. In the longer term, even the ground-state Lamb shift should be accessible using ALPHA’s trapped antiatoms.

It is clearly “game on” for precision comparisons of matter and antimatter at the AD. It is fair to say that the facility has already exceeded its expectations, and the physics programme is in full bloom. We have some way to go before we reach hydrogen-like precision in ALPHA, but the road ahead is clear. With the commissioning of the very challenging gravity experiments GBAR, AEGIS and ALPHA-g over the next few years, and the advent of the new low-energy ELENA ring at the AD (CERN Courier December 2016 p16), low-energy antimatter physics at CERN promises a steady stream of groundbreaking results, and perhaps a few surprises.

The post Illuminating antimatter appeared first on CERN Courier.

The enigma of why the universe contains more matter than antimatter has been with us for more than half a century. While charge–parity (CP) violation can, in principle, account for the existence of such an imbalance, the observed matter excess is about nine orders of magnitude larger than what is expected from known CP-violating sources within the Standard Model (SM). This striking discrepancy inspires searches for additional mechanisms for the universe’s baryon asymmetry, among which are experiments that test fundamental charge–parity–time (CPT) invariance by comparing matter and antimatter with great precision. Any measured difference between the two would constitute a dramatic sign of new physics. Moreover, experiments with antimatter systems provide unique tests of hypothetical processes beyond the SM that cannot be uncovered with ordinary matter systems.

The Baryon Antibaryon Symmetry Experiment (BASE) at CERN, in addition to several other collaborations at the Antiproton Decelerator (AD), probes the universe through exclusive antimatter “microscopes” with ever higher resolution. In 2017, following many years of effort at CERN and the University of Mainz in Germany, the BASE team measured the magnetic moment of the antiproton with a precision 350 times better than by any other experiment before, reaching a relative precision of 1.5 parts per billion (figure 1). The result followed the development of a multi-Penning-trap system and a novel two-particle measurement method and, for a short period, represented the first time that antimatter had been measured more precisely than matter.

Non-destructive physics

The BASE result relies on a quantum measurement scheme to observe spin transitions of a single antiproton in a non-destructive manner. In experimental physics, non-destructive observations of quantum effects are usually accompanied by a tremendous increase in measurement precision. For example, the non-destructive observation of electronic transitions in atoms or ions led to the development of optical frequency standards that achieve fractional precisions on the 10^–18 level. Another example, allowing one of the most precise tests of CPT invariance to date, is the comparison of the electron and positron g-factors. Based on quantum non-demolition detection of the spin state, such studies during the 1980s reached a fractional accuracy on the parts-per-trillion level.

The latest BASE measurement follows the same scheme but targets the magnetic moment of protons and antiprotons instead of electrons and positrons. This opens tests of CPT in a totally different particle system, which could behave entirely differently. In practice, however, the transfer of quantum measurement methods from the electron/positron to the proton/antiproton system constitutes a considerable challenge owing to the smaller magnetic moments and higher masses involved.

The idea is to store single particles in ultra-stable, high-precision Penning traps, where they oscillate at characteristic frequencies. By measuring those frequencies, we can access the cyclotron frequency, ν_c, which defines the particle’s revolutions per second in the trap’s magnetic field. Together with a measurement of the spin precession frequency ν_L, the g-factor can be extracted from the relation: 

$\frac{g_{\overline{p}}}{2}=\frac{{\nu }_L}{{\nu }_c}$

To determine ν_c we use a technique called image-current detection. The oscillation of the antiproton in the trap induces tiny image currents in the trap electrodes, which are picked up by highly sensitive superconducting tuned circuits.

The measurement of ν_L, on the other hand, relies on single-particle spin-transition spectroscopy – comparable to performing NMR with a single antiproton. The idea is to switch the spin of the individual antiproton from one state to the other and then detect the flip. To this end a smart trick is used: the continuous Stern–Gerlach effect, which imprints the collapsed spin state of the single antiproton on its axial oscillation frequency (a parameter that can be measured non-destructively). We use a special Penning trap configuration in which an inhomogeneous magnetic bottle is superimposed on the homogeneous magnetic field of the ideal Penning trap (figure 2, top). The inhomogeneous  field adds a spin-dependent quadratic magnetic potential to the axial electrostatic trapping potential and, consequently, the continuously measured axial oscillation frequency of the trapped antiproton becomes a function of the spin eigenstate.

In practice, to detect spin quantum-transitions we first measure the axial frequency, then inject a magnetic radio-frequency to drive spin transitions, and finally measure the axial frequency again. The observation of an axial frequency jump corresponds to the clear signature that a spin-transition was driven, and by repeating such measurements many times and for different drive frequencies, we obtain the spin-flip probability as a function of the drive frequency. The corresponding resonance curve gives ν_L (figure 2, bottom).

Doubling up

This challenge has become the passion of the members of the BASE collaboration for the past decade. A trap was developed at Mainz with a superimposed magnetic inhomogeneity of 300,000 T/m², which corresponds to a magnetic field change of about 1 T over a distance of about 1.5 mm! In this extreme magnetic environment, a proton/antiproton spin transition induces an axial frequency shift of only 170 mHz when driven at a frequency of around 650 kHz.

Using this unique device, in 2011 we reported the first observation of spin flips with a single trapped proton. This was followed by the unambiguous quantum-non-demolition detection of proton spin-transitions, which was later also demonstrated with antiprotons (figure 3). The high-fidelity detection of the spin state, however, requires the particle to be cooled to temperatures of the order of 100 mK. This was achieved by sub-thermal cooling of the particle’s cyclotron mode by means of cryogenic resistors, but is an inconceivably time-consuming procedure.

The high-fidelity resolution of single-spin quantum transitions is the key to measuring the antiproton magnetic moment at the parts-per-billion level. The elegant double-trap technique that makes this possible was invented at Mainz and applied with great success in tests of bound-state quantum electrodynamics, in collaboration with GSI Darmstadt and the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, both institutes also being part of the BASE collaboration. This double Penning-trap technology separates the sensitive frequency measurements of ν_L and ν_c, and the spin analysis measurements into two traps: a homogeneous “precision trap” (PT) and the spin state “analysis trap” (AT) with the superimposed strong magnetic bottle. The magnetic field in the PT is about 100,000 times more homogeneous than that of the AT and allows sampling of the spin-flip resonance at much higher resolution, compared to measurements solely carried out in the inhomogeneous AT.

The single-particle “double-trap method”, however, comes with the drawback that each frequency measurement in the PT heats the particle’s radial mode to about room-temperature and requires repeated particle preparation to sub-thermal radial energy, a condition that is ultimately required for the high-fidelity detection of spin transitions. Each of these sub-thermal-energy preparation cycles takes several hours, while a well resolved g-factor resonance contains at least 400 individual data points. We applied this method at BASE to measure the proton magnetic moment with parts-per-billion precision in a measurement campaign that took, including systematic studies and maintenance of the instrument, about half a year.

To reduce the total measurement time, we invented the novel two-particle method in which the precision frequency measurements and the high-fidelity spin-state analysis are carried out using two particles: a hot “cyclotron particle” and a cold “Larmor particle”, in addition to adding a third trap called the “park trap” (figure 4). We first identify the spin state of the cold antiproton in the AT. Then we measure the cyclotron frequency with the hot particle in the PT, move this particle to the park trap and transport the cold antiproton to the PT, where spin-flip drives are irradiated. Afterwards, the cold particle is shuttled back to the AT and the hot particle to the PT. There, the cyclotron frequency is measured again, and in a last step the spin state of the cold particle in the AT is identified. By repeating this scheme many times and for different drive frequencies, the spin-flip probability as a function of the spin-flip drive frequency, normalized to the measured cyclotron frequency, is obtained – a g-factor resonance – with all the required frequency information sampled in the homogeneous PT. This novel two-particle scheme drastically reduces the measurement time, since it avoids the time-consuming preparation of sub-thermal radial energy-states.

Successfully implementing this new method, we were able to sample about 1000 data points over a period of just two months. From this campaign we extracted the antiproton magnetic moment as µ_p̅ = –2.792 847 344 1 (42) μ_N, the value having a fractional precision of 1.5 parts per billion and thereby improving the previous best value by BASE by a factor of 350. The result is consistent with our most precise measurement of the proton magnetic moment, μ_p = 2.792 847 350 (9) µ_N, and thus supports CPT invariance.

Trappings of success

Underpinning this rapid achievement of the initially defined major experimental goal of the BASE collaboration was another BASE invention called the reservoir trap (RT) method. This RT, being one of four traps in the BASE trap-stack, is loaded with a shot of antiprotons and provides single particles to the precision measurement traps on request. The method allows BASE to operate antiproton experiments even during the winter shut-down of CERN’s accelerators and practically doubles the available experiment time. Indeed, we have demonstrated antiproton trapping and experiment optimisation for a period of more than 400 days and operated the entire 2016 run with antiprotons captured in 2015. This long storage time also allows us to set limits on directly measured antiproton lifetime.

Together with the proton-to-antiproton charge-to-mass ratio comparison with a fractional precision of 69 parts in a trillion CERN Courier September 2015 p7), which was carried out during the 2014 antiproton run, BASE has set tighter constraints on all the fundamental antiproton parameters that are directly accessible by this type of experiment. So far, all the BASE results are consistent with CPT invariance.

The latest triple-trap measurement of the antiproton magnetic moment sets new constraints on CPT violating coefficients in the Standard Model extension (SME) – an effective theory that allows the sensitivities of different experiments at different locations to be compared with respect to CPT violation. The recent BASE magnetic-moment measurement addresses a total of six combinations of SME coefficients and improves the limits on all of them by more than two orders of magnitude. Finding a non-zero coefficient would, for example, indicate the discovery of a new type of exchange boson that couples exclusively to antimatter and immediately raise the question of its role in the universal baryon asymmetry.

Although up to now all results are CPT-consistent, this not-yet-understood asymmetry is one of the motivations to further improve the experimental resolution of the AD experiments. The recent successes reported by the ALPHA collaboration herald the first ultra-high-precision measurements on the optical spectrum of antihydrogen. Improved methods in measurements on antiprotonic helium by the ASACUSA collaboration will lead to even higher resolution results in comparisons of the antiproton-to-electron mass ratio, while the ATRAP collaboration continues to contribute independent measurements of antiprotons and antihydrogen.

Gravitational sensitivity

A new branch of experiments at CERN’s AD, AEgIS, GBAR and ALPHA-g, will soon investigate the gravitational acceleration of antimatter in Earth’s gravitational field – which has never been directly observed before. Indirect measurements were carried out with antiprotons by the TRAP collaboration at the AD’s predecessor, LEAR, and by BASE, which set constrains on antigravity effects.

The AD community aims to verify the laws of physics with antimatter in various ways, thereby testing fundamental CPT invariance. The experiments are striving to access yet unmeasured quantities, or to improve their sensitivities to new physics. In this respect, the BASE–Mainz experiment succeeded recently in measuring the proton magnetic moment at an 11-fold improved precision, reaching a fractional uncertainty of 0.3 parts per billion. By applying these even further advanced methods to the antiproton, BASE will improve the sensitivity of the CPT invariance test by at least another factor of five.

The post Experiment of the moment appeared first on CERN Courier.

Since 2008, astronomers have been puzzled by a mysterious feature in the cosmic-ray energy spectrum. Data from the PAMELA satellite showed a significant increase in the ratio of positrons to electrons at energies above 10 GeV. This unexpected positron excess was subsequently confirmed by both the Fermi-LAT satellite and the AMS-02 experiment onboard the ISS (CERN Courier December 2016 p26–30), sparking many explanations, ranging from dark-matter annihilation to positron emission by nearby pulsars. New measurements by the High-Altitude Water Cherenkov (HAWC) experiment now seem to rule out the second explanation, hinting at a more exotic origin of the positron excess.

Although standard cosmic-ray propagation models predict the production of positrons from interactions of high-energy protons travelling through the galaxy, the positron fraction is expected to decrease as a function of energy. One explanation for the excess is the annihilation of dark-matter particles with masses of several TeV, which would result in a bump in the electron–positron fraction, with the measured increase perhaps being the rising part of such a bump. According to other models, however, the excess is the result of positron production by astrophysical sources such as pulsars (rapidly spinning neutron stars). Since these charged particles lose energy due to interactions with interstellar magnetic and radiation fields they must be produced relatively close to Earth, making nearby pulsars a prime suspect.

HAWC, situated near the city of Puebla in Mexico, detects charged particles created in the Earth’s atmosphere from collisions between high-energy photons and atmospheric nuclei. The charged particles produced in the resulting shower produce Cherenkov radiation in HAWC’s 300 water tanks, their high altitude location making HAWC the most sensitive survey instrument to measure astrophysical photons in the TeV range. This allows the study of TeV-scale photon emission from nearby pulsars, such as Geminga and PSR B0656+14, to investigate if these objects could be responsible for the positron excess.

Pulsars are thought to emit electrons and positrons with energies up to several hundred TeV, which diffuse into the interstellar medium, but the details of the emission, acceleration and propagation of these leptons are not well understood. The TeV photons measured by HAWC are produced as the electrons and positrons emitted by the pulsars interact with low energy photons in the interstellar medium. One can, therefore, use the intensity of the TeV photon emission and the size of the emitting region to indirectly measure the high-energy positrons. The HAWC data show the large emitting regions of both the pulsars Geminga and PSR B0656+14 (see figure). The spectral and spatial features of the TeV emission were then inserted in a diffusion model for the positrons, allowing the team to calculate the positron flux from these sources reaching Earth. The results, published in Science, indicate that the positron flux from these sources reaching Earth is significantly smaller than that measured by PAMELA and AMS-02.

These indirect measurements of the positron emission appear to rule out a significant contribution of the local positron flux by these two pulsars, making it unlikely that pulsars are the origin of the positron excess. More exotic explanations such as dark matter, or other astrophysical sources such as micro-quasars and supernovae remnants, are not ruled out, however. Results from gamma-ray observations of such sources, along with more detailed measurements of the lepton flux at even higher energies by AMS-02, DAMPE or CALET, are therefore highly anticipated to fully solve the mystery of the cosmic positron excess.

The post HAWC clarifies cosmic positron excess appeared first on CERN Courier.

Since the discovery of the positron in 1932 and the antiproton in 1955, physicists have striven to confront the properties of leptonic and baryonic matter and antimatter. A major advance in the story took place in 1995 when the first antihydrogen atoms were observed at CERN’s LEAR facility. Then, in 2002, the ATHENA and ATRAP collaborations produced cold (trappable) antihydrogen at CERN’s Antiproton Decelerator (AD), paving the way to the first measurement of antihydrogen’s atomic transitions. An intense research programme at the AD has followed to compare the atomic states of antimatter with the most well-known atomic transitions in matter.

The physical properties of antimatter particles are tightly constrained within the Standard Model of particle physics (SM). For all local Lorentz-invariant quantum-field theories of point-like particles like the SM, the combination of the discrete symmetries charge-conjugation, parity and time-reversal (CPT) is conserved. An implication of the CPT theorem is that the properties of matter and antimatter are equal in absolute value. In this respect the lack of observation of primordial antimatter in the universe is tantalising, hinting that the universe has a preference for matter over antimatter despite their perfect symmetry on the microscopic scale as imposed by the SM. Although violations of CP symmetry, from which an imbalance in matter and antimatter can arise, have been observed in several systems, the effect is many orders of magnitude too small to account for the observed cosmological mismatch.

In the quest for a quantitative explanation to the baryon asymmetry in the universe, one could question the validity of our formulation of the laws of physics in terms of quantum-field theory. This is additionally motivated by the notable absence of the gravitational force in the SM and would suggest that CPT symmetry (or Lorentz invariance) need not be conserved. A framework called Standard Model Extension (SME), an effective field theory that contains the SM and general relativity but also possible CPT and Lorentz violating terms, allows researchers to interpret the results of experiments designed to search for such effects.

Any measurement with antihydrogen atoms constitutes a model-independent test of CPT invariance. Given the precision at which they have been measured in hydrogen, two atomic transitions in antihydrogen are of particular interest: the 1S–2S transition and the ground-state hyperfine splitting (which corresponds to the 21 cm microwave-emission line between parallel and antiparallel antiproton and positron spins). These were determined over the past few decades in hydrogen with an absolute (relative) precision of 10 Hz (4 × 10^–15) and 2 mHz (1.4 × 10^–12), respectively. Reaching similar precision in antihydrogen, hydrogen’s CPT conjugate would provide one of the most sensitive CPT tests in what was until recently a yet unprobed atomic domain. But this is a daunting challenge.

Status and prospects

Measurements of the hyperfine splitting of hydrogen reached their apogee in the 1970s. It is only recently that interest in such measurements has been revived, motivated by the possibility to further develop methods that can be applied to antihydrogen. Hydrogen’s hyperfine splitting was originally measured using a maser to interrogate atoms held in a Teflon-coated storage bulb, but this technique is not transferable to antihydrogen because unavoidable interactions between the antiatoms and the walls would lead to annihilations.

A precision of a few Hz can, however, be envisioned using the “beam-resonance” method of Rabi. This technique involves a polarised beam, microwave fields to drive spin flips, magnetic-field gradients to select a spin state, and a detector to measure the flux of atoms as a function of the microwave frequency. While less precise than the maser technique, the in-beam method can be directly applied to antihydrogen with a foreseen initial precision of a few kHz (10^–6 relative precision). The leading order of the hyperfine splitting can be calculated from the known properties of the antiproton and positron, but a 10^–6 level measurement would be sensitive to the antiproton magnetic and electric form factors that are so far unknown.

Earlier this year, the ALPHA experiment at CERN’s AD measured the hyperfine splitting of trapped antihydrogen. Following a long campaign that saw ALPHA determine antihydrogen’s 1S–2S transition in 2016 (CERN Courier January/February 2017 p8), the collaboration achieved a precision of 4 × 10^–4 (0.5 MHz) on the hyperfine measurement. Ultimately the precision of in-trap measurements will be limited by the presence of strong magnetic-field gradients, however. The in-beam technique, by contrast, probes the hyperfine transition far away from the strong inhomogeneous magnetic trapping fields. In the 1950s this technique enabled hydrogen’s hyperfine structure to be determined to a precision of 50 Hz. The recent measurement of this transition by the ASACUSA experiment using a similar technique has now improved on this precision by more than an order of magnitude.

The ASACUSA collaboration was formed in 1997 to investigate antiprotonic atoms and collisions involving slow antiprotons. Its antihydrogen programme started in 2005 at the AD and in recent years the collaboration has focused on two topics. One is laser spectroscopy of antiprotonic helium, which allows the determination of the antiproton mass (CERN Courier September 2011 p7) and the antiproton magnetic moment. The latter value was recently measured to higher precision in Penning traps first by the ATRAP experiment (CERN Courier May 2013 p6) and, as announced in October, further improved by more than three orders of magnitude by the BASE experiment, both also located at the AD.

The second focus of ASACUSA, led by the CUSP group, is to measure the hyperfine structure of antihydrogen in a polarised beam. ASACUSA employs a multi-trap set-up to produce an antihydrogen beam (CERN Courier March 2014 p5) for Rabi-type spectroscopy on the hyperfine transition. The spectroscopy apparatus was designed to match the expected properties of an antihydrogen beam and called for a test of the apparatus with a hydrogen beam of similar characteristics.

Hydrogen first

The spectroscopy technique relies on the dependency of the atomic energy levels on a magnetic field, also known as the Zeeman effect (figure 1). In the presence of a magnetic field, the degeneracy of the hyperfine triplet states is lifted. Two of the states, called low-field seekers (lfs), have a rising energy with rising magnetic field, while the third state of the triplet and the singlet state decrease their energies with rising magnetic field (they are called high-field seekers, hfs). These distinguishing properties are used to first polarise the beam by means of a magnetic-field gradient (figure 2), which leads to opposite forces on lfs and hfs. As a result, only lfs arrive at the interaction region, where a microwave cavity provides an oscillating magnetic field. This field can then induce state conversions from lfs to hfs if tuned to the right frequency. Atoms in hfs states are subsequently removed from the beam by a second section of magnetic-field gradients, thus leading to a reduced count rate at the detector when the transition is induced.

In the apparatus design chosen, large geometrical openings compensate for the low antihydrogen flux and a superconducting magnet is used to generate sufficiently selective magnetic-field gradients over such a large area. The oscillating microwave field needed to drive the hyperfine transition must be homogenous over the large geometrical opening, which dictated the design of the cavity leading to a particular resonance spectrum (figure 3). The functionality of the spectroscopy apparatus and other technical developments were tested by coupling a cold and polarised hydrogen source and a quadrupole mass spectrometer as hydrogen detector to the spectroscopy apparatus envisioned for the antihydrogen experiment (figure 2).

The measurement led to the determination of the hydrogen’s so-called σ₁ hyperfine transition (figure 1), the transition frequency of which was measured as a function of an externally applied magnetic field. From a set of frequency determinations, the zero-field value could be extracted and such measurements were repeated under 10 distinct conditions to investigate systematic effects. In total more than 500 resonances (an example is shown in figure 3) were acquired to extract the zero-field hydrogen ground-state hyperfine splitting. Numerical methods developed to assist the analysis of the transition line shape contributed to the improvement by more than an order of magnitude, leading to a precision of 3.8 Hz and a value consistent with the more precise maser result.

A measurement of hydrogen’s hyperfine splitting at the Hz level implies an absolute precision of 10^–15 eV. Given the scarcity of antihydrogen and the yet unprobed properties (namely velocity and atomic states) of the antihydrogen beam, a measurement at this level of precision on antihydrogen is not possible in the short-term. However, the analysis of ASACUSA data collected with hydrogen enabled the collaboration to assess the necessary number of antiatoms to reach a 10^–6 sensitivity, assuming plausible beam properties. The conclusion is that a measurement at the peV level (kHz precision) should be possible if 8000 antiatoms can be detected after the spectrometer. That would require at least an order-of-magnitude increase in the antihydrogen flux.

The Rabi-type spectroscopy approach chosen by ASACUSA has the capability to test individual transitions in hydrogen and antihydrogen under well-controlled external conditions and, if successful, will immediately result in a precision of 10^–6 or better. At this level, the hyperfine transitions would provide yet unknown information on the internal structure of the antiproton. However, much work remains to be done for the ASACUSA experiment to gather the needed number of antihydrogen atoms in a reasonable time.

Until then, more measurements can be performed with the hydrogen set-up. The apparatus has recently been modified to allow for the simultaneous measurement of σ₁ and π₁ transitions (figure 1). Within the SME, the latter transition could reveal CPT and Lorentz violations while the σ₁ transition is insensitive to these effects and would serve as a monitor of potential systematic errors. This would give access to a number of so-far-unconstrained SME parameters that can be probed by hydrogen alone. While the antihydrogen experiment focuses on increasing the cold, ground- state antihydrogen flux, the hydrogen experiment is about to start a new measurement campaign for which results are expected in the next 18–24 months. The hydrogen atom has been a source of profound theoretical developments for some time, and history has shown that it is well worth the effort to study it ever more closely.

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On 1 March, the first component of a new CERN experiment called GBAR (Gravitational Behaviour of Antihydrogen at Rest) was installed: a 1.2 m-long linear accelerator that will be used to generate positrons. Located in the Antiproton Decelerator (AD) hall, GBAR is the first of five experiments that will be connected to the new ELENA deceleration ring and it is specifically designed to measure the effect of gravity on antihydrogen atoms. The experiment will use antiprotons supplied by ELENA and positrons created by the linac to produce antihydrogen ions, which will be slowed almost to a standstill using lasers and then allowed to fall under gravity over a vertical distance of 20 cm.

Although antimatter is not expected to fall “up”, detecting even the tiniest difference between the rate at which matter and antimatter fall would have profound implications for fundamental laws such as Einstein’s equivalence principle. Two further experiments that are based at the AD, AEGIS and ALPHA, are also studying the effect of gravity on antimatter. First results on anti-ion production are expected next year, with gravity studies following later.

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The Baryon Antibaryon Symmetry Experiment (BASE) collaboration at CERN has made the most precise direct measurement of the magnetic moment of the antiproton, allowing a fundamental comparison between matter and antimatter.

The BASE measurement shows that the magnetic g-factors (which relate the magnetic moment of a particle to the nuclear magneton) of the proton and antiproton are identical within the experimental uncertainty of 0.8 parts per million: 2.7928465(23) for the antiproton, compared to 2.792847350(9) for the proton. The result improves the precision of the previous best measurement by the ATRAP collaboration in 2013, also at CERN, by a factor of six.

Comparisons of the magnetic moments of the proton and antiproton at this level of precision provide a powerful test of CPT invariance. Were even slight differences to be found, it would point to physics beyond the Standard Model. It could imply, for example, the existence of a new vector boson that couples only to antimatter, which could have a direct effect on the lifetime of baryons. Such effects more generally could also shed light on the mystery of the missing antimatter observed on cosmological scales.

BASE uses antiprotons from CERN’s Antiproton Decelerator (AD), which serves several other experiments making rapid progress in precision antimatter measurements (CERN Courier December 2016 p16). By trapping the particles in electromagnetic containers called Penning traps and cooling them to temperatures below 1 K, the BASE team can measure the cyclotron and Larmor frequencies of single trapped antiprotons. By measuring the ratio of these two frequencies the magnetic moment of the antiproton is obtained in units of the nuclear magneton.

Similar techniques have been successfully applied in the past to electrons and positrons. However, antiprotons present a much bigger challenge because their magnetic moments are considerably weaker, requiring BASE to design Penning traps with about 2000 times higher sensitivity with respect to magnetic moments. BASE now plans to measure the antiproton magnetic moment using a new double-Penning trap technique, which should enable a precision at the level of a few parts per billion in the future.

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Measuring the effect of gravity on antimatter is a long-standing story. It started with a project at Stanford in 1968 that attempted to measure the free fall of positrons, but a trial experiment with electrons showed that environmental effects swamped the effect of gravity and the final experiment was not performed. In the 1990s, the PS200 experiment at CERN’s LEAR facility attempted the same feat with antiprotons, but the project ended with the termination of LEAR before any robust measurement could be made. To date, indirect measurements have set limits on the deviation from standard gravity at the level of 10^–6.

Thanks to advances in cooling and trapping technology, and the construction of a new synchrotron at CERN called ELENA, three collaborations are now preparing experiments at CERN’s Antiproton Decelerator (AD) facility to measure the behaviour of antihydrogen (a positron orbiting an antiproton) under gravity. The ALPHA experiment has already analysed its data on the trapping of antihydrogen atoms to set upper limits on differences in the free-fall rate of matter and antimatter, and is now designing a new set-up. AEgIS is currently putting its apparatus through its paces, while GBAR will start installation in 2017.

Given that most of the mass of antinuclei comes from massless gluons, it is extremely unlikely that antimatter experiences an opposite gravitational force to matter and therefore “falls” up. Nevertheless, precise measurements of the free fall of antiatoms could reveal subtle differences that point to a crack in our current understanding.

Violating equivalence

To date, most efforts at the AD have focused on looking for CPT violation by comparing the spectroscopy of antihydrogen to its well-known matter counterpart, hydrogen. Now we are in a position to test Einstein’s equivalence principle with antimatter by directly measuring the free fall of antiatoms on Earth. The equivalence principle is the keystone of general relativity and states that all particles with the same initial position and velocity should follow the same trajectories in a given gravitational field. On the other hand, quantum theories such as supersymmetry or superstrings do not necessarily lead to an equivalent force on matter and antimatter (technically, the terms related to gravity in the Lagrangians are not bound to be the same for matter and antimatter). This is also the case when Lorentz-symmetry violating terms are included in the Standard Model of particle physics.

Any difference seen in the behaviour of antimatter and matter with respect to gravity would mean that the equivalence principle is not perfect and force us to understand quantum effects in the gravitational arena. Experiments performed with free-falling matter atoms have so far found no difference to that of macroscopic objects. Such tests have set limits at the level of one part in 10¹³, but have not yet been able to test the equivalence principle at the level where supersymmetric or other quantum effects would appear. Since the amplitude of these effects could be different for antimatter, the AD experiments might have a better opportunity to test such quantum effects. Any difference would probably not change anything in the observable universe, but it would point to the necessity of having a quantum theory of gravity.

AEgIS plans to measure the vertical deviation of a pulsed horizontal beam of cold antihydrogen atoms, generated by bringing laser-excited positronium moving at several km/s into contact with cold antiprotons, travelling with a velocity of a few hundred m/s. The resulting highly excited antihydrogen atoms are then accelerated horizontally and a moiré deflectometer used to measure the vertical deviation, which is expected to be a few microns given the approximately 1 m-long flight tube of AEgIS. Reaching the lowest possible antiproton temperature minimises the divergence of the beam and therefore maximises the flux of antihydrogen atoms that end up on the downstream detector.

In GBAR, which takes advantage of advances in ion-cooling techniques, antihydrogen ions (H⁺) are produced with velocities of the order of 0.5 m/s. In a second step, the anti-ions will be stripped of one positron to give an ultra-slow neutral antiatom that is allowed to enter free fall. The time of free fall over a height of 20 cm is as long as 200 ms, which is easily measurable. These numbers correspond to the gravitational acceleration known for matter atoms, and the expected sensitivity to small deviations is 1% in the first phase of operation.

The ALPHA-g experiment will release antihydrogen atoms from a vertical magnetic atom trap and record their positions when they annihilate on the walls of the experiment. In a proof-of-principle experiment using the original ALPHA atom trap, the acceleration of antihydrogen atoms by gravity was constrained to lie anywhere between –110 g and 65 g. ALPHA-g improves on this original demonstration by orienting the trap vertically, thereby enabling better control of the antiatom release and improving sensitivity to the vertical annihilation position. In the new arrangement, antihydrogen gravitation can be measured at the 10% level, which would already settle the question of whether antimatter falls up or down, but improvements in cooling techniques will allow measurements at the 1% level. A long-term aspiration of the ALPHA-g project is to use techniques that cause antihydrogen atoms to interact with a beam of photons, promising a sensitivity in the 10^–6 range.

Cooling matter

In the case of AEgIS, the deflectometer principle that underpins the measurement has already been demonstrated with matter atoms and with antiprotons, while the time-of-flight measurement is straightforward in the case of GBAR. The difficulty for the experiments lies in preparing sufficient numbers of antiatoms at the required low velocities. ALPHA has already demonstrated trapping of several hundred antiatoms at a temperature below 0.5 K, corresponding to random velocities of the order 10 m/s. The antiatoms are formed by letting the antiprotons traverse a plasma of positrons located within the same Penning trap.

A different scheme is used in AEgIS and GBAR to form and possibly cool the antiatoms and anti-ions. In AEgIS, antiprotons are cooled within a Penning trap and receive a shower of positronium atoms (bound e⁺e^– pairs) to form the antiatoms. These are then slightly accelerated by electric fields (which act on the atoms’ induced electric-dipole moments) so that they exit the charged particle trap axially in the form of a neutral beam. For GBAR, the antiproton beam traverses a cloud of positronium to form the anti-ions, which are then cooled to a few μK by forcing them to interact with laser-cooled beryllium ions.

In this race towards low energies, ALPHA and AEgIS are located on the beam at the AD, which delivers 5 MeV antiprotons. While AEgIS is already commissioning its dedicated gravity experiment, ALPHA will move from spectroscopy to gravity in the coming months. GBAR, which will be the first experiment to make use of the beam delivered by ELENA, is now beginning installation and expects first attempts at anti-ion production in 2018. ELENA will decelerate antiprotons coming from the AD from 5 MeV to just 100 keV, making it more efficient to trap and store antimatter. Following commissioning first with protons and then with hydrogen ions, ELENA should receive its first antiprotons in the middle of 2017 (CERN Courier December 2016 p16). Along with precision tests of CPT invariance, this facility will help to ensure that any differences in the gravitational antics of antimatter are not missed.

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Following 20 years of research and development by the CERN antimatter community, the ALPHA collaboration has reported the first ever measurement of the optical spectrum of an antimatter atom. The result, published in Nature in December, involves technological developments that open a completely new era in high-precision antimatter research.

Comprising a single electron orbiting a single proton, hydrogen is the simplest and most well-understood atom, and has played a central role in fundamental physics for more than a century. Its spectrum is characterised by well-known spectral lines at certain wavelengths, corresponding to the emission of photons when electrons jump between different orbits. Measurements of the hydrogen spectrum agree with the predictions of quantum electrodynamics at the level of a few parts in 10¹⁵, and CPT invariance requires that antihydrogen has exactly the same spectrum.

The ALPHA team has now succeeded in observing the first spectral line in an atom of antihydrogen, made up of an antiproton and a positron. The measurement concerned the 1S–2S transition, which has a lifetime on the order of a tenth of a second and therefore leads to a narrow spectral line that is particularly suitable for precision measurements. The measurement was found to be in agreement with the hydrogen spectrum, and therefore consistent with CPT invariance, with a relative precision of around 2 × 10^–10.

Comparing the spectra of hydrogen and antihydrogen was one of the main scientific motivations for CERN’s Antiproton Decelerator (AD), since it offers an extraordinary new tool to test whether matter behaves differently from antimatter and thus test the robustness of the Standard Model. The ALPHA collaboration, which expects to improve the precision of its measurements, generates roughly 25,000 antihydrogen atoms per trial by mixing antiprotons from the AD with positrons. Around 14 antiatoms per trial are trapped and interrogated by a laser at a precisely tuned frequency to measure their internal states.

Low-energy antihydrogen was first synthesised by the ATHENA collaboration in 2002, later repeated by the ATRAP, ALPHA and ASACUSA collaborations, and ALPHA trapped the first antihydrogen atoms in 2010. The new result, along with recent limits on the antiproton–electron mass ratio by the ASACUSA collaboration and antiproton charge-to-mass ratio by the BASE collaboration, demonstrates that tests of fundamental symmetries with antimatter at CERN are maturing rapidly.

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The Antiproton Decelerator (AD) facility at CERN, which has been operational since 2000, is a unique source of antimatter. It delivers antiprotons with very low kinetic energies, enabling physicists to study the fundamental properties of baryonic antimatter – namely antiprotons, antiprotonic helium and antihydrogen – with great precision. Comparing the properties of these simple systems to those of their respective matter conjugates therefore provides highly sensitive tests of CPT invariance, which is the most fundamental symmetry underpinning the relativistic quantum-field theories of the Standard Model (SM). Any observed difference between baryonic matter and antimatter would hint at new physics, for instance due to the existence of quantum fields beyond the SM.

In the case of matter particles, physicists have developed advanced experimental techniques to characterise simple baryonic systems with extraordinary precision. The mass of the proton, for example, has been determined with a fractional precision of 89 parts in a trillion (ppt) and its magnetic moment is known to a fractional precision of three parts in a billion. Electromagnetic spectroscopy on hydrogen atoms, meanwhile, has allowed the ground-state hyperfine splitting of the hydrogen atom to be determined with a relative accuracy of 0.7 ppt and the 1S/2S electron transition in hydrogen to be determined with a fractional precision of four parts in a quadrillion – a number that has 15 digits.

ELENA will lead to an increase by one to two orders of magnitude in the number of antiprotons captured by experiments

In the antimatter sector, on the other hand, only the mass of the antiproton has been determined at a level comparable to that in the baryon world (see table). In the late 1990s, the TRAP collaboration at CERN’s LEAR experiment used advanced trapping and cooling methods to compare the charge-to-mass ratios of the antiproton and the proton with a fractional uncertainty of 90 ppt. This was, among others, one of the crucial steps that inspired CERN to start the AD programme. Over the past 20 years, CERN has made huge strides towards our understanding of antimatter (see panel). This includes the first ever production of anti-atoms – antihydrogen, which comprises an antiproton orbited by a positron – in 1995 and the production of antiprotonic helium (in which an antiproton and an electron orbit a normal helium nucleus).

CERN has decided to boost its AD programme by building a brand new synchrotron that will improve the performance of its antiproton source. Called the Extra Low ENergy Antiproton ring (ELENA), this new facility is now in the commissioning phase. Once it enters operation, ELENA will lead to an increase by one to two orders of magnitude in the number of antiprotons captured by experiments using traps and also make new types of experiments possible (see figure). This will provide an even more powerful probe of new physics beyond the SM.

Combined technologies

The production and investigation of antimatter relies on combining two key technologies: high-energy particle-physics sources and classical low-energy atomic-physics techniques such as traps and lasers. One of the workhorses of experiments in the AD facility is the Penning trap. This static electromagnetic cage for antiprotons serves for both high-precision measurements of the fundamental properties of single trapped antiprotons and for trapping large amounts of antiprotons and positrons for antihydrogen production.

The AD routinely provides low-energy antiprotons to a dynamic and growing user community. It comprises a ring with a circumference of 182.4 m, which currently supplies five operational experiments devoted to studying the properties of antihydrogen, antiprotonic helium and bare antiprotons with high precision: ALPHA, ASACUSA, ATRAP, AEgIS and BASE (see panel). All of these experiments are located in the existing experimental zone, covering approximately one half of the space inside the AD ring. With this present scheme, one bunch containing about 3 × 10⁷ antiprotons is extracted roughly every 120 seconds at a kinetic energy of 5.3 MeV and sent to a particular experiment.

Although there is no hard limit for the lowest energy that can be achieved in a synchrotron, operating a large machine at low energies requires magnets with low field strengths and is therefore subject to perturbations due to remanence, hysteresis and external stray-field effects. The AD extraction energy of 5.3 MeV is a compromise: it allows beam to be delivered under good conditions given the machine’s circumference, while enabling the experiments to capture a reasonable quantity of antiprotons. Most experiments further decelerate the antiprotons by sending them through foils or using a radiofrequency quadrupole to take them down to a few keV so that they can be captured. This present scheme is inefficient, however, and less than one antiproton in 100 that have been decelerated with a foil can be trapped and used by the experiments.

The ELENA project aims to further decelerate the antiprotons from 5.3 MeV down to 100 keV in a controlled way. This is achieved via a synchrotron equipped with an electron cooler to avoid losses during deceleration and to generate dense bunches of antiprotons for users. To achieve this goal, the machine has to be smaller than the AD; a circumference of 30.4 metres has been chosen, which is one sixth of the AD. The experiments still have to further decelerate the beam either using thinner foils or other means, but the lower energy from the synchrotron makes this process more efficient and therefore increases the number of captured antiprotons dramatically.

With ELENA, the available intensity will be distributed to several (the current baseline is four) bunches, which are sent to several experiments simultaneously. Despite the reduction in intensity, the higher beam availability for a given experiment means that a given experiment will receive beam almost continuously 24 hours per day, as opposed to during an eight-hour-long shift a few times per week, as is the case with the present AD.

The ELENA project started in 2012 with the detailed design of the machine and components. Installations inside the AD hall and inside the AD ring itself began in spring 2015, in parallel to AD operation for the existing experiments. Installing ELENA inside the AD ring is a simple cost-effective solution because no large additional building to house a synchrotron and a new experimental area had to be constructed, plus the existing experiments have been able to remain at their present locations. Significant external contributions from the user community include a H^– ion and proton source for commissioning, and very sensitive profile monitors for the transfer lines.

Low-energy challenges

Most of the challenges and possible issues of the ELENA project are a consequence of its low energy, small size and low intensity. The low beam energy makes the beam very sensitive to perturbations such that even the Earth’s magnetic field has a significant impact, for instance deforming the “closed orbit” such that the beam is no longer located at the centre of the vacuum chamber. The circumference of the machine has therefore been chosen to be as small as possible, thus demanding higher-field magnets, to mitigate these effects. On the other hand, the ring has to be long enough to install all necessary components.

For similar reasons, magnets have to be designed very carefully to ensure a sufficiently good field quality at very low field levels, where hysteresis effects and remanence become important. This challenge triggered thorough investigations by the CERN magnet experts and involved several prototypes using different types of yokes, resulting in unexpected conclusions relevant for any project that relies on low-field magnets. The initially foreseen bending magnets based on “diluted” yokes, with laminations made of electrical steel alternated with thicker non-magnetic stainless steel laminations, were found to have larger remnant fields and to be less suitable. Based on this unexpected empirical observation, which was later explained by theoretical considerations, it has been decided that most ELENA magnets will be built with conventional yokes. The corrector magnets have been built without magnetic yoke to completely suppress hysteresis effects.

Electron cooling is an essential ingredient for ELENA: cooling on an intermediate plateau is applied to reduce emittances and losses during deceleration to the final energy. Once the final energy is reached, electron cooling is applied again to generate dense bunches with low emittances and energy spread, which are then transported to the experiments. At the final energy, so-called intra beam scattering (IBS) caused by Coulomb interactions between different particles in the beam increases the beam “emittances” and the energy spread, which, in turn, increases the beam size. This phenomenon will be the dominant source of beam degradation in ELENA, and the equilibrium between IBS and electron cooling will determine the characteristics of the bunches sent to the experiments.

Another possible limitation for a low-energy machine such as ELENA is the large cross-section for scattering between antiprotons and the nuclei of at-rest gas molecules, which leads to beam loss and degradation. This phenomenon is mitigated by a carefully designed vacuum system that can reach pressures as low as a few 10^–12 mbar. Furthermore, ELENA’s low intensities and energy mean that the beam can generate only very small signals and therefore makes beam diagnostics challenging. For example, the currents of the circulating beam are less than 1 μA, which is well below what can be measured with standard beam-current transformers and therefore demands that we seek alternative techniques to estimate the intensity.

An external source capable of providing 100 keV H^– and proton beams will be used for a large part of the commissioning. Although this allows commissioning to be carried out in parallel with AD operation for the experiments, it means that commissioning starts at the most delicate low-energy part of the ELENA cycle where perturbations have the most impact. Another advantage of ELENA’s low energy is that the transfer lines to the experiments are electrostatic – a low-cost solution that allows for the installation of many focusing quadrupoles and makes the lines less sensitive to perturbations.

CERN's AD facility opens new era of precision anitmatter studies

CERN’s Antiproton Decelerator (AD) was approved in 1997, just two years after the production of the first antihydrogen atoms at the Low Energy Antiproton Ring (LEAR), and entered operation in 2000. Its debut discovery was the production of cold antihydrogen in 2002 by the ATHENA and ATRAP collaborations. These experiments were joined by the ASACUSA collaboration, which aims at precision spectroscopy of antiprotonic helium and Rabi-like spectroscopy of the antihydrogen ground-state hyperfine splitting. Since then, techniques have been developed that allow trapping of antihydrogen atoms and the production of a beam of cold antihydrogen atoms. This culminated in 2010 in the first report on trapped antihydrogen by the ALPHA collaboration (the successor of ATHENA). In the same year, ASACUSA produced antihydrogen using a cusp trap, and in 2012 the ATRAP collaboration also reported on trapped antihydrogen.

TRAP, which was based at LEAR and was the predecessor of ATRAP, is one of two CERN experiments that have allowed the first direct investigations of the fundamental properties of antiprotons. In 1999, the collaboration published a proton-to-antiproton charge-to-mass ratio with a factional precision of 90 ppt based on single-charged-particle spectroscopy in a Penning trap using data taken up to 1996. Then, published in 2013, ATRAP measured the magnetic moment of the antiproton with a fractional precision of 4.4 ppm. The BASE collaboration, which was approved in the same year, is now preparing to improve the ATRAP value to the ppb level. In addition, in 2015 BASE reported on a comparison of the proton-to-antiproton charge-to-mass ratio with a fractional precision of 69 ppm. So far, all measured results are consistent with CPT invariance.

The ALPHA, ASACUSA and ATRAP experiments, with the goal of performing precise antihydrogen spectroscopy, are challenging because they need antihydrogen first to be produced and then to be trapped. This requires the accumulation of both antiprotons and positrons, in addition to antihydrogen production via three-body reactions in a nested Penning trap. In 2012, ALPHA reported on a first spectroscopy-type experiment and published the observation of resonant quantum transitions in antihydrogen (see figure) and, later, ASACUSA reported in 2014 on the first production of a beam of cold antihydrogen atoms. The reliable production/trapping scheme of ALPHA, meanwhile, enabled several high-resolution studies, including the precise investigation of the charge neutrality of antihydrogen with a precision at the 0.7 ppb level.

The ASACUSA, ALPHA and ATRAP collaborations are now preparing their experiments to produce the first electromagnetic spectroscopy results on antihydrogen. This is difficult because ALPHA typically reports on about one trapped antihydrogen atom per mixing cycle, while ASACUSA detects approximately six antihydrogen atoms per shot. Both numbers demand for higher antihydrogen production rates, and to further boost AD physics, CERN built the new low-energy antiproton synchrotron ELENA. In parallel to these efforts, proposals to study gravity with antihydrogen were approved. This led to the formation of the AEgIS collaboration in 2008, which is currently being commissioned, and the GBAR project in 2012.

Towards first beam

As of the end of October 2016, all sectors of the ELENA ring –except for the electron cooler, which has temporarily been replaced by a simple vacuum chamber, and a few transfer lines required for the commissioning of the ring – have been installed and baked to reach the very low rest-gas density required. Following hardware tests, commissioning with beam is under way and will be resumed in early 2017, only interrupted for the installation of the electron cooler some time in spring.

ELENA will be ready from 2017 to provide beam to the GBAR experiment, which will be installed in the new experimental area (see panel). The existing AD experiments, however, will be connected only during CERN’s Long Shutdown 2 in 2019–2020 to minimise the period without antiprotons and to optimise the exploitation of the experiments. GBAR, along with another AD experiment called AEgIS, will target direct tests of the weak-equivalence principle by measuring gravitational acceleration based on antihydrogen. This is another powerful way to test for any violations between the way the fundamental forces affect matter and antimatter. Although the first antimatter fall experiments were reported by the ALPHA collaboration in 2013, these results will potentially be improved by several orders of magnitude using the dedicated gravity experiments offered by ELENA.

ELENA is expected to operate for at least 10 years and be exploited by a user community consisting of six approved experiments. This will take physicists towards the ultimate goal of performing spectroscopy on antihydrogen atoms at rest, and also to investigate the effect of gravity on matter and antimatter. A potential discovery of CPT violation will constitute a dramatic challenge to the relativistic quantum-field theories of the SM and will potentially contribute to an understanding of the striking imbalance of matter and antimatter observed on cosmological scales.

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The ALPHA collaboration has just published a new measurement of the charge of the antihydrogen atom. Although the Standard Model predicts that antihydrogen must be strictly neutral, only a few actual direct measurements have been performed so far to test this conjecture.

A glance at the Particle Data Book reveals that, according to the latest measurements, the antiproton charge can differ from the charge of the electron by at most 7 × 10^–10 times the fundamental charge. The comparable number for the positron is somewhat larger, at 4 × 10^–8. Note that studies with atoms of normal matter show that they are neutral to about one part in 10²¹. We are, therefore, unsurprisingly, way behind in our ability to study antimatter. Given that we still do not understand the baryon asymmetry, it is generally a good idea to take a hard look at antimatter, if you can get your hands on some.

Antihydrogen is unique in the laboratory in that it should be neutral, stable antimatter. Indeed, the charge–parity–time (CPT) symmetry requires antihydrogen to have the same properties as hydrogen, including charge neutrality. In ALPHA, we can produce antihydrogen atoms and catch them in a trap formed by superconducting magnets, and we can hold them for at least 1000 s.

The current article in Nature results from experiments in the recently commissioned ALPHA-2 machine, and uses a new technique proposed by ALPHA member Joel Fajans and colleagues at UC Berkeley. The new method, known as stochastic acceleration, involves subjecting the trapped antihydrogen atoms to electric-field pulses at various time intervals. If the antihydrogen is not really neutral, it will be “heated” by the repeated pulses until it finally escapes the trap and annihilates. Comparing the results of trials with and without the pulsed field, we can derive a limit on how “charged” antihydrogen might be. The answer so far: antihydrogen is neutral to 0.7 ppb (one standard deviation) of the fundamental charge. This is a factor of 20 improvement over our previous limit, set by using static electric fields to try to deflect antihydrogen when it is released from the trap.

If we take another approach and assume that antihydrogen is indeed neutral, we can combine this result with ASACUSA’S measurement of the antiproton charge anomaly to improve the limit on the positron charge anomaly by a factor of about 25. Of course, we are looking for signs of new physics in the antihydrogen system – it is probably best not to assume anything.

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On 21 September 1955, Owen Chamberlain, Emilio Segrè, Clyde Wiegand and Tom Ypsilantis found their first evidence of the antiproton, gathered through measurements of its momentum and its velocity. Working at what was known as the “Rad Lab” at Berkeley, they had set up their experiment at a new accelerator, the Bevatron – a proton synchrotron designed to reach an energy of 6.5 GeV, sufficient to produce an antiproton in a fixed-target experiment (CERN Courier November 2005 p27). Soon after, a related experiment led by Gerson Goldhaber and Edoardo Amaldi found the expected annihilation “stars”, recorded in stacks of nuclear emulsions (figure 1). Forty years later, by combing antiprotons and positrons, an experiment at the Low Energy Antiproton Ring (LEAR) at CERN gathered evidence in September 1995 for the production of the first few atoms of antihydrogen.

Over the decades, antiprotons have become a standard tool for studies in particle physics; the word “antimatter” has entered into mainstream language; and antihydrogen is fast becoming a laboratory for investigations in fundamental physics. At CERN, the Antiproton Decelerator (AD) is now an important facility for studies in fundamental physics at low energies, which complement the investigations at the LHC’s high-energy frontier. This article looks back at some of the highlights in the studies of the antiworld at CERN, and takes a glimpse at what lies in store at the AD.

Back at the Bevatron, the discovery of the antineutron through neutral particle annihilation followed in 1956, setting the scene for studies of real antimatter. Initially, everyone expected perfect symmetry between matter and antimatter through the combination of the operations of charge conjugation (C), parity (P) and time reversal (T). However, following the observation of CP violation in 1964, it was not obvious that nuclear forces were CPT invariant and that antinucleons should bind to build antinuclei. These doubts were laid to rest with the discovery of the antideuteron at CERN by a team led by Antonino Zichichi, and at Brookhaven by a team from Columbia University, including Leon Lederman and Sam Ting (CERN Courier May 2009 p15and October 2009 p22). A decade later, evidence emerged for antihelium-3 and antitritium in the WA33 experiment at CERN’s Super Proton Synchrotron, following the sighting of a few candidates at the 70 GeV proton synchroton at the Institute for High Energy Physics near Serpukhov. More recently, the availability of colliding beams of heavy ions has led to the observation of antihelium-4 by the STAR experiment at Brookhaven’s Relativistic Heavy-Ion Collider (CERN Courier June 2011 p8). At CERN, the ALICE experiment at the LHC observes the production of light nuclei and antinuclei with comparable masses and therefore compatible binding energies (figure 2).

Exit baryonium, enter new mesons

Back in 1949, before the discovery of the antiproton, Enrico Fermi and Chen-Ning Yang predicted the existence of bound nucleon–antinucleon states (baryonium), when they noted that certain repulsive forces between two nucleons could become attractive in the nucleon–antinucleon system. Later, quark models based on duality predicted the existence of states made of two quarks and two antiquarks, which should be observed when a proton annihilates with an antiproton. In the 1970s, nuclear-potential models went on to predict a plethora of bound states and resonance excitations around the two-nucleon mass. There were indeed reports of such states, among them narrow states observed in antiproton–proton (pp) annihilation at CERN’s Proton Synchrotron (PS) and in measurements of the pp cross-section as a function of energy (the S meson with a mass of 1940 MeV).

Baryonium was the main motivation for the construction at CERN of LEAR, which ran for more than a decade from 1982 to 1996 (see box). However, none of the baryonium states were confirmed at LEAR. The S meson was not observed with a sensitivity 10 times below the signal reported earlier in the pp total cross-section. Monoenergetic transitions to bound states were also not observed. The death of baryonium was a key topic for the Antiproton 86 Conference in Thessaloniki. What had happened? The high quality of the antiproton beams from LEAR meant that all of the pions had decayed. The high intensity of antiprotons (10⁶/s compared with about 10²/s in extracted beams at the PS) and a high momentum resolution of 10^–3–10^–4 was crucial at low energies for antiprotons stopping with very small range-straggling.

The spectroscopy of mesons produced in pp annihilation at rest in several experiments at LEAR proved to be much more fruitful. This continued a tradition that had begun in the 1960s with antiprotons annihilating in the 81 cm Hydrogen Bubble Chamber at the PS, leading to the discovery of the E meson (E for Europe, now the η(1440)) and the D meson (now the f₁(1285)) in pp → (E, D →  KKπ)ππ. The former led to the long-standing controversy about the existence in this mass region of a glueball candidate – a state made only of gluons – which was observed in radiative J/ψ decay at SLAC’s e⁺e^– collider, SPEAR. With the start up of LEAR, the experiments ASTERIX, OBELIX, Crystal Barrel and JETSET took over the baton of meson spectroscopy in pp annihilation. ASTERIX discovered a tensor meson – the AX, now the f₂(1565) – which was also reported by OBELIX; its structure is still unclear, although it could be the predicted tensor baryonium state.

Crystal Barrel specialized in the detection of multineutral events. The antiprotons were stopped in a liquid-hydrogen target and π⁰ mesons were detected through their γγ decays in a barrel-shaped assembly of 1380 CsI (Tl) crystals. Figure 3 shows the detector together with a Dalitz plot of pp annihilation into π⁰π⁰π⁰, measured by the experiment. The non-uniform distribution of events indicates the presence of intermediate resonances that decay into π⁰π⁰, such as the spin-0 mesons f₀(980) and f₀(1500), and the spin-2 mesons f₂(1270) and f₂(1565). The f₀(1500) is a good candidate for a glueball.

Fundamental symmetries

The CPT theorem postulates that physical laws remain the same when the combined operation of CPT is performed. CPT invariance arises from the assumption in quantum field theories of certain requirements, such as Lorentz invariance and point-like elementary particles. However, CPT violation is possible at very small length scales, and could lead to slight differences between the properties of particles and antiparticles, such as lifetime, inertial mass and magnetic moment.

At LEAR, the TRAP collaboration (PS196) performed a series of pioneering experiments to compare precisely the charge-to-mass ratios of the proton and antiproton, using antiprotons stored in a cold electromagnetic (Penning) trap. The signal from a single stored antiproton could be observed, and antiprotons were stored in the trap for up to two months. By measuring the cyclotron frequency of the orbiting antiprotons with an oscillator and comparing it with the cyclotron frequency of H^– ions in the same trap, the team finally achieved a result at the level of 9 × 10^–11. The experiment used H^– ions instead of protons to avoid biases when reversing the signs of the electric and magnetic fields.

Under the assumption of CPT invariance, the violation of CP symmetry first observed in the neutral kaon system in 1964 implies that T invariance is also violated. However, in 1998 the CPLEAR experiment demonstrated the violation of T in the neutral kaon system without assuming CPT conservation (CERN Courier March 1999 p21). The K⁰ and K⁰ morph into one another as a function of time, and T violation implies that, at a given time t, the probability of finding a K⁰ when initially a K⁰ was produced is not equal to the probability of finding a K⁰ when a K⁰ was produced. CPLEAR established the identity of the initial kaon by measuring the sign of the associated charged kaon in the annihilation pp → K⁺K⁰π^– or K^–K⁰π⁺; that of the kaon at time t was inferred by detecting the decays K⁰ → π⁺e^– ν and K⁰ → π^–e⁺ν. Figure 4 shows that a small asymmetry was indeed observed, consistent with expectations from CP violation, assuming CPT invariance.

The CPT theorem also predicts that matter and antimatter should have identical atomic excitation spectra. Antihydrogen – the simplest form of neutral antimatter consisting of a positron orbiting an antiproton – was observed for the first time in the PS210 experiment at LEAR. The circulating 1.9 GeV/c internal antiproton beam traversed a xenon-cluster jet target, allowing the possibility for an e⁺e^– pair to be produced as an antiproton passed through the Coulomb field of a xenon nucleus. The e⁺ could then be captured by the antiproton to form electrically neutral antihydrogen with a momentum of 1.9 GeV/c, which could be detected further downstream through its annihilation into pions and photons. This production process is rather rare, but nonetheless the PS210 collaboration reported evidence for nine antihydrogen atoms, following about two months of data taking in August–September 1995, and only months before LEAR was shut down. The observation of antihydrogen was confirmed two years later at Fermilab’s Antiproton Accumulator, albeit with a much smaller production cross-section.

At the AD

A new chapter in the story of antihydrogen at CERN opened in 2000 with the start up of the AD, which decelerates antiprotons to 100 MeV/c, before extracting them for experiments on antimatter and atomic physics (CERN Courier November 1999 p17). The PS210 experiment had tried to make antihydrogen in flight, but to study, for example, the spectroscopy of antihydrogen, it is far more convenient to store antihydrogen atoms in electromagnetic traps, just as TRAP had done in its antiproton experiments. This requires antihydrogen to be produced at very low energies, which the AD helps to achieve.

In 2002, the ATHENA and ATRAP experiments at the AD demonstrated the production of large numbers of slow antihydrogen atoms (CERN Courier November 2002 p5and December 2002 p5). ATHENA used absorbing foils to reduce the energy of the antiprotons from the AD to a few kilo-electron-volts. A small fraction of the antiproton beam was then captured in a Penning trap, while positrons from a radioactive sodium source were stored in a second trap. The antiproton and positron clouds were then transferred to a third trap and made to overlap to produce electrically neutral antihydrogen, which migrated to the cryostat walls and annihilated. The antihydrogen detector contained two layers of silicon microstrips to track the charged pions from the antiproton annihilation; an array of 192 CsI crystals detected and measured the energies of the photons from the positron annihilation (figure 5). About a million antihydrogen atoms were produced during the course of the experiment, corresponding to an average rate of 10 antiatoms per second.

Antihydrogen has a magnetic dipole moment (that of the positron), which means that it can be captured in an inhomogeneous magnetic field. The first attempt to do this was carried out at the AD by the ALPHA experiment, which successfully captured 38 antihydrogen atoms in an octupolar magnetic field (CERN Courier March 2011 p13). The initial antihydrogen storage time of 172 ms was increased later to some 15 minutes, thus paving the way to atomic spectroscopy experiments. A sensitive test of CPT is to induce transitions from singlet to triplet spin states (hyperfine splitting, or HfS) in the antihydrogen atom, and to compare the transition energy with that for hydrogen, which is known with very high precision. ALPHA made the first successful attempts to measure the HfS with microwave radiation, managing to flip the positron spin and to eject 23 antihydrogen atoms from the trap (CERN Courier April 2012 p7).

An alternative approach is to perform a Stern–Gerlach-type experiment with an antihydrogen beam. The ASACUSA experiment has used an anti-Helmholtz coil (cusp trap) to exert forces on the antihydrogen atoms and to select those in a given positron spin state. The polarization can then be flipped with microwaves of the appropriate frequency. In a first successful test, 80 antihydrogen atoms were detected downstream from the production region (CERN Courier March 2014 p5).

The ASACUSA collaboration has also tested CPT, using antiprotons stopped in helium. The antiproton was captured by ejecting one of the two orbiting electrons, the ensuing antiprotonic helium atom being left in a high-level, long-lived atomic state that is amenable to laser excitation. By using two counter-propagating laser beams (to reduce the Doppler broadening caused by thermal motion), the group was able to determine the antiproton-to-electron mass ratio with a precision of 1.3 ppb (CERN Courier September 2011 p7). An earlier comparison of the charge-to-mass ratio between the proton and the antiproton had been performed with a precision of 0.09 ppb by the TRAP collaboration at LEAR, as described above. When the results from ASACUSA and TRAP are combined, the masses and charges of the proton and antiproton are determined to be equal at a level below 0.7 ppb.

CPT also requires the magnetic moment of a particle to be equal to (minus) that of its antiparticle. The BASE experiment now under way at the AD will determine the magnetic moment of the antiproton to 1 ppb by measuring the spin-dependent axial oscillation frequency in a Penning trap subjected to a strong magnetic-field gradient. The experimental approach is similar to the one used to measure the magnetic moment of the proton to a precision of 3 ppb (CERN Courier July/August 2014 p8). The collaboration has already compared the charge-to-mass ratios of the antiproton and proton, with a fractional precision of 6.9 × 10^–11 (p7).

The weak equivalence principle (WEP), which states that all objects are accelerated in exactly the same way in gravitational fields, has never been tested with antimatter. Attempts using positrons or antiprotons have so far failed, as a result of stray electric or magnetic fields. In contrast, the electrically neutral antihydrogen atom is an ideal probe to test the WEP. The AEgIS collaboration at the AD plans to measure the sagging of an antihydrogen beam over a distance of typically 1 m with a two-grating deflectometer. The displacement of the moiré pattern induced by gravity will be measured with high resolution (around 1 μm) by using nuclear emulsions (figure 6) – the same detection technique that was used to demonstrate the annihilation of the antiproton at the Bevatron, back in 1956.

The future is ELENA

Future experiments with antimatter at CERN will benefit from the Extra Low ENergy Antiproton (ELENA) project, which will become operational at the end of 2017. The capture efficiency of antiprotons in experiments at the AD is currently very low (less than 0.1%), because most of them are lost when degrading the 5 MeV beam from the AD to the few kilo-electron-volts required by the confinement voltage of electromagnetic traps. To overcome this, ELENA – a 30 m circumference electron-cooled storage ring that will be located in the AD hall – will decelerate antiprotons down to, typically, 100 keV. Fast extraction (as opposed to the slow extraction that was available at LEAR) is foreseen to supply the trap experiments.

One experiment that will profit from this new facility is GBAR, which also aims to measure the gravitational acceleration of antihydrogen. Positrons will be produced by a 4.3 MeV electron linac and used to create positive antihydrogen ions (i.e. an antiproton with two positrons) that can be transferred to an electromagnetic trap and cooled to 10 mK. After transfer to another trap, where one of the positrons is detached, the antihydrogen will be launched vertically with a mean velocity of about 1 m/s (CERN Courier March 2014 p31).

It is worth recalling that the discovery of the antiproton in Berkeley was based on some 60 antiprotons observed during a seven-hour run. The 1.2 GeV/c beam contained 5 × 10⁴ more pions than antiprotons. Today, the AD delivers pure beams of some 3 × 10⁷ antiprotons every 100 s at 100 MeV/c, which makes the CERN laboratory unique in the world for antimatter studies. Over the decades, antiproton beams have led to the discovery of new mesons and enabled precise tests of symmetries between matter and antimatter. Now, the properties of hydrogen and antihydrogen are being compared, and accurate tests will be performed with ELENA. The odds to see any violation of exact symmetry are slim, the CPT theorem being a fundamental law of physics. However, experience shows that – as with the surprising discovery of the non-conservation of parity in 1957 and CP violation in 1964 – experiments will, ultimately, have the last word.

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The Japanese/German BASE collaboration at CERN’s Antiproton Decelerator (AD) has compared the charge-to-mass ratios of the antiproton and proton with a fractional precision of 69 parts in a trillion (ppt). This high-precision measurement was achieved by comparing the cyclotron frequencies of antiprotons and negatively charged hydrogen ions in a Penning trap. The result is consistent with charge–parity–time-reversal (CPT) invariance, which is one of the cornerstones of the Standard Model of particle physics, and constitutes the most precise test comparing baryons and antibaryons performed to date.

In their experiment, the BASE collaboration has profited from techniques pioneered in the 1990s by the TRAP collaboration at the Low Energy Antiproton Ring at CERN. The advanced cryogenic Penning-trap system used in BASE consists of four traps, two of which were used in this measurement – a measurement trap and a reservoir trap (figure 1). When the experiment receives a pulse of 5.3 MeV antiprotons from the AD, they strike the degrader structure, which is designed to slow them down, and release hydrogen. Negatively charged hydrogen ions (H^–) can form in the process, producing a composite cloud with the antiprotons that is shuttled to the reservoir trap. BASE has developed techniques to extract single antiprotons and negative hydrogen ions from this cloud whenever needed. Moreover, the reservoir has a lifetime of more than a year, making the BASE experiment almost independent from AD cycles.

Using this extraction technique, and taking the timing from the AD cycle, BASE prepares a single antiproton in the measurement trap, while an H^– ion is held in the downstream park electrode, as shown in figure 1. The cyclotron frequency of the antiproton is then measured in exactly 120 s, which corresponds to one AD cycle. The particles are subsequently exchanged by performing appropriate potential ramps, and the cyclotron frequency of the H^– ion is measured. Thus, a single comparison of the charge-to-mass ratios takes only 240 s. This is much faster than in previous experiments, enabling BASE to perform about 6500 ratio comparisons in 35 days of measurement time (figure 2). The result is a value of the ratio-comparison: (q/m)_p-/(q/m)_p – 1 = 1(69) × 10^–12, thus confirming CPT at the level of ppt.

The high sampling rate has also enabled the first high-resolution study of diurnal variations in a baryon/antibaryon comparison, which could be introduced by Lorentz-violating cosmic-background fields. The measurement sets constraints on such variations at the level of less than 720 ppt. In addition, by assuming that CPT invariance holds, the measurement can be interpreted as a test of the weak equivalence principle using baryonic antimatter. If matter respects weak equivalence while antimatter experiences an anomalous coupling to the gravitational field, this gravitational anomaly would contribute to a possible difference in the measured cyclotron frequencies. Thus, by following these assumptions, the result from BASE can be used to set a limit on the gravitational anomaly parameter, α_g: |α_g – 1| < 8.7 × 10^–7.

The main goal for the BASE experiment, which was approved in June 2013, is to measure the magnetic moment of the antiproton with a precision of parts per billion. Using the double Penning trap system, the collaboration recently performed the most precise measurement of the magnetic moment of the proton.

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The Alpha Magnetic Spectrometer (AMS) on the International Space Station (ISS) has new results on energetic cosmic-ray electrons and positrons, based on analysis of the first 41 billion events. These results provide a deeper understanding of the nature of high-energy cosmic rays and could shed more light on the existence of dark matter.

Of the 41 × 10⁹ primary cosmic-ray events analysed so far, 10.9 × 10⁶ have been identified as electrons and positrons. Using these, the AMS collaboration has measured the positron fraction – the ratio of the number of positrons to the combined number of positrons and electrons – in the energy range 0.5–500 GeV (Accardo et al. 2014). When compared with the expectation based on the production of positrons in standard cosmic-ray collisions, the results show that the fraction starts to increase rapidly at 8 GeV (figure 1). This indicates the existence of a new source of positrons.

AMS has also accurately determined the exact rate at which the positron fraction increases with energy, and for the first time observed the fraction reach a maximum (figure 2). The data show that the rate of change of the positron fraction crosses zero at 275±32 GeV – indicating the energy at which the fraction reaches its maximum (Aguilar et al. 2014). The results also show that the excess of the positron fraction is isotropic within 3%, suggesting strongly that the energetic positrons might not be coming from a preferred direction in space. Moreover, the fraction shows no observable sharp structures.

AMS has also precisely determined the flux of electrons (figure 3) as well as for positrons (figure 4). These measurements reveal that the fluxes differ significantly in both their magnitude and energy dependence. The positron flux first increases (0.5–10 GeV) and then levels out (10–30 GeV), before increasing again (30–200 GeV). Above 200 GeV, it has a tendency to decrease. This is totally different from the scaled electron flux. The results show that neither flux can be described with a constant spectral index (figure 4, bottom). In particular, between 20 and 200 GeV, the rate of change of the positron flux is surprisingly higher than the rate for electrons. This is important proof that the excess seen in the positron fraction is from a relative excess of high-energy positrons, and not the loss of high-energy electrons.

Different models for dark matter predict different behaviours for the positron-fraction excess. The new results from AMS put much tighter constraints on the validity of these models. The results are consistent with a dark-matter particle (neutralino) of mass of the order of 1 TeV. To determine if the observed new phenomenon is indeed from dark matter or from astrophysical sources such as pulsars, AMS is now making measurements to determine the rate at which the positron fraction decreases beyond the turning point, as well as to determine the antiproton fraction.

• Fifteen countries from Europe, Asia and America participated in the construction of AMS: Finland, France, Germany, the Netherlands, Italy, Portugal, Spain, Switzerland, Turkey, China, Korea, Taiwan, Russia, Mexico and the US. AMS was launched by NASA to the ISS on 16 May 2011. Data are transmitted to the AMS Payload Operations Control Center, located at CERN.

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Antiprotons returned to CERN’s Antiproton Decelerator (AD) on 5 August and experiments have been receiving beams since mid-September, following an intensive consolidation programme during the first long shutdown (LS1) of the accelerator complex. Work has involved some of the most vital parts of the decelerator, such as the target area, the ring magnets, the stochastic cooling system, vacuum system, control system and various aspects of the instrumentation.

The AD uses antiprotons produced by directing the 26 GeV/c proton beam extracted from the Proton Synchrotron (PS) onto an iridium target. In the AD target area, these antiprotons are produced, collimated and momentum-selected to prepare for their injection into the decelerator, where their energy is reduced to the level requested by the experiments.

Although the AD started operations for the antimatter programme in 2000, it reuses almost entirely the components and configuration of an older machine – the Antiproton Collector (AC) – built in 1986. When the AC was designed, the target area needed a high repetition rate of one proton pulse every 2.4 s. Now, the AD’s repetition rate is just 90 s, so components wear out more slowly. Nevertheless, at the beginning of LS1 a problem was found in the transmission line for the electric pulse that goes into the magnetic horn – the device invented by Nobel laureate Simon van der Meer that focusses the diverging antiproton beam. As well as this, after 20 years of operation, the magnetic horn itself had been severely damaged by electric arcs.

The LS1 programme, involving teams of specialists from CERN’s technology, engineering and beam departments, replaced the transmission line and magnetic horn. The horn assembly is composed of three main parts: the horn itself, which consists of two concentric aluminium conductors, a 6-m-long aluminium strip line that carries the current from the generators to the horn, and a movable clamping system that ensures the electrical continuity between the horn and the stripline. Given the critical situation, the teams decided to replace all three components. They had only six months to re-assemble and test spares more than 20 years old, and to construct additional pieces. The consolidated system was assembled and tested on the surface before being installed underground in the target area.

While repairing the damaged components, the teams also examined the 20-tonne dipole magnets. One magnet was removed from the ring and opened up for the first time in 30 years. The coils were in good condition, but the shimming that holds the coils had been completely transformed into dust and needed repair.

The consolidation work on the AD was completed at the end of July, and the first beam was sent to the target on 5 August. Debugging, adjustments and fine tuning were then carried out to deliver antiproton beams to the experiments in mid-September. The work also included the installation of a brand-new beam line for the new Baryon Antibaryon Symmetry Experiment (BASE) experiment, which aims to take ultra-high-precision measurements of the antiproton magnetic moment. The programme has been prompted by the start of the Extra Low ENergy Antiproton ring (ELENA) project. Planned to be operational in 2017, ELENA will allow further deceleration, together with beam cooling of the antiprotons, resulting in an increased number of particles trapped downstream in the experiments.

Elsewhere at CERN, 12 September saw the Super Proton Synchrotron accelerate its first proton beam after LS1. At the LHC, work continues towards the restart. Of the eight sectors, sector 6-7 is the first to have been cooled down to its nominal temperature of 1.9 K. The first powering tests began there on 15 September. Five other sectors were in the process of being cooled during September, with the seventh on track to begin its cool down in early October. All sectors are first cooled to 20 K for the copper-stabilizer continuity measurement tests, which allow the performance of the circuits to be checked when they are not superconducting. The finish line is in sight for the LHC’s restart in spring 2015.

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A German/Japanese collaboration working at the University of Mainz has performed the first direct high-precision measurement of the magnetic moment of the proton – which is by far the most accurate to date. The result is consistent with the currently accepted value of the Committee on Data for Science and Technology (CODATA), but is 2.5 times more precise and 760 times more accurate than any previous direct measurement. The techniques used will feature in the Baryon-Antibaryon Symmetry Experiment (BASE) – recently approved to run at CERN’s Antiproton Decelerator (AD) – which aims at the direct high-precision measurement of the magnetic moments of the proton and the antiproton with fractional precisions at the parts-per-billion (ppb) level, or better.

Prior to this work, the record for the most precise measurement of the proton’s magnetic moment had stood for more than 40 years. In 1972, a group at Massachusetts Institute of Technology measured its value indirectly by performing ground-state hyperfine spectroscopy with a hydrogen maser in a magnetic field. This experiment measured the ratio of the magnetic moments of the proton and the electron. The results, combined with theoretical corrections and two additional independent measurements, enabled the calculation of the proton magnetic moment with a precision of about 10 parts in a billion.

In an attempt to surpass the record, the collaboration of scientists from Mainz University, the Max Planck Institute for Nuclear Physics in Heidelberg, GSI Darmstadt and the Japanese RIKEN institute applied the so-called double Penning trap technique to a single proton for the first time (see figure 1). One Penning trap – called the analysis trap – is used for the non-destructive detection of the spin state, through the continuous Stern-Gerlach effect. In this elegant approach, a strong magnetic inhomogeneity is superimposed on the trap, so coupling the particle’s spin-magnetic-moment to its axial oscillation frequency in the trap. By measuring the axial frequency, the spin quantum state of the trapped particle can be determined. And by recording the quantum-jump rate as a function of a spin-flip drive frequency, the spin precession frequency ν_L is obtained. Together with a measurement of the cyclotron frequency ν_c of the trapped particle, the magnetic moment of the proton μ_p is obtained finally in units of the nuclear magneton, μ_p/μ_N = ν_L/ν_c.

This approach has already been applied with great success in measurements of the magnetic moments of the electron and the positron. However, the magnetic moment of the proton is about 660 times smaller than that of the electron, so the proton measurement requires an apparatus that is orders of magnitude more sensitive. To detect the proton’s spin state, the collaboration used an extremely strong magnetic inhomogeneity of 300,000 T/m². However, this limits the experimental precision in the frequency measurements to the parts-per-million (ppm) level. Therefore a second trap – the precision trap – was added about 45 mm away from the strong magnetic-field inhomogeneity. In this trap the magnetic field is about 75,000 times more homogeneous than in the analysis trap.

To determine the magnetic moment of the proton, the first step was to identify the spin state of the single particle in the analysis trap. Afterwards the particle was transported to the precision trap, where the cyclotron frequency was measured and a spin flip induced. Subsequently the particle was transported back to the analysis trap and the spin state was analysed again. By repeating this procedure several hundred times, the magnetic moment was measured in the homogeneous magnetic field of the precision trap. The result, extracted from the normalized resonance curve (figure 2), is the value μ_p = 2.792847350(9)μ_N, with a relative precision of 3.3 ppb.

In the BASE experiment at the AD the technique will be applied directly to a single trapped antiproton and will potentially improve the currently accepted value of the magnetic moment by at least a factor of 1000. This will constitute a stringent test with baryons of CPT symmetry – the most fundamental symmetry underlying the quantum field theories of the Standard Model of particle physics. CPT invariance implies the exact equality of the properties of matter–antimatter conjugates and any measured difference could contribute to understanding the striking imbalance of matter and antimatter observed on cosmological scales.

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Aristotle said that ‘‘An iron ball of one hundred pounds, falling from a height of one hundred cubits [about 5.2 m], reaches the ground before a one-pound ball has fallen a single cubit.” Galileo Galilei replied, “I say that they arrive at the same time.” The universality of free fall illustrated by the latter’s legendary experiment at the tower of Pisa was formulated by Isaac Newton in his Principia and became, with Albert Einstein, the weak equivalence principle (WEP): the motion of any object under the influence of gravity does not depend on its mass or composition. This principle is the cornerstone of general relativity.

The WEP has been verified to incredible precision by dropping experiments and Eötvös-type torsion balances, the latter reaching an amazing accuracy of one part in 10¹³. The acceleration of the Earth and the Moon towards the Sun has also been determined to the same accuracy by measuring the transit time of laser pulses between the planet and the reflectors left on the Moon by the Apollo and Soviet space missions. But does the WEP also hold for antimatter for which no direct measurement has been performed, in particular for antimatter particles such as positrons or antiprotons? Or does antimatter even fall up?

The purpose of the 2nd International Workshop on Antimatter and Gravity, which took place on 13–15 November, was to review the experimental and theoretical aspects of antimatter interaction with gravity. The meeting was hosted by the Albert Einstein Center for Fundamental Physics of the University of Bern, following the success of the first workshop held in 2011 at the Institut Henri Poincaré in Paris. The highlights are summarized here.

Free-fall experiments with charged particles are notoriously difficult because they must be carefully shielded from electromagnetic fields

Free-fall experiments with charged particles are notoriously difficult because they must be carefully shielded from electromagnetic fields. For example, the sagging of the gas of free electrons in metallic shielding induces an electric field that can counterbalance the effect of gravity. Indeed, measurements based on dropping electrons led to a value of the acceleration of gravity, g, consistent with zero (instead of g = 9.8 m/s²). A free-fall experiment with positrons has not yet been performed, owing to the lack of suitable sources of slow positrons. In the 1980s, a team proposed a free-fall measurement of g with antiprotons at CERN’s Low Energy Antiproton Ring (LEAR), but it could not be performed before the closure of LEAR in 1996.

Using neutral antimatter such as antihydrogen can alleviate the disturbance from electromagnetic fields. The ALPHA collaboration at CERN’s Antiproton Decelerator (AD) has set the first free-fall limit on g with a few hundred antihydrogen atoms held for more than 400 ms in an octupolar magnetic field. The results exclude a ratio of antimatter to matter acceleration larger than 110 (normal gravity) and smaller than −65 (antigravity). Plans to measure this ratio at the level of 1% by using a vertical trap are under way.

Positronium matters

The AEgIS collaboration at the AD uses positronium produced by bombarding a nanoporous material with a positron pulse derived from a radioactive sodium source. Positronium (Ps) is then brought to highly excited states with lasers and mixed with captured antiprotons to produce antihydrogen (H) through the reaction Ps + p → e^– + H. The highly excited antihydrogen atoms possess large electric dipole moments and can be accelerated with inhomogeneous electric fields to form an antihydrogen beam. The sagging of the beam over a distance of typically 1 m is measured with a two-grating deflectometer by observing the intensity pattern with high-resolution (around 1 μm) nuclear emulsions. AEgIS is currently setting up, with antiprotons (around 10⁵) and positrons (3 × 10⁷) successfully stacked. A first measurement of g is planned in 2015 and the initial goal is to reach 1% uncertainty.

As a neutral system, positronium is also suitable for gravity measurements, but free-fall experiments are not easy because positronium lives for 140 ns only. Such studies require sufficiently cold positronium in long-lived, highly excited states and the appropriate atom optics. Preparations for a free-fall experiment at University College London are under way.

At ETH Zurich, a team is measuring the 1s → 2s atomic transition in positronium with a precision better than one part per billion (1 ppb) by using a high-intensity positron beam that traverses a solid neon moderator and impinges on a porous silica target. The positronium ejected from the target is laser-excited to the 2s state and the γ-decay rate is measured by scintillating crystals, as a function of laser frequency. The 1s → 2s frequency can be calculated from hydrogen data. For hydrogen, the frequency is redshifted in the gravitational potential of the Sun, but the shift cannot be observed because the clocks used to measure the frequency are equally redshifted. However, for positronium (equal amounts of matter and antimatter) and assuming antigravity, measurements should yield a higher frequency than is calculated from hydrogen. At the level of 0.1 ppb, such studies could even test the hypothesis of antigravity as the Earth revolves around the Sun.

A similar experiment with muonium – an electron orbiting a positive muon – is planned at PSI in Switzerland. Ultra-slow muon beams with sub-millimetre sizes and sub-electronvolt energy for re-acceleration could also be used in a free-fall experiment employing gratings (a Mach–Zehnder interferometer).

Free-fall experiments

At CERN, the AD delivers bunches of 5.3 MeV antiprotons (3 × 10⁷) every 100 s. However, storing antiprotons requires lower energies, which are reachable by inserting thin foils, albeit at the expense of substantial losses and degradation in beam size. Prospects for improved experiments are now bright with ELENA, a 30 m circumference electron-cooled ring that decelerates the AD beam further to 100 keV (figure 1). ELENA will be installed in 2015 and will be available for physics in summer 2017.

The first free-fall experiment to profit from this new facility will be GBAR. Antihydrogen atoms will be obtained by the interaction of antiprotons from ELENA with a positronium cloud. The positrons will be produced by a 4.3 MeV electron linac. In contrast to AEgIS, the antihydrogen atom will capture a further positron to become a positively charged ion, which can be transferred to an electromagnetic trap, cooled to 10 mK with cold beryllium ions and then transported to a launching trap where the additional positron will be photodetached. The mean velocity of the antihydrogen atoms will be around 1 m/s and the fall distance will be about 30 cm. GBAR will be commissioned in 2017 with the initial goal of reaching 1% accuracy on g.

The sensitivity of GBAR, limited by the velocity distribution of the antihydrogen atoms, could be improved substantially by using quantum reflection, a fascinating effect that was discussed at the workshop. Antihydrogen atoms dropped towards a surface experience a repulsive force, which leads to gravitational quantum states. A similar phenomenon was observed with cold neutrons at the Institut Laue–Langevin (ILL) in Grenoble. Now, the ILL team proposes to bounce the atoms in GBAR between two layers – a smooth lower surface to reflect slow enough antihydrogen atoms and a rough upper surface to annihilate the fast ones. Transition frequencies between the gravitational levels – which depend on g – could also be measured by recording the annihilation rate on the bottom surface. Provided that the lifetime of these antihydrogen levels is long enough, orders of magnitude improvements could be obtained on the determination of g.

Atom interferometers might be able to measure g to within 10^–6. In a Ramsey–Bordé interferometer, the falling atom interacts with pulses from two counter-propagating vertical laser beams. Having absorbed a photon from the first beam, the atom is stimulated to emit another photon with the frequency of the second beam, thereby modifying its momentum. The signal from the annihilating antihydrogen atom, for example at the top of the interferometer, interferes with the one from another atom that has equal momentum but was not subject to the laser kick. The interference pattern will depend on the value of g.

At FLAIR the antiproton flux will be an order of magnitude higher than at ELENA

In the more distant future, the Facility for Low-energy Antiproton and Ion Research (FLAIR) will become operational at GSI. As an extension to the high-energy antiproton facility, FLAIR will consist of a low-energy storage ring decelerating antiprotons from 30 MeV to 300 keV, followed by an electrostatic ring capable of reducing the energy even further, down to 20 keV. At FLAIR the antiproton flux will be an order of magnitude higher than at ELENA, and slow extracted antiproton beams will be available for experiments in nuclear and particle physics.

The question of how large an effect these free-fall experiments could measure cannot be answered without theoretical assumptions, such as exact symmetry between matter and antimatter (the CPT theorem). However, string theory can break CPT. The standard model extension proposed by the Indiana/Carleton group involves Lorentz and CPT violation. Also, atoms and nuclei contain virtual antiparticles in amounts that depend on the atomic number. The calculable quantum corrections agree with measurements, arguing against antigravity. However, there is a huge discrepancy in the value of the cosmological constant estimated from vacuum particle–antiparticle pair fluctuations, which might question our understanding of the interaction between gravity and virtual particles. As pointed out at the workshop, if all of the theoretical assumptions are valid, then antimatter experiments should not expect to see discrepancies in g at a level larger than 10^–7. Ultimately, the issue must be settled by experiments.

To compare with matter, a presentation was given on the 10^–9 precision achievable on g at the Swiss Federal Institute of Metrology (METAS) using a free-fall interferometer. Together with improved measurements of Planck’s constant with a watt balance, this might lead to a re-definition of the kilogram based on natural units.

The workshop also included a session on antimatter in the universe. Is there any antimatter and could it repel matter (the Dirac–Milne universe) and provide the accelerating expansion? Can the excess of positrons observed above 10 GeV by balloon experiments, the PAMELA satellite experiment and, more recently, the Fermi Gamma-ray Space Telescope and the Alpha Magnetic Spectrometer (AMS-02), be explained by antimatter annihilation?

In his summary talk, Mike Charlton of Swansea University concluded that “the challenge of measuring gravity on antihydrogen remains formidable”, but that “in the past decade the prospects have advanced from the totally visionary to the merely very difficult”.

The workshop, with 28 plenary talks, was attended by 70 participants. A visit to the house where Einstein spent the years 1903–1905 and dinner at Altes Tramdepot were part of the social programme.

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A beam of antihydrogen atoms has for the first time been successfully produced by an experiment at CERN’s Antiproton Decelerator (AD). The ASACUSA collaboration reports the unambiguous detection of antihydrogen atoms 2.7 m downstream from their production, where the perturbing influence of the magnetic fields used to produce the antiatoms is negligibly small. This result is a significant step towards precise hyperfine spectroscopy of antihydrogen atoms.

High-precision microwave spectroscopy of ground-state hyperfine transitions in antihydrogen atoms is a main focus of the Japanese-European ASACUSA collaboration. The research aims at investigating differences between matter and antimatter to test CPT symmetry (the combination of charge conjugation, C, parity, P, and time reversal, T) by comparing the spectra of antihydrogen with those of hydrogen, one of the most precisely investigated and best understood systems in modern physics.

One of the key challenges in studying antiatoms is to keep them away from ordinary matter. To do so, other collaborations take advantage of antihydrogen’s magnetic properties and use strong, non-uniform magnetic fields to trap the antiatoms long enough to study them. However, the strong magnetic-field gradients degrade the spectroscopic properties of the antihydrogen. To allow for clean, high-resolution spectroscopy, the ASACUSA collaboration has developed an innovative set-up to transfer antihydrogen atoms to a region where they can be studied in flight, far from the strong magnetic field regions.

In ASACUSA, the antihydrogen atoms are formed by loading antiprotons and positrons into the so-called cusp trap, which combines the magnetic field of a pair of superconducting anti-Helmholtz coils (i.e., coils with antiparallel excitation currents) with the electrostatic potential of an assembly of multi-ring electrodes (CERN Courier March 2011 p17). The magnetic-field gradient allows the flow of spin-polarized antihydrogen atoms along the axis of the cusp trap. Downstream there is a spectrometer consisting of a microwave cavity to induce spin-flips in the antiatoms, a superconducting sextupole magnet to focus the neutral beam and an antihydrogen detector. (The microwave cavity was not installed in the 2012 experiment.)

The detector, located 2.7 m from the antihydrogen-production region, consists of single-crystal bismuth germanium oxide (BGO) surrounded by five plates of plastic scintillator. Antihydrogen atoms annihilating in the crystal emit three charged pions on average, so the signal required consists of a coincidence between the crystal and at least two plastic scintillators. Simulations show that this requirement reduces the background, from antiprotons annihilating upstream and from cosmic rays, by three orders of magnitude.

The ASACUSA researchers investigate the principal quantum number, n, of the antihydrogen atoms that reach the detector, because their goal is to perform hyperfine spectroscopy on the ground state, n = 1. For these measurements, field-ionization electrodes were positioned in front of the BGO, so that only antihydrogen atoms with n < 43 or n < 29 reached the detector, depending on the average electric field. The analysis indicates that 80 antihydrogen atoms were unambiguously detected with n < 43, with a significant number having n < 29.

This analysis was based on data collected in 2012, before the accelerator complex at CERN entered its current long shutdown. Since then, the collaboration has also been preparing for the restart of the experiment at the AD in October this year. A new cusp magnet is under construction, which will provide a much stronger focusing force on the spin-polarized antihydrogen beam. A cylindrical high-resolution tracker and a new antihydrogen-beam detector are also under development. In addition, the positron accumulator will lead to an order of magnitude more positrons. The team eventually needs a beam of antihydrogen in its ground state (n = 1) so the updated experiment will employ an ionizer with higher fields to extract antihydrogen atoms that are in effect in the ground state.

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Measuring a total of 434 atoms, they found that in the absence of systematic errors, F must be < 75 at a statistical significance level of 5%; the worst-case systematic errors increase this limit to < 110. A similar search places somewhat tighter bounds on a negative F, that is, on antigravity. Refinements of the technique, coupled with larger numbers of cold-trapped antiatoms, should allow future measurements to place tighter bounds on F and approach the interesting region around 1.

Meanwhile, the antimatter programme at CERN is expanding. AEgIS and GBAR, two experiments currently under construction, will focus on measuring how gravity affects antihydrogen.

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The Antihydrogen TRAP (ATRAP) experiment at CERN’s Antiproton Decelerator has reported a new measurement of the antiproton’s magnetic moment made with an unprecedented uncertainty of 4.4 parts per million (ppm) – a result that is 680 times more precise than previous measurements. The unusual increase in precision results from the experiment’s ability to trap individual protons and antiprotons, as well as from using a large magnetic gradient to gain sensitivity to the tiny magnetic moment.

By applying its single particle approach to the study of antiprotons, the ATRAP experiment has been able make precise measurements of the charge, mass and magnetic moment of the antiproton. Using a Penning trap, the antiproton is suspended at the centre of an iron ring-electrode that is sandwiched between copper electrodes. Thermal contact with liquid helium keeps the electrodes at 4.2 K, providing a nearly perfect vacuum that eliminates the stray matter atoms that could otherwise annihilate the antiproton. Static and oscillating voltages applied to the electrodes allow the antiproton to be manipulated and its properties to be measured.

The result is part of an attempt to understand the matter–antimatter imbalance of the universe. In particular, a comparison of the antiproton’s magnetic moment with that of the proton, tests the Standard Model and its CPT theorem at high precision. The ATRAP team found that the magnetic moments of the antiproton and proton are “exactly opposite”: equal in strength but opposite in direction with respect to the particle spins and consistent with the prediction of the Standard Model and the CPT theorem to 5 parts per million.

However, the potential for much greater measurement precision puts ATRAP in position to test the Standard Model prediction much more stringently. Combining the single particle methods with new quantum methods that make it possible to observe individual antiproton spin flips should make it feasible to compare an antiproton and a proton to 1 part per billion or better.

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The international team running the Alpha Magnetic Spectrometer (AMS) has announced the first results in its search for dark matter. They indicate the observation of an excess of positrons in the cosmic-ray flux. The results were presented by Samuel Ting, the spokesperson of AMS, in a seminar at CERN on 3 April, the date of publication in Physical Review Letters.

The AMS results are based on an analysis of some 2.5 × 10¹⁰ events, recorded over a year and a half. Cuts to reject protons, as well as electrons and positrons produced in the interactions of cosmic rays in the Earth’s atmosphere, reduce this to around 6.8 × 10⁶ positron and electron events, including 400,000 positrons with energies between 0.5 GeV and 350 GeV. This represents the largest collection of antimatter particles detected in space.

The data reveal that the fraction of positrons increases from 10 GeV to 250 GeV, with the slope of the increase reducing by an order of magnitude over the range 20–250 GeV. The data also show no significant variation over time, or any preferred incoming direction. These results are consistent with the positrons’ origin in the annihilation of dark-matter particles in space but they are not yet sufficiently conclusive to rule out other explanations.

The AMS detector is operated by a large international collaboration led by Nobel laureate Samuel Ting. The collaboration involves some 600 researchers from China, Denmark, Finland, France, Germany, Italy, Korea, Mexico, the Netherlands, Portugal, Spain, Switzerland, Taiwan and the US. The detector was assembled at CERN, tested at ESA’s ESTEC centre in the Netherlands and launched into space on 16 May 2011 on board NASA’s Space Shuttle Endeavour. Designed to study cosmic rays before they interact with the Earth’s atmosphere, the experiment is installed on the International Space Station. It tracks incoming charged particles such as protons and electrons, as well as antimatter particles such as positrons, mapping the flux of cosmic rays with unprecedented precision.

An excess of antimatter within the cosmic-ray flux was first observed around two decades ago in experiments flown on high-altitude balloons and has since been seen by the PAMELA detector in space and the Large Area Telescope on the Fermi Gamma-ray Space Telescope. The origin of the excess, however, remains unexplained.

One possibility, predicted by theories involving supersymmetry, is that positrons could be produced when two particles of dark matter collide and annihilate. Assuming an isotropic distribution of dark-matter particles, these theories predict the observations made by AMS. However, the measurement by AMS does not yet rule out the alternative explanation that the positrons originate from pulsars distributed around the galactic plane. Moreover, supersymmetry theories also predict a cut-off at higher energies above the mass range of dark-matter particles and this has not yet been observed.

AMS is the first experiment to measure to 1% accuracy in space – a level of precision that should allow it to discover whether the positron observation has an origin in dark matter or in pulsars. The experiment will further refine the measurement’s precision over the coming years and clarify the behaviour of the positron fraction at energies above 250 GeV.

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Located in the experimental hall at the Antiproton Decelerator (AD), the AEgIS experiment is designed to make the first direct measurement of Earth’s gravitational effect on antimatter. By sending a beam of antihydrogen atoms through very thin gratings, the experiment will be able to measure how far the antihydrogen atoms fall and in how much time – giving the AEgIS team a measurement of the gravitational coupling. The team finished putting all of the elements of the experiment together by the end of 2012, but they will have to wait for two years for beams to return to the AD hall following the Long Shutdown (LS1), which has just begun.

To make progress in the meantime, the AEgIS team has decided to try out the experiment with hydrogen instead of antihydrogen. By replacing antiprotons with their own proton source, the team will be able to manufacture its own hydrogen beam to use for commissioning and testing the set-up. Surprisingly, carrying out the experiment with hydrogen will be more difficult technically than with antihydrogen. Another challenge will be in the production of the positronium that will be used in creating the hydrogen. The positronium needs to be moving fast enough to ensure that it does not decay before it meets the protons/antiprotons, but not so fast as to pass the protons/antiprotons altogether. The AEgIS team will be carrying out this commissioning during the coming months, opening up their set-up next month to make any necessary adjustments and to install a hydrogen detector and proton source.

• For more, see the article in CERN Bulletin.

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The last day of September saw an exciting coincidence of three competing experiments simultaneously releasing three new and directly similar results. The occasion was the CKM2012 workshop in Cincinnati and the subject of interest: excellent new measurements of the CKM phase, γ.

Two of the contenders were well known to each other, having battled for supremacy in B physics for more than a decade. The “B factory” experiments, Belle and BaBar, were designed on the same principle: e⁺e^– collisions at the Υ(4S) resonance produce large numbers of BB pairs, which can be cleanly reconstructed in isolation. Except for a few selective technology choices, their most obvious dissimilarity is their location: Belle is at KEK in Japan while BaBar resides at SLAC in the US.

The meeting in Cincinnati saw these old foes joined by a new competitor, LHCb, which unlike the B factories collects its huge samples of bottom hadrons from high-energy proton–proton collisions at the LHC. Although there is little doubt that the CERN-based experiment will ultimately triumph with precision measurements of γ, on the morning of 30 September no one yet knew if that time had come.

Among the fundamental forces of nature, the weak force is special. Not only does it have a unique structure that gives rise to fascinating and often counter-intuitive physical effects, it is also highly predictive, making it excellent territory for searches for new physics. Perhaps the most celebrated phenomenon is CP violation – a common short-hand for saying that weak interactions of matter differ subtly from those of antimatter. Discovered in 1964 as a small effect (10^–3) in K_L⁰ decays, CP violation has more recently been observed as a large effect (10^–2–10^–1) in several B-meson decay modes.

The CKM matrix

The size and variety of CP violation in b-quark transitions is widely acknowledged as a triumphant validation of the Cabibbo-Kobayashi-Maskawa (CKM) description of quarks coupling to W^± bosons. This mechanism explains three-generation quark-mixing – up-type quarks (u, c, t) transmuting to and from down-type quarks (d, s, b) via the charged weak current – in terms of a 3 × 3 matrix rotation of the quarks’ mass eigenstates into their weak-interaction eigenstates. CP violation arises naturally through the mathematically mandatory presence of one complex phase in this generically complex matrix. Furthermore, if nature indeed has only three quark generations and probability is conserved, then the CKM transformation must be unitary.

Unitary matrices have a property that the scalar product of any two rows or columns must equate to zero. In the case of the 3 × 3 CKM matrix, six equations can be written down that must hold true if there are three – and only three – generations of quarks. Of these six relations, which are all triangles on the Argand plane, the most celebrated is

V^*_ubV_ud +V^*_cbV_cd +V^*_tbV_td = 0

where each V_XY is one of nine CKM matrix elements that encode the strength with which quark X couples to quark Y. This triangle, whose internal angles are usually labelled α, β and γ, is widely publicized because it summarizes concisely the largest CP-violating processes in B mesons. Studying the geometry of this unitarity triangle (UT) tests the internal consistency of the three-generation CKM picture of quark mixing. The lengths of the sides of the UT are measured in CP-conserving processes, whereas the size of the angles (or phases) can be measured only via CP-violating decays.

In Cincinnati, the BaBar collaboration announced that it had achieved a measurement of γ = 69⁺¹⁷_–16° from a combination of many analyses of B^± → D^(*)K^± decays. The precision of around 25% can be compared with the precision with which the other two UT angles are known. The smallest of the three angles, β, is known to less than 4%, β = 21.4 ± 0.8°, principally from measuring the time-dependent CP asymmetry in the mixing and decay of B⁰ → J/ψK⁰ decays. The angle subtended by the apex of the triangle, α, is known to around 5%, α = 88.7^+4.6_–4.2°, from similar, time-dependent analyses of B⁰ → ππ and B⁰ → ρρ decays. Remembering that the three angles of a triangle always add up to 180°, it is clear that BaBar’s central value is remarkably close to the CKM expectation.

The Belle collaboration’s presentation quickly followed and explained a similar measurement of γ = 68⁺¹⁵_–14°, the modest improvement perhaps being a result of the almost twice-as-large data set. As with BaBar, this number results from the careful combination of various measurements of CP-violating properties of B^± → DK^± and B^± → D^*K^± decays.

Interfering amplitudes

The B factories’ common choice of B^± → DK^± decays is not a coincidence. Among the current UT angle analyses, only γ measurements use direct CP violation in charged B decays. This promises a simple asymmetry of matter versus antimatter but requires two interfering amplitudes resulting in the same, indistinguishable final state. They must have different CP-conserving phases (generally true for any two quantum processes) and be of similar magnitude, or the influence of the less-likely process is too hard to detect.

In the UT definition, γ is identified as the weak phase difference between b → c and b → u quark transitions. Figure 2 shows Feynman diagrams for two paths of B^± → DK^±. The one involving a b → c quark transition is labelled “favoured” because a b quark is most likely to decay to a c quark. The second diagram involves a b → u quark transition and is labelled “suppressed” because the chance of its occurrence is around 1% of that of the favoured process (i.e. the ratio of amplitudes, r_B is around 0.1).

This all looks good except for the detail in figure 2 that the favoured diagram results in a D⁰ while the suppressed diagram yields a D⁰. For the two B decays to interfere, the two neutral particles must be reconstructed in a final state that is common to both, i.e. the D⁰ and D⁰ should be indistinguishable. This might occur in the following ways, all of which are studied by Belle, BaBar and to some extent, LHCb.

• CP-eigenstate decays of neutral D mesons are by definition equally accessible to D⁰ and D⁰. In this case, the interference – and hence the size of the direct CP violation – is around 10% (from r_B in figure 2). Examples of this type are B^± → [K⁺K^–]_DK^± and B^± → [K_S⁰π⁰]_DK^± decays, where the D indicates that the particles in parentheses originated from a D meson.

• The unequal rate of the favoured and suppressed B decays can be redressed by selecting D final states that have an opposite suppression. Such combinations are referred to as ADS decays, after their original proponents. The most obvious example is B^± → [π^±K^+–]_DK^± decays where, importantly, the kaon from the D decay is of an opposite charge to that emanating from the B decay. In this particular case, the favoured B decay from figure 2 is followed by the doubly Cabibbo-suppressed D⁰ → π^–K⁺ decay, whereas the suppressed B decay precedes a favoured D⁰ → K⁺π^– decay. With this opposite suppression, the total ratio of amplitudes (r_B/r_D) is much closer to unity than the first case, so larger CP violation, and hence greater sensitivity to γ, is achieved.

• A third possibility considers multi-body D decays such as B^± → [K_S⁰π⁺π^–]_DK^±. In this case, the kinematics of the three-body D decay is studied across a 2D histogram, the Dalitz plot. When the D → K_S⁰π⁺π^– Dalitz plot for B^– → DK^– decays is compared with that of B⁺ → DK⁺ decays, they look identical except for a few places where γ has induced CP violation. Some places on the Dalitz plot have large sensitivity to γ, others less, but a big advantage comes from understanding the CP-conserving phases that vary smoothly across the Dalitz plot. Such an analysis is complicated, but worth it as the patterns of CP asymmetry across the Dalitz plane can be solved by only one value of γ (modulo 180°). This compares well to the first two cases whose interpretations suffer from trigonometric ambiguities because of their non-trivial sinusoidal dependence on γ.

Both the Belle and BaBar results combine all of these methods using B^± → DK^± and B^± → D^*K^± decays. This diversity is vital since the branching fraction of γ-sensitive decays is so small (proportional to |V_ub|²) and only a few hundred events have been collected in these experiments, even after a decade of operation.

LHCb has different advantages and challenges. On one hand the huge cross-section for B production at the LHC means that LHCb has a considerable advantage in the number of charged-track-only decays that it can gather. On the other hand, because of the hadronic environment LHCb fairs less well with modes containing neutral particles. The D → K_S⁰π⁺π^– mode is still useful, but cannot be relied on as heavily as at the B factories. Modes with a π⁰ or a photon, notably the otherwise important B^± → D^*K^±, D^* → D⁰π⁰/D⁰γ suite of modes, have not yet been attempted at LHCb.

Nevertheless for the charged-track final states, such as the easiest ADS modes, LHCb has triumphed with first observations of the B^± → [π^±K^+–]_DK^± mode (see figure 3), as well as the similarly interesting B^± → [π^±K^+–π^–π⁺]_DK^± mode. By measuring the large CP asymmetries in these modes, and with the help of an ambiguity-busting B^± → [K_S⁰π⁺π^–]_DK^± analysis, the LHCb collaboration concluded the CKM2012 session by announcing a measurement of γ = (71.1^+16.6_–15.7)° from B^± → DK^± decays.

Such exotic processes are the reason for well established phenomena such as B-mixing and flavour-changing neutral-current decays

The simple combination of these three independent results (neglecting their common systematics) leads to the conclusion that γ is known to better than 14% accuracy: γ = 69.3^+9.4_–8.8°. This is illustrated in figure 1, which also shows the remarkable similarity of the three measurements and their mutual agreement with the expectation based on the world-average values of β and α.

The concluding theme in Cincinnati was that despite LHCb’s coming of age since CKM2010, the CKM description of the quarks’ weak interactions continues to prove impressively complete. It was noted however, that many flagship B-physics measurements, including the UT angles α and β, involve processes that contain quantum loops and/or boxes. Such exotic processes are the reason for well established phenomena such as B-mixing and flavour-changing neutral-current decays. Standard Model loop-processes contain the virtual existence of high-mass particles such as W^±, top quarks and by extension, possibly non-Standard Model particles too. If they exist, and if they couple to quarks, such new-physics particles could be altering the physical behaviour of B mesons from the CKM-based expectation.

Detection of non-CKM effects is possible only if loop-sensitive observations can be compared with a gold-standard CKM process. B^± → DK^± decays provide exactly this. They are “tree-level” measurements (meaning, no loops) that are almost unique in heavy-flavour physics for their theoretical cleanliness. The measurement of γ in these modes is a measurement of γ_CKM, something the other two angles of the UT cannot boast with such certainty.

Though γ is currently the least well known UT property, by the end of this decade LHCb will have reduced its uncertainty to less than 5° (less than about 8%). By the end of the epoch of the Belle and LHCb upgrades, sub-degree precision looks likely. Such stunning precision will mean that this phase will become the CKM standard candle against which loop processes will be compared increasingly carefully.

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The ALPHA collaboration has reported the first-ever resonant interaction with the antihydrogen atom, observed in their experiment at the Antiproton Decelerator (AD) at CERN.

ALPHA synthesizes antihydrogen from cryogenic plasmas of antiprotons and positrons. While the charged constituents can be easily confined through their interactions with electric and magnetic fields, confining neutral antihydrogen is much more difficult. It can be held in a highly inhomogeneous magnetic field (a “minimum-B” configuration) because it has a magnetic dipole moment, but the interaction is so weak that only atoms with kinetic energy equivalent to 0.5 K or less in temperature can be trapped, using superconducting magnets. This is how ALPHA has already held antihydrogen atoms for up to 1000 s (CERN Courier March 2011 p13 and July/August 2011 p6).

Assuming that antihydrogen behaves like hydrogen, the 1s ground state will exhibit both hyperfine splitting (through the interaction between the spins of the positron and the antiproton) and splitting in a magnetic field (see figure). In a high magnetic field, these states are characterized by the direction of the spins of the antiproton and positron with respect to the field direction. The “low-field-seeking” states labelled |c〉and |d〉 can be trapped, because their energy increases with magnetic field strength. Atoms that end up in the |a〉and |b〉 states (“high-field-seekers”) are expelled from the trap and annihilate in the surrounding apparatus.

In the latest experiment, a horn antenna directed microwaves into the atom trap so as to flip the spin of the positron in the stored atoms, thus driving the transitions |c〉 → |b〉and |d〉→ |a〉. The experimental sequence was as follows: produce and trap antihydrogen (of the order of one trapped atom at a time on average); irradiate the trapped atom with microwaves resonant on either the |c〉 → |b〉or

|d〉→ |a〉transition (these are excited alternately for 15 s each over a total of 180 s); look for evidence of “lost” antihydrogen. To conduct control experiments, it was repeated without microwaves or with microwaves at a shifted off-resonance frequency. Each sequence took about 10 minutes of real time.

The collaboration used two methods to look for evidence of ejected antihydrogen. At the end of each sequence, the atom trap is rapidly de-energized, the fields falling with a time constant of about 9 ms. Any antihydrogen remaining in the trap is released and detected by ALPHA’s three-layer silicon detector over a 30 ms time window. It is then possible to compare the survival rate of anti-atoms for the three cases: no microwaves present, resonant microwaves present, or off-resonant microwaves present. The other detection measurement involves looking for direct annihilations from ejected antihydrogen during the times in which resonant microwaves are present; background (primarily cosmic rays) discrimination here is more challenging because of the longer observation time.

In both types of measurement, ALPHA finds a strong signal for resonant interaction. For example, in 110 trials with off-resonance microwaves, 23 annihilations were observed when the trap was de-energized; with microwaves on resonance, 2 annihilations were observed in 103 trials. (Detection efficiency is about 50% for both cases). The on- and off-resonance measurements localize the resonance to no better than 100 MHz in about 29 GHz; the collaboration has not yet attempted to scan the lineshape to further localize a resonant peak.

This measurement marks the beginning of anti-atom spectroscopy and illustrates that it is possible to make measurements on antimatter atoms using only a few atoms. In 2012 the ALPHA apparatus will give way to ALPHA-2, a new device that is further optimized for precision microwave and laser spectroscopy. ALPHA-2 will be commissioned during the upcoming run of the AD, from May to November.

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The recent successes of the AD experiments are just the latest in a long list of important scientific results with low-energy antiprotons at CERN that started in the 1990s with the Low Energy Antiproton Ring. Over the years, the scientific demand for antiprotons at the AD has continued to grow. There are now four experiments running there (ATRAP, ALPHA, ASACUSA and ACE). A fifth, AEGIS, has been approved and will take beam for the first time at the end of the year; further proposals are also under consideration. The AD is approaching the stage where it can no longer provide the number of antiprotons needed. As antihydrogen studies evolve into antihydrogen spectroscopy and gravitational measurements, the shortage will become even more acute.

The solution is a small ring of magnets that will fit inside the current AD hall – in other words, ELENA, the recently approved upgrade. ELENA will be a 30 m-circumference decelerator that will slow down the 5.3 MeV antiprotons from the AD to an energy of only 100 keV. Receiving slower antiprotons will help the experiments to improve their efficiency in creating antimatter atoms.

Currently, around 99.9% of the antiprotons produced by the AD are lost because of the experiments’ use of degrader foils, which are needed to decelerate the particles from the AD ejection energy down to around 5 keV – the energy needed for trapping. ELENA will increase the experiments’ efficiency by a factor of 10–100 as well as offer the possibility to accommodate an extra experimental area.

The new ring will be located such that its assembly and commissioning will have a minimal impact on operation of the AD. Indeed, the commissioning of the ELENA ring will take place in parallel with the current research programme, with short periods dedicated to commissioning during the physics run. The layout of the experimental area at the AD will not be significantly modified, but the much lower beam energies involved require the design and construction of completely new electrostatic transfer lines.

The construction of ELENA should begin in 2013 and the first physics injection should follow about three years later. The initial phase of the work will include the installation and commissioning of the ELENA ring while using the existing AD beam lines. The old ejection lines in all of the experimental areas will then be replaced with new electrostatic beam lines that will deliver antiprotons at the design energy of 100 keV. In its final configuration, ELENA will be able to deliver beams almost simultaneously to four experiments, resulting in a vital gain in total beam time.

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Low-energy antiproton physics is an interdisciplinary field that spans particle, nuclear, atomic and applied physics, as well as astrophysics. It confronts directly the relationship between matter and antimatter, in particular CPT symmetry, one of the foundations of the theory of particle physics. CPT is so fundamental that its violation would require a complete rewriting of particle-physics textbooks. Precision studies with antiprotons may also shed light on the question of why the universe is made almost exclusively of matter but not antimatter. Recent months have witnessed dramatic breakthroughs in the field at CERN’s Antiproton Decelerator (AD), including the trapping of antihydrogen atoms and developments towards an antihydrogen beam. Satellite and balloon experiments are searching for cosmic antimatter, the results of which could have profound implications on cosmology. Antiprotons are also being used to study the properties and structures of atoms, nuclei and hadrons, for which the start of the Facility for Antiproton and Ion Research (FAIR) in Darmstadt will usher in a new era.

Dialogue across disciplines

It was against this stimulating backdrop that LEAP 2011 – the 10th International Conference on Low Energy Antiproton Physics – took place at TRIUMF in Vancouver on 27 April – 1 May. The conference was organized and supported by the Canadian institutions involved in the ALPHA experiment at the AD (the universities of British Columbia, Calgary, Simon Fraser, York and TRIUMF), with additional support from the Canadian Institute of Nuclear Physics, and was chaired by Makoto Fujiwara of TRIUMF/

Calgary, with Mary Alberg of Seattle as co-chair. LEAP 2011 was the first of the series in North America; the conferences have traditionally been held in Europe, with the exception of Yokohama in 2003. It attracted nearly 100 participants and featured more than 60 invited plenary speakers, with an emphasis on promoting young researchers. Several review talks by senior physicists facilitated dialogue across the disciplines. In addition, a dozen posters were presented and presenters were allowed a two-minute talk to advertise their work at a plenary, a format that worked quite effectively. This report presents some of the highlights of a packed programme.

The conference began with a session on antihydrogen physics, with reports on the recent trapping of antihydrogen by the ALPHA experiment and the ASACUSA collaboration’s developments towards an antihydrogen beam, both at the AD. The two results were together voted the number one physics breakthrough for 2010 by Physics World. Key techniques that enabled ALPHA’s trapping of antihydrogen are evaporative cooling and autoresonant excitation of antiproton plasmas. The conference heard how the collaboration’s work has led to the successful confinement of antihydrogen for 1000 s. The next major goal for ALPHA is to perform microwave spectroscopy on trapped antihydrogen. ASACUSA also has plans to use microwave spectroscopy to measure ground-state hyperfine splitting with an antihydrogen beam.

The ATRAP collaboration, again at the AD, presented new results on adiabatic cooling of antiprotons, with up to 3 × 10⁶ antiprotons cooled to 3.5 K, and described the first demonstration of centrifugal separation of antiprotons and electrons, suggesting a new method for isolating low-energy antiprotons. The team also has a scheme for improved antihydrogen production via interactions with positronium atoms, created in the interactions of excited caesium atoms with positrons. Other talks described new possibilities for antimatter gravity experiments with antihydrogen at the AD: AEGIS, already under preparation, and the proposed Gbar.

Ion traps with single-particle sensitivity are another powerful tool. A team from Heidelberg and Mainz has recently observed a single proton spin-flip, a result that paves the path for the comparison of the magnetic moments of protons and antiprotons. At TRIUMF, an ion trap system, TITAN, is being used at the ISAC facility for precision studies of radioactive nuclei.

Talks on applications and new techniques with antiprotons included the ACE experiment at the AD, which is studying the possible use of antiprotons for cancer therapy, and developments towards spin-polarized antiprotons. The session on atomic physics also covered some novel techniques that have possible applications to antihydrogen. One proposal concerns a new pulsed Sisyphus scheme for (anti)hydrogen laser cooling. Another involves using an atomic coil-gun, which can stop beams of paramagnetic species, to trap hydrogen isotopes, followed by single-photon cooling techniques. A Lyman-α laser for antihydrogen cooling is being developed at Mainz.

The positron, or anti-electron, is the other ingredient in antihydrogen atoms. A review on positron accumulation techniques was given by Clifford Surko of the University of California, San Diego – the inventor of the Surko trap now used by many of the antihydrogen experiments. Studies were reported using variations of the Surko trap by ATRAP and the University of Swansea groups. Measurement of hyperfine splitting in positronium could provide precision tests of QED. One experiment on positronium atoms at the University of Tokyo has made the first direct measurement of this splitting, employing a novel sub-THz source, while another aims at precise measurements via the Zeeman effect.

This year marks the 20th anniversary of the discovery of long-lived antiprotonic helium at KEK. Studies of such exotic atoms and fundamental symmetries are an important part of antiproton physics. ASACUSA has made recent progress on precision studies on antiprotonic helium and on microwave measurements of antiprotonic ³He atoms. Recent but still controversial results on muonic hydrogen spectroscopy at the Paul Scherrer Institute indicate a much smaller size for the proton radius than is generally accepted. Hadronic and radioactive atoms were featured in review talks at the conference, focusing on pionic and kaonic atoms, as well as on the fundamental symmetries programme at TRIUMF. The final results of the TWIST experiment at TRIUMF, a precision measurement of muon decay parameters, have greatly reduced systematic uncertainties, providing improved limits for constraining extensions to the Standard Model.

An important pillar of antiproton physics is hadron and QCD physics at “low energy”, ranging from stopped antiprotons to a beam of 15 GeV. At the lower energy end, ASACUSA is studying antiproton in-flight annihilation on nuclei. Following hints from an experiment at KEK, an experiment in a low-momentum antiproton beam at the Japan Proton Accelerator Research Complex (J-PARC) will search for a φ-meson–nucleus bound state using antiproton annihilation on nuclei. Also at J-PARC, a study of double anti-kaonic nuclear clusters in antiproton–³He annihilation has been proposed. Further into the future, the research programme for the major PANDA detector at FAIR, which is expected to start running in 2018, encompasses a breadth of physics that includes searches for exotic states and studies of double Λ hypernuclei. Back to the present, hot news from the Brookhaven National Laboratory concerned the discovery of the anti-alpha nucleus, the heaviest anti-nucleus observed.

The theory talks at the conference covered topics ranging from atomic collisions to cosmology. There were reviews on atomic collision physics with antiprotons and on interactions of antihydrogen with ordinary matter atoms. Calculations of gravitational effects on the interaction between antihydrogen and a solid surface suggest that the antiatoms would settle in long-lived quantum states, the study of which could provide a new way to measure the gravitational force on antihydrogen. Theoretical ideas based on the so-called Standard Model Extension, an effective theory that incorporates CPT and Lorentz violation, could offer the opportunity for probing Planck-scale physics as well as antimatter gravity in antihydrogen experiments. On the hadron physics side, antiproton–proton and antiproton–nucleus collisions provide ways to test theories of strangeness production, the latter offering a window onto the behaviour of strange particles in the nuclear medium that complements heavy-ion studies. In cosmology, baryon asymmetry – or the dominance of matter over antimatter – is a long-standing puzzle, as is the nature of dark matter. Could hidden antibaryons be the dark matter? Such a possibility could explain the two mysteries in one go.

LEAP 2011 featured two dedicated sessions on the universe. In the first, CERN’s John Ellis discussed the nature of dark matter and its connection to low-energy hadron physics and William Unruh, from the University of British Columbia, reported on fascinating experimental work that confirms aspects of Hawking radiation in an analogue system, confirming his own theoretical prediction from some 30 years ago. The second of the sessions focused on experimental searches for antimatter in the universe – a hot topic as the conference was held not long before the launch into space of the Alpha Magnetic Spectrometer. The latest results from the PAMELA detector, which has been in space since 2006, continue to show an anomaly in the positron flux at high energies (PAMELA’s quest for answers to cosmic questions). BESS-Polar II, the second flight of the Balloon-borne Experiment with a Superconducting Spectrometer (BESS) over Antarctica, has a new measurement of the antiproton spectrum based on 24.5 days in which 4.7 × 10⁹ cosmic-ray events were collected, yielding a sensitivity complementary to satellite experiments. The proposed General Antiparticle Spectrometer (GAPS) would be a balloon experiment to search for anti-deuterons from dark-matter annihilations using exotic atom techniques.

Looking to the future, the construction of FAIR at Darmstadt will allow for a dedicated Facility for Low-energy Antiproton and Ion Research (FLAIR), while Fermilab has a proposal to use its Antiproton Source – the world’s most intense – for low-energy experiments once the Tevatron programme comes to an end later this year. Finally the conference returned to the AD, when the proposal for the Extra Low ENergy Antiproton ring (ELENA) was described by Walter Oelert, from the Jülich Research Centre, whose experiment at CERN observed the first antihydrogen atoms in 1996. The conference ended with his remarks on the prospects for antiproton physics. Just a few weeks after the conference, CERN Council approved the construction of ELENA, which will provide significantly enhanced opportunities for antiproton physics at CERN in the coming decade (ELENA prepares a bright future for antimatter research).

This successful conference was capped off by a social programme that included a dinner cruise in Vancouver’s spectacular English bay, and a well-attended public lecture by John Ellis at the University of British Columbia. The future of low-energy antiproton physics appears bright. The next LEAP meeting is planned for Uppsala in 2013, chaired by Tord Johansson.

• For full details of the speakers and many of the presentations, see http://leap2011.triumf.ca. The proceedings will be published in Hyperfine Interactions.

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To make these measurements, the ASACUSA team first traps antiprotons inside antiprotonic helium, in which the negatively charged antiproton takes the place of an electron and occupies a Rydberg state, keeping it relatively far from the nucleus. The antiprotonic helium atoms thus live long enough to allow the frequencies of atomic transitions to be measured by laser spectroscopy. The frequencies depend on the ratio of the antiproton mass to the electron mass and ASACUSA has already used this technique to achieve record precision.

However, an important source of imprecision comes from Doppler broadening of the resonance observed when the laser is tuned to the transition frequency. The atoms move around, so that those moving towards and away from the laser beam experience slightly different frequencies. In the previous measurement in 2006, the ASACUSA team used just one laser beam, and the achievable accuracy was dominated by this effect. This time they have used two beams moving in opposite directions, with the result that the broadening for the two beams partly cancels out.

The resulting narrow spectral lines allowed the team to measure three transition frequencies with fractional precisions of 2.3–5 parts in 10⁹. By comparing the results with three-body QED calculations, they find an antiproton-to-electron mass ratio of 1836.1526736(23), where the error (23) represents one standard deviation. This agrees with the proton-to-electron value, which is known to a similar precision.

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Roberto Petronzio, president of INFN has announced that the SuperB Factory, will be built at the University of Rome ‘Tor Vergata’. The facility tops the list of 14 flagship projects of the National Research Plan of the Italian Ministry for Education, Universities and Research.

The SuperB project involves the construction underground of a new asymmetric high-luminosity electron–positron collider. It will occupy approximately 30 hectares on the campus of the University of Rome ‘Tor Vergata’ and be closely linked to the INFN Frascati National Laboratories, located nearby. The project, which will ultimately cost a few hundred-million euros, obtained funding approval for €250 million in the Italian government’s CIPE Economic Planning Document. It has also attracted interest from physicists in many other countries. At the end of May, some 300 physicists from all over the world gathered on the island of Elba for a meeting that started the formal formation of the SuperB collaboration, a crucial milestone on the road towards realization of the accelerator.

SuperB will be a major international research centre for fundamental and applied physics. The high a design luminosity of 10³⁶ cm^–2 s^–1 will allow the indirect exploration of new effects in the physics of heavy quarks and flavours through the studies of large samples of B, D and τ decays. The same infrastructure will also provide new technologies and advanced experimental instruments for research in solid-state physics, biology, nanotechnologies and biomedicine.

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In November of 2010, the ALPHA collaboration at CERN’s Antiproton Decelerator (AD) grabbed the world’s headlines by trapping a handful of atoms of antihydrogen (CERN Courier January/February 2011 p7). The result demonstrated that it was, indeed, possible to produce trappable antihydrogen atoms. Now, the ALPHA team has shown that it can hold on to the trapped antiatoms for up to 1000 seconds and has succeeded in measuring the energy distribution of the trapped antihydrogen (ALPHA collaboration 2011).

Antihydrogen has been produced at CERN since 2002 by allowing antiprotons from the AD to mix with positrons in a Penning trap comprised of a strong solenoid magnet and a set of hollow, cylindrical electrodes for manipulating the particles. However, being neutral, the antiatoms are not confined by the fields of the Penning trap and annihilate in the apparatus. It has taken eight years to learn how to trap the antihydrogen, mainly because of the weakness of the magnetic dipole interaction that holds the antiatoms. The antihydrogen must be produced with a kinetic energy, in temperature units, of less than 0.5 K, otherwise it will escape ALPHA’s “magnetic bottle”. By contrast, the plasma of antiprotons used to synthesize the antihydrogen begins its time in ALPHA with an energy of up to 4 keV (about 50 million K).

The ALPHA antiatom trap consists of a transverse octupole magnet and two short solenoid or “mirror” coils – all fabricated at the Brookhaven National Laboratory (figure 1). This configuration produces a magnetic minimum at the centre of the device (CERN Courier March 2011 p13). Antihydrogen forms at the magnetic minimum and cannot escape if its energy is below 0.5 K. To see if there is any antihydrogen in the trap, the team rapidly shuts down the magnets (9 ms time constant). Any escaping antiatoms are revealed by their annihilation, which is registered in a three-layer, silicon vertex detector. In 2010, antiatoms were trapped for 172 ms, the minimum time necessary to make certain that no bare antiprotons remained in the trap, and the experiment detected 38 events consistent with the release of trapped antihydrogen.

The ALPHA team has subsequently worked to improve the trapping techniques, succeeding in particular in increasing by a factor of five the number of antiatoms trapped in each attempt; the total number trapped has now risen to 309. The improvements include the addition of evaporative antiproton cooling and optimization of the autoresonant mixing that helps to produce the coldest-possible antiatoms. The team then made measurements in which they increased the time in the trap from 0.4 to 2000 s, yielding 112 detected annihilations in 201 attempts (figure 2). The probability that the detected events are background from cosmic rays is less than 10^–15 (8 σ) at 100s, and 4 × 10^–3 (2.6 σ) at 2000s. Calculations indicate that most of these trapped antiatoms reach the ground state – which is crucial for future studies with laser and microwave spectroscopy.

The distributions in space and time of the annihilations of the escaping antiatoms are already providing information about their energy distribution in the trap. This can be compared with a theoretical model of how the team thinks the antihydrogen is being produced in the first place.

The long storage time implies that the team can begin almost immediately to look for resonant interactions with antihydrogen – even if only one or two atoms occupy the trap at any given time. For example, resonant microwaves will flip the spin of the positron in the trap, causing a trapped atom to become untrapped, and annihilate. The ALPHA collaboration hopes to begin studies with microwaves in 2011, aiming for the first resonant interaction of an antiatom with electromagnetic radiation. In the longer term, the ALPHA2 device will allow laser interaction with the trapped antiatoms in 2012 – the first step in what the team hopes will be a steady stream of laser experiments with ever-increasing precision.

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Members of the international STAR collaboration at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory have observed antihelium-4. This is the heaviest antinucleus detected so far, following the discovery of the first antihypernucleus (an antiproton, an antineutron and a Λ) by the same collaboration just a year ago. After sifting through 0.5 × 10¹² tracks in data for 10⁹ gold–gold collisions at centre-of-mass energies of 200 GeV and 62 GeV per nucleon–nucleon pair, the STAR collaboration found 18 events with the signature of the antihelium-4 nucleus, which is distinguished by its mass together with its charge of -2.

While the curvature of the tracks in the magnetic field of the STAR detector provide a momentum measurement, key information also comes from the mean energy-loss per unit track length, 〈dE/dx〉, in the gas of the TPC and from the time of flight of particles arriving at the time-of-flight barrel that surrounds the TPC. The 〈dE/dx〉 information helps in identification by distinguishing particles with different masses or charges, the time of flight being needed for identification at higher momenta, above 1.75 GeV/c. The figure shows the identification of isotopes based on energy loss and mass calculated from momentum in the region of helium-3 and helium-4 for both positive and negative particles, with 18 counts for antihelium-4.

The team used this observation to calculate the antimatter yield at RHIC and found that the production rate falls by a factor of 1.6 +1.0/–0.6 × 10³ (1.1 +0.3/–0.2 × 10³) for each additional antinucleon (nucleon). This is in line with the expectations from coalescent nucleosynthesis models, as well as from thermodynamic models.

The finding ties in with the scientific goals of the Alpha Magnetic Spectrometer launched on 16 May (AMS takes off), which will search for antimatter in space. It also nicely marks the centenary of the paper by Ernest Rutherford in which he analysed the scattering of helium nuclei (alpha particles) on gold and first established the existence of the atomic nucleus.

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Last December, the cusp-trap group of the Japanese–European ASACUSA collaboration demonstrated for the first time the efficient synthesis of antihydrogen, in a major step towards the production of a spin-polarized antihydrogen beam. Such a beam will allow, for the first time, high-precision microwave spectroscopy of ground-state hyperfine transitions in antihydrogen atoms, enabling tests of CPT symmetry (the combination of charge conjugation, C, parity, P, and time reversal, T) – the most fundamental symmetry of nature. The new experiment may also shed light on some of the most profound mysteries of our universe: the asymmetry of matter and antimatter in our universe. Why is it that the universe today is made up almost exclusively of matter, and not antimatter? Scientists believe that the answer may lie in tiny differences between the properties of matter and antimatter, manifested in violations of CPT symmetry.

Testing CPT symmetry

Antihydrogen, made up of an antiproton and a positron, is attractive for testing CPT symmetry given its simple structure. In particular, comparisons of antihydrogen’s transition frequencies with those of ordinary hydrogen atoms will provide stringent tests of CPT symmetry. For this purpose, the ATRAP and ALPHA experiments under way at CERN’s Antiproton Decelerator (AD) aim to make high-precision measurements of the transition frequency between the ground state (1s) and first excited state (2s) of antihydrogen, which is close to 2466 THz, in the realm of laser spectroscopy. The ALPHA collaboration made an essential breakthrough in this approach when they successfully trapped antihydrogen for the first time in November.

The ASACUSA experiment, also at the AD, is taking the complementary approach of measuring precisely the transition frequency between the two substates of the ground state that arise from hyperfine splitting as a result of the interaction between the two magnetic moments associated with the spins of the antiproton and the positron. The collaboration aims to measure the ground-state hyperfine transition frequency, which is about 1420 MHz in the microwave region, by extracting a spin-polarized antihydrogen beam in a field-free region. Last December, the cusp-trap group of ASACUSA reported that the cusp trap, which is designed not to trap antihydrogen but to concentrate spin-polarized antiatoms into a beam, succeeded in synthesizing antihydrogen atoms with an efficiency as high as 7%. This is a big step towards the realization of high-precision microwave spectroscopy of the ground-state hyperfine transition in antihydrogen.

The cusp trap uses anti-Helmholtz coils, which are like Helmholtz coils but with the excitation currents antiparallel rather than parallel to each other. This arrangement yields a magnetic quadrupole field that has axial symmetry about the coil axis: a so-called cusp magnetic field (figure 1). In addition, an axially symmetric electric field is generated by an assembly of multi-ring electrodes (MREs) that is coaxially arranged with respect to the coils. Having axial symmetry, these magnetic and electric fields guarantee the stable storage and manipulation of a large number of antiprotons and positrons simultaneously – one of the unique features of the cusp trap. Furthermore, the magnetic field distribution of the cusp trap can produce an intensified antihydrogen beam with high spin-polarization in low-field-seeking (LFS) states. In other words, antihydrogen atoms can be tested for CPT symmetry in a field-free (or weak field) region – a vital condition for making high-precision spectroscopy a reality. These properties are exclusive to the cusp-trap scheme.

As figure 1 shows, the extracted beam is injected into a microwave cavity, followed by a sextupole magnet and a spin analyser, and then focused on an antihydrogen detector (shown in red). When the microwave frequency is in resonance with one of the hyperfine transition frequencies, it induces a spin flip, which converts the LFS state into a high-field-seeking (HFS) state. In this case, the antihydrogen beam becomes defocused (shown in purple), a transition that is easily monitored by an intensity drop in the antihydrogen detector. As is evident from this description, the cusp trap scheme does not need to trap antihydrogen atoms, but it can do so if necessary. The big advantage is that a large number of antihydrogen atoms with higher temperatures can participate in the measurements.

The AD at CERN supplies a pulsed antiproton beam of around 3 × 10⁷ particles per pulse at 5.3 MeV, which is slowed down to 120 keV in ASACUSA by the radio-frequency quadrupole decelerator. For the antihydrogen experiments the beam is then injected into an antiproton catching trap (called the MUSASHI trap) through two layers of thin degrader foil. In this way, about 1.5 × 10⁶ antiprotons per AD shot are accumulated in the trap where they are cooled with preloaded electrons. The antiproton cloud is then radially compressed by a “rotating wall” technique to allow efficient transportation into the cusp trap. The positrons that make up the antihydrogen are supplied via a compact all-in-one positron accumulator that was designed and developed for this research. Both antiprotons and positrons are then injected into the cusp trap to synthesize cold antihydrogen atoms. A 3D track detector monitors the cusp track to determine the annihilation position of antiprotons by tracking charged pions. The detector comprises two pairs of two modules, each with 64 horizontal and 64 vertical scintillator bars that are 1.5 cm wide.

Inside the cusp trap

Figure 2 shows schematically the structure of the central part of the cusp trap. The MRE is housed in a cryogenic ultrahigh vacuum tube held at a temperature of several Kelvin with a good heat contact, while still being insulated electrically. Thermal shields at 30 K located on both ends of the MRE prevent room-temperature radiation creeping in from the beamline. Outside the MRE part of the bore tube, five superconducting coils installed symmetrically with respect to the MRE centre provide the cusp magnetic field. On the downstream side, the bore diameter is expanded for efficient extraction of the antihydrogen beam.

In the recent experiment, antihydrogen atoms were synthesized by mixing antiprotons and positrons at the nested trap region, as shown by the blue solid line (φ₁) in figure 3. As antihydrogen atoms are neutral, they are not trapped and move more or less freely so some of them reached the field-ionization trap (FIT). If the antihydrogen atoms were formed via a three-body-recombination process in high Rydberg states, i.e. relatively loosely bound, they are field-ionized and their antiprotons are accumulated in the FIT. During the experiment, the FIT was opened (as indicated by the dash dotted line, φ₂) every 5 s and the antiprotons accumulated were released and counted by the 3D tracker through their annihilations. This gave the antihydrogen synthesis rate as a function of time since the start of the mixing process. Figure 4 shows an example of the evolution of the synthesis rate for 3 × 10⁵ antiprotons and 3 × 10⁶ positrons, in which the rate grew in the first 20–30 s, and then gradually decreased. In this case, a total of around 7 × 10³ antihydrogen atoms were synthesized.

The ASACUSA collaboration is now looking forward to starting the microwave spectroscopy of hyperfine transition frequencies – which may lead to groundbreaking insights into the nature of antimatter and symmetry.

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On 17 November 2010 the ALPHA collaboration at CERN’s Antiproton Decelerator (AD) reported online in the journal Nature that they had observed trapped antihydrogen atoms by releasing them quickly from the magnetic trap in which they were produced and detecting the annihilation of the antiproton – the nucleus of the antihydrogen atom (Andresen et al. 2010a). This exciting result from a proof-of-principle experiment paves the way to detailed study of antimatter atoms.

Do matter and antimatter obey the same laws of physics? One intriguing way to test this would be to compare the spectra of hydrogen and its antimatter twin: antihydrogen. Such studies would build on almost a century of detailed theoretical and experimental investigation of the hydrogen atom, from the Bohr model to the ultraprecise measurements of Nobel laureate Theodor Hänsch and colleagues. The frequency of the 1s–2s transition in hydrogen has been measured with a precision of about 2 parts in 10¹⁴. The CPT theorem requires that this frequency must be exactly the same in antihydrogen. The goal of the ALPHA experiment is to test this claim – at least from the high-energy physics point of view. To the atomic physicist, for whom hydrogen is the basic, elegant workhorse of the evolution of quantum mechanics, the question is perhaps: “How could you possibly have access to antihydrogen and not try to measure that?”

While our colleagues at the LHC have been busily setting new records for the highest energy stored hadrons, we at the AD have been headed in the other direction – setting a new record for the lowest energy anti-hadrons. The antihydrogen atoms in ALPHA can be trapped only if their kinetic energy, in temperature units, is less than 0.5 K. This corresponds to about 9 × 10^–5 eV, or 3 × 10^–17 times the energy of protons in the LHC, which represents quite a dynamic range for CERN.

The low temperature necessary has been a daunting challenge for the ALPHA experimenters. Antihydrogen is formed by mixing antiprotons from the AD with positrons from a special accumulator fuelled by a ²²Na positron emitter. The particles are mixed in cryogenic Penning traps, which feature strong solenoidal magnetic fields for transverse confinement and electrostatic fields for longitudinal confinement (figure 1). The resultant antihydrogen, which is electrically neutral, can be confined only by the weak interaction of its magnetic dipole moment with an external magnetic trapping-field. The strength of this dipole interaction is such that, for ground state antihydrogen, a 1 T deep magnetic well can confine atoms with kinetic energy up to 0.7 K.

The atom trap in ALPHA comprises an octupole magnet and two solenoidal “mirror coils” (figure 1). These produce a magnetic minimum at the position at which the antihydrogen atoms are formed. If the atoms are formed with a kinetic energy of less than about 0.5 K (in temperature units), they are trapped. (This is for the ground state; excited atoms can have a larger magnetic moment and experience a deeper well.)

The difficulty lies in the transition from plasmas of charged particles to neutral atoms. The space-charge potential energies in the plasmas can be of order 10 eV – about 120 000 K in temperature equivalent. So one of the experimental challenges for antihydrogen trapping has been to learn how to cool and carefully manipulate the charged species to produce cold, trappable atoms.

At ALPHA, we mix about 30 000 antiprotons with about two million positrons in each attempt to trap antihydrogen. The two plasmas are placed in adjacent potential wells, as in figure 1, and the antiprotons are then driven into the positron plasma using a frequency-swept, axial electric field (Andresen et al. 2011). This drive is “autoresonant”, i.e. the amplitude of the antiproton oscillation automatically matches the corresponding drive frequency in the nonlinear potential well. The idea is to control the energy of the antiprotons precisely by carefully tailoring the drive frequencies. The antiprotons enter the positron cloud with low relative energy and do not heat the positron cloud on entry.

The positrons themselves are self-cooling: they lose energy by radiation in the 1 T magnetic field in the Penning trap. We supplement this process using evaporative cooling. Starting with an equilibrated positron plasma in a potential well, we lower one side of the well, allowing the hottest positrons to escape. The remaining positrons re-equilibrate through collisions, settling to a lower temperature. The technique, which is well known in the field of Bose-Einstein condensation for neutral atoms, was also demonstrated by ALPHA on antiprotons in 2009 (Andresen et al. 2010b). After evaporative cooling, the positrons in ALPHA are at about 40 K. Under ALPHA conditions, the antiprotons can enter the positron plasma and come into thermal equilibrium before making antihydrogen. Thus, only a small fraction of the antihydrogen atoms produced will have a kinetic energy equivalent to less than 0.5 K.

Antiprotons and positrons are allowed to interact or “mix” for 1s to produce antihydrogen, after which we remove any charged particles that remain trapped in the potential wells and then ground the electrodes of the Penning trap. The decisive step is to shut down the magnetic atom trap quickly to see if there are any trapped antihydrogen atoms that escape and annihilate on the walls of the device. However, even with the Penning trap’s electric fields turned off, there is still a small chance that antiprotons could be magnetically trapped due to the mirror effect in the strong magnetic field gradients in the atom trap. To eliminate this possibility, we apply pulsed electric fields along the axis of the trap, in alternating directions, so as to kick any stubborn antiprotons out of the trapping volume.

The ALPHA experiment’s superconducting atom-trap magnets, manufactured at Brookhaven National Laboratory, can be turned off with a time constant of about 9 ms. This fast shutdown helps to discriminate between antihydrogen annihilations and cosmic rays.

Antiproton annihilations are detected by an imaging, three-layer silicon vertex detector (see figure 2) that surrounds the cryostat for the traps and magnets. To be absolutely sure that any annihilations observed come from neutral antimatter and not from charged antiprotons, we apply an axial electric “bias” field to the trap while it is shutting off. While antiprotons would be deflected by this field, antihydrogen is not, and we can see the result using the position-sensitive silicon vertex detector. The silicon detector is also extremely useful in topologically rejecting cosmic rays.

The result of many trapping attempts is shown in figure 3, reproduced from the article in Nature. Each trapping attempt takes about 20 minutes of real time. In 335 trapping attempts, we observed 38 annihilations consistent with the controlled release of trapped antihydrogen atoms. The spatial distribution of these annihilations is not consistent with the expected behaviour of charged particles (figure 3). We can conclude that neutral antihydrogen atoms were trapped for at least 172 ms, which is the time it took to eject the charged particles from the trap and to apply the multiple field pulses to ensure the clearing of mirror-trapped antiprotons.

In subsequent experiments, we made good progress on improving the trapping probability and investigated the storage lifetime of antihydrogen atoms in the trap. At holding times up to 1000s, we still see the signal for release of trapped atoms. This is an encouraging result that leads us to be optimistic about the future of spectroscopic studies with trapped antihydrogen.

When the AD starts up again in 2011, we hope to pick up where we left off in 2010. The first step is to continue to improve the trapping probability for produced antihydrogen atoms, by, for example, working on reducing the positron temperature and studying improvements in the mixing manipulations to make colder antihydrogen. As regards the spectrum of antihydrogen, the 1s to 2s laser-frequency transition described above is not the only game in town. Microwaves can interact with antiatoms in the magnetic trap, either with the positron spin (positron spin resonance) or with the antiproton spin (antinuclear magnetic resonance). Paradoxically, using rare atoms of antimatter can offer a detection bonus for such experiments, as a resonant interaction can lead to loss and annihilation of the trapped atom – an event that can be detected with high efficiency. At ALPHA we hope to take the first steps towards microwave spectroscopy – the first resonant look at the inner workings of an antiatom – in 2011. At the same time we will be working on a new atom-trapping device that is optimized for precision measurements with both lasers and microwaves.

Having demonstrated trapping of antihydrogen atoms, the ALPHA collaboration was able to finish off the year by celebrating the honour of being recognized as the Physics Breakthrough of the Year for 2010 by Physics World magazine. We shared this honour with our friends across the wall at the AD in the ASACUSA collaboration, who produced antihydrogen in a new type of device that could lead to in-flight studies of the antiatoms. Finally, the American Physical Society news staff named our trapping of antihydrogen as one of the top ten physics-related news stories of 2010. All in all, 2010 was a vintage year for antimatter at the AD.

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Research at CERN’s Antiproton Decelerator (AD) has made important breakthroughs in experimental techniques for studying antihydrogen in the laboratory. On 17 November, in a paper published in Nature, the ALPHA collaboration announced that it had successfully trapped atoms of antihydrogen for the first time. Then, on 6 December, the ASACUSA collaboration published results in Physical Review Letters on a technique that should allow the production of a beam of antihydrogen. Recognition of these achievements soon followed in the scientific media, with the award of Physics World‘s “2010 Breakthrough of the Year” on 20 December.

Both ALPHA and ASACUSA aim to measure precisely the spectrum of antihydrogen and compare it with that of hydrogen. Any small difference would cast light on the imbalance between matter and antimatter in the universe today. The first nine atoms of antihydrogen were produced at CERN in 1995. Then, in 2002, the ATHENA and ATRAP experiments at the AD showed that it was possible to produce large quantities of cold (i.e. very low velocity) antihydrogen, thus opening up the possibility of conducting detailed studies. However, the challenge remained of producing the antihydrogen in such a way that its spectrum could be analysed.

The strategy being pursued in the ALPHA experiment, which evolved from ATHENA, is to make cold antihydrogen and then hold the neutral antiatoms in a superconducting magnetic trap similar to those used for high-precision atomic spectroscopy. The ultimate aim is to measure 1s–2s transitions for comparison with the latest results in hydrogen. The ALPHA trap consists of an octupole and two solenoidal “mirrors”, which together create a magnetic field that confines the antiatoms by interacting with their magnetic moments. Silicon detectors surrounding the trap record the annihilations of any trapped antihydrogen once it is released. In the studies reported in November, the collaboration observed 38 annihilations (Andreson et al. 2010).

The ASACUSA experiment is following a different approach aimed at studying hyperfine transitions in antihydrogen, which involve much smaller energy differences and hence microwave rather than laser spectroscopy. The technique does not require the antiatoms to be trapped, so the collaboration is taking steps towards extracting a beam of antihydrogen in a field-free region for high-resolution spectroscopy. The December paper reports success in producing cold antihydrogen in a so-called “cusp” trap, an essential precursor to making a beam. This trap consists of a superconducting anti-Helmholtz coil and a stack of multiple ring electrodes (Enomoto et al. 2010). The next step will involve extracting a spin-polarized antihydrogen beam along the axis of the trap.

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The new state is related to antihelium-3, with the Λ replacing one of the neutrons. The STAR team identified it via its decay into antihelium-3 and a positive pion. Altogether, in an analysis of hundred million collisions, they found 70 ± 17 antihypertritons and 157 ± 30 hypertritons (consisting of pnΛ).

In heavy-ion collisions only a tiny fraction of the emitted fragments are light nuclei, but these states are of fundamental interest. The STAR team finds that the measured yields of hypertritons (antihypertritons) and helium-3 (antihelium-3) are similar. This suggests an equilibrium in the populations of up, down, and strange quarks and antiquarks, contrary to what is observed at lower collision energies.

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“Those who say that antihydrogen is antimatter should realize that we are not made of hydrogen and we drink water, not liquid hydrogen.” These are words spoken by Paul Dirac to physicists gathered around him after his lecture “My life as a Physicist” at the Ettore Majorana Foundation and Centre for Scientific Culture in Erice in 1981 – 53 years after he had, with a single equation, opened new horizons to human knowledge. To obtain water, hydrogen is, of course, not sufficient; oxygen with a nucleus of eight protons and eight neutrons is also needed. Hydrogen is the only element in the Periodic Table to consist of two charged particles (the electron and the proton) without any role being played by the nuclear forces. These two particles need only electromagnetic glue (the photon) to form the hydrogen atom. The antihydrogen atom needs two antiparticles (antiproton and antielectron) plus electromagnetic antiglue (antiphoton). Quantum electrodynamics (QED) dictates that the photon and the antiphoton are both eigenstates of the C-operator (see later) and therefore electromagnetic antiglue must exist and act like electromagnetic glue.

If matter were made with hydrogen, the existence of antimatter would be assured by the existence of the two antiparticles (antiproton and antielectron), the existence of the antiphoton being assured by QED. As Dirac emphasized, to have matter it is necessary to have another particle (the neutron) and another glue (the nuclear glue) to allow protons and neutrons to stay together in a nucleus. This problem first comes into play in heavy hydrogen, which has a nucleus – the deuteron – made of one proton and one neutron. For these two particles to remain together there needs to be some sort of “nuclear glue”. We have no fundamental theory (like QED) to prove that the nuclear antiglue must exist and act like the nuclear glue. It can be experimentally established, however, by looking at the existence of the first example of nuclear antimatter: the antideuteron, made with an antiproton, an antineutron and nuclear antiglue. If the antideuteron exists, all other antielements beyond heavy antihydrogen must exist. Their nuclei must contain antiprotons, antineutrons and nuclear antiglue. But if the antideuteron did not exist, nothing but light antihydrogen could exist: farewell anti-water and farewell all forms of antimatter.

Dirac’s statement takes into consideration half a century of theoretical and experimental discoveries, which have ultimately concluded that the existence of antimatter is supported exclusively by experiment. The CPT theorem implies that if matter exists then so should antimatter, but T D Lee has shown that the theorem is invalid at the Planck scale (around 10¹⁹ GeV) where all of nature’s fundamental forces converge (Lee 1995). Because this grand unification is the source of everything, if CPT collapses at the energy scale where it occurs, then we can bid farewell to all that derives from CPT.

CPT and the existence of antimatter

The CPT theorem states that physical laws are invariant under simultaneous transformations that involve inversions of charge (C), parity (P) and time (T). The first of these invariance operators to be discovered was C, by Hermann Weyl in 1931. This says that physical reality remains invariable if we replace the charges that are additively conserved by their corresponding anticharges – the first known example being that of the electron and the antielectron. The P operator, discovered by Eugene Wigner, Gian-Carlo Wick and Arthur Wightman, tells us that in replacing right-handed systems with left-handed ones, the results of any fundamental experiment will not change. The T operator, discovered by Wigner, Julian Schwinger and John Bell, established that inverting the time axis will also not alter physical reality.

The mathematical formulation of relativistic quantum-field theory (RQFT), which is supposed to be the basic description of nature’s fundamental forces, possesses the property of CPT invariance whereby inverting all does not change the physical results. In other words, if we invert all charges using C, the three space reference axes (x, y, z) using P, and the time axis using T, all will remain as before. However, matter is made of masses coupled to quantum numbers that are additively conserved: electric charges, lepton numbers, baryon numbers, “flavour” charges etc. If we were to apply the three CPT operators to matter in a certain state we would obtain an antimatter state. This means that if the CPT theorem is valid then the existence of matter implies the existence of antimatter and that the mass of a piece of matter must be identical to that of the corresponding piece of antimatter.

Suppose that nature obeys the C invariance law; in this case, the existence of matter implies the existence of antimatter. If C invariance is broken, the existence of antimatter is guaranteed by CPT. Now, suppose that CP is valid; again, the existence of matter dictates the existence of antimatter. If CP is not valid, then the existence of antimatter is still guaranteed by CPT. If CPT collapses, however, only experimental physics can guarantee the existence of antimatter. This summarizes what effectively happened during the decades after Dirac’s famous equation of 1928 – until we finally understood that CPT is not an impervious bulwark governing all of the fundamental forces of nature.

Three years after Dirac came up with his equation, Weyl discovered C and it was thought at the time that the existence of the antielectron and the production of electron–antielectron pairs were the consequences of C invariance. The equality of the mean life of positive and negative muons was also thought to be an unavoidable consequence of the validity of C. These ideas continued with the discoveries of the antiproton, the antineutron and, finally, of the neutral strange meson called θ₂. This apparent triumph of the invariance operators came in parallel with the success in identifying a “point-like” mathematical formulation that was capable of describing the fundamental forces of nature. Building on the four Maxwell equations the marvellous construction of RQFT was finally achieved. This theory should have been able to describe not only the electromagnetic force (from which it was derived) but also the weak force and the nuclear force. Two great achievements reinforced these convictions: Enrico Fermi’s mathematical formulation of the weak force and Hideki Yukawa’s triumphant discovery of the “nuclear glue” – the famous π meson – thanks to Cesare Lattes, Hugh Muirhead, Giuseppe Occhialini and Cecil Powell (CERN Courier September 2007 p43).

These initial extraordinary successes were, however, later confronted with enormous difficulties. In QED, there were the so-called “Landau poles” and the conclusion that the fundamental “bare” electric charge had to be zero; for the weak forces, unitarity fell apart at an energy of 300 GeV; and in the realm of the nuclear force, the enormous proliferation of baryons and mesons was totally beyond understanding in terms of RQFT. This is when a different mathematical formulation, the “scattering matrix” or S-matrix, was brought in, and with it the total negation of the “field” concept. It required three conditions: analyticity, unitarity and crossing. So, why bother with RQFT if the S-matrix is enough? On the other hand, if RQFT does not exist, how do we cope with the existence of CPT invariance? This opened the field related to the breaking of the invariance laws, C, P, T.

The shock of CP violation

In 1953 Dick Dalitz discovered the famous θ–τpuzzle: two mesons, with identical properties, had to be of opposite parity. Intrigued by this “puzzle”, T D Lee and C N Yang analysed experimental results in 1956 and found that there was no proof confirming the validity of C and P in weak interactions. Within one year of their findings, Chien-Sung Wu and her collaborators discovered that the invariance laws of C and P are violated in weak interactions. So how could we cope with the existence of antimatter? This is why Lev Landau proposed in 1957 that if both the C and P operators are violated then their product, CP, may be conserved; the existence of antimatter is then guaranteed by the validity of CP (Landau 1957).

There is one small detail that was always overlooked. In 1957, in a paper that not many had read (or understood), Lee, Reinhard Oehme and Yang demonstrated that, contrary to what had been said and repeated, the existence of the two neutral strange mesons, θ₁ and θ₂, was not a proof of the validity of C or P, or of their product CP (Lee, Oehme and Yang 1957).

I was in Dubna in 1964 when Jim Cronin presented the results on CP violation that he had obtained together with Val Fitch, James Christenson and René Turley. On my right I had Bruno Touschek and on my left Bruno Pontecorvo. Both said to me of Cronin and his colleagues, “they have ruined their reputation”. The validity of Landau’s proposal of CP invariance, with antimatter as the mirror image of matter, was highly attractive; to put it in doubt found very few supporters. Dirac, however, was one of the latter and he fell into a spell of deep “scientific depression”. He, who was well known for his caution, had total belief in C invariance, which had led him to predict the existence of antiparticles, antimatter, antistars and antigalaxies. Now even CP was breaking.

If the CPT product is to remain conserved, the breaking of CP involves that of T. For some of the founding fathers of modern physics, however, invariance relative to time inversion at the level of the fundamental laws had to remain untouched. So, if CP breaks and T does not, then CPT must break. After all, why not? In fact, the bulwark of CPT was RQFT, but it already seemed as if this mathematical formulation had to be replaced by the S-matrix. The breaking of the invariance operators (C, P, CP) and the apparent triumph of the S-matrix were coupled at the time to experimental results that indicated no trace of antideuterons, even among the production of 10 million pions in proton collisions.

To obtain the first example of antimatter it seemed that CPT had to be proved right, which meant proving the violation of T. No one back in 1964 could imagine that physics would open the new horizons that we know today. The only actions left to us then were of a technological–experimental nature. It turned out that the discovery of true antimatter required the realization of the most powerful beam of negative particles at CERN’s PS, as well as the invention of a new technology capable of measuring, with a precision never achieved before, the time-of-flight of charged particles. This is how we came to discover an antideuteron produced, not after 10 million pions, but only after a 100 million (Massam et al. 1965).

The crucial experiment

The search for the existence of the first example of nuclear antimatter needed a high-intensity beam of negative particles produced in high-energy interactions. This negative beam was dominated by pions, with a fraction of K mesons and a few antiprotons. It was necessary to separate particles with different masses, starting with pions and then going up with mass to K mesons, antiprotons and (it was hoped) antideuterons. To accomplish this the first step was a combined system of bending magnets coupled with magnetic quadrupoles – for focusing purposes – and a strong electrostatic separator. This high-intensity beam of negative “partially separated” particles was the result of a special project made and carried out with two friends of mine, Mario Morpurgo and Guido Petrucci. The second vital step was a sophisticated time-of-flight system capable of achieving the time resolution needed to detect one negative particle (the antideuteron) in a background of a 100 million other negative particles (essentially, π mesons). The results, which showed the existence of a negative particle with mass equal to that of the deuteron, were obtained on 11 March 1965, the same day as the 41st birthday of the PS director, Peter Standley.

Dirac came out of his depression when he received a phone call from his friend Abdus Salam, saying: “Relax Paul, my friend Nino Zichichi has discovered the antideuteron”. Dirac called me and invited me for lunch at his place, and this started a friendship that led us to the realization of the Erice Seminars on Nuclear Wars.

To understand the importance of this discovery we need to have a clear idea of what is meant by “matter”. Particles are not sufficient to constitute matter; we also need “glues”. With electromagnetic glue we can make atoms and molecules; to make the nucleus, we need protons, neutrons and nuclear glue. To make antimatter requires antiprotons, antineutrons and nuclear “antiglue”; but we also need to know that nuclear antiglue allows these constituents of antimatter to stick together just as protons and neutrons do to form matter. A fundamental law is needed that establishes the existence of nuclear antiglue that is exactly identical to the nuclear glue in matter. This fundamental law is missing.

In fact, we know today that the strengths of all of the fundamental forces converge at the Planck energy, where CPT invariance breaks down. Moreover, if we replace the “points” with “strings”, nothing changes. CPT results from the “point-like” mathematical formulation of RQFT but it collapses at the energy scale at which the fundamental forces originate, i.e. at the Planck energy. If we replace “points” with “strings” then relativistic quantum string theory results; but it cannot validate CPT. This implies that no theory exists that can guarantee that if we have matter then antimatter must exist. This is why the fact that all anti-atoms with their antinuclei must exist with certitude resulted from the experiment at CERN in March 1965.

In 1995, during his opening lecture for the symposium celebrating the 30th Anniversary of the Discovery of Antimatter in Bologna, T D Lee said: “Werner Heisenberg discovered quantum mechanics in 1925 and by 1972 he had witnessed almost all of the big jumps in modern physics. Yet he ranked the discovery of antimatter as the biggest jump of all. In fact in his book The Physicist’s Conception of Nature (1972), Heisenberg writes, ‘I think that this discovery of antimatter was perhaps the biggest jump of all big jumps in physics in our century.’.”

• This article is based on the opening talk given at the event to celebrate the 50th anniversary of the Karlsruhe Nuclide Chart, held in Karlsruhe on 9 December 2008 (see www.nucleonica.net:81/wiki/index.php/Help:Karlsruhe_Nuclide_Chart). For the full article with complete references, see www.nucleonica.net:81/wiki/images/a/aa/05_Zichichi_Karlsruhe.pdf.

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The latest record for antiparticle density created in the laboratory has not come from an accelerator facility but from the Lawrence Livermore National Laboratory’s Jupiter laser facility. Hui Chen and colleagues blasted picosecond laser pulses carrying 10²⁰ Wcm^–2 from the Titan laser onto gold targets some 1 mm thick. Part of each laser pulse created a plasma and part drove the plasma’s electrons into the gold. The gold nuclei then slowed down the electrons, producing photons that converted into electron–positron pairs. The result was an estimated 10¹⁶ positrons/cubic centimetre.

In addition to being intrinsically interesting this work could aid better understanding of astrophysical phenomena such as gamma-ray bursts. It could also lead to new ways to produce positron sources, which at present are limited to positron-emitting radioisotopes and pair-creation from high-energy photons at accelerators.

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The satellite experiment Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA) has made a new measurement of the antiproton-to-proton flux ratio in cosmic rays with energies up to 100 GeV. The results, which represent a great improvement in statistics compared with data published previously, provide significant constraints on exotic sources of cosmic antimatter.

The PAMELA experiment has been in low Earth-orbit on the Resurs-DK1 satellite since its launch in June 2006. During 500 days of data collection it has identified 1000 antiprotons with energies in the range 1–100 GeV, including 100 antiprotons with an energy above 20 GeV. This is a larger data sample at higher energies than any other experiment has obtained.

Cosmic antiprotons can be made in particle (mainly proton) collisions with interstellar gas but they could also have more exotic origins, for example, in the annihilation of dark-matter particles. Finding out more about the actual production mechanisms requires detailed studies of the antiproton energy spectrum over a wide energy range, which in turn depend on data with good statistics, as PAMELA now provides.

Analysis of the data from PAMELA show that the antiproton-to-proton flux ratio rises smoothly to about 10 GeV, before tending to level off. The results match well with theoretical calculations that assume only secondary production of antiprotons by cosmic rays propagating through the galaxy. This places limits on contributions from other, more exotic sources.

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Recently, the Japanese–European ASACUSA group made the first steps towards producing a low-velocity antihydrogen beam, which may be used to measure the hyperfine transition frequency between the two spin substates of antihydrogen’s ground state. Its value for ordinary hydrogen is near 1420 MHz.

Before this can be done antiprotons must be confined and cooled in an evacuated container in which magnetic and/or electric fields produce restoring forces that stop the antiprotons drifting to the container walls, where they would annihilate. To do this, the MUSASHI group of the ASACUSA collaboration has introduced a novel variant of the familiar Helmholtz coils. The MUSASHI coils differ from the usual configuration by having antiparallel rather than parallel excitation currents. This produces a magnetic quadrupole field rather than the normal constant one, and is symmetric about the coil axis. If a suitable electrostatic multipole field is added to this so-called “magnetic cusp” field, all of the restoring forces needed to confine both positive and negative charges are present within the container.

This “cusp trap” can thus also hold positrons, with which the antiprotons recombine to create the antihydrogen, as well as electrons. The latter can be used to cool the antiprotons to the extremely low temperature at which recombination occurs. In the recent tests, some 3 million antiprotons were stored in the trap and cooled with electrons.

A well known obstacle to CPT tests with antihydrogen is that both the hyperfine and the 1S–2S frequency measurements must be performed on ground-state atoms, while it appears that positron–antiproton recombination produces them in very highly excited states. One great advantage of the cusp trap is that if these neutral atoms are cold enough its quadrupole field pulls on their large magnetic moment, causing them to seek the field minimum at the trap centre. They remain confined there until they reach the ground state. However, since their magnetic moment falls as they de-excite, the pull weakens. This means that in the ground state, only antihydrogen atoms in one of the two possible spin states are pulled to the centre, while those in the other state are expelled along the trap axis, emerging as a spin-polarized, ground-state antihydrogen beam.

This is ideal for the classical type of slow atomic beam experiment in which a microwave cavity induces spin flips when tuned to the correct hyperfine frequency (see figure). The resonant frequency can then be detected using a sextupole magnet which focuses flipped atoms onto a detector but defocuses unflipped ones. Comparison with the well measured hydrogen frequency then gives a stringent test of CPT symmetry.

Although much of this remains to be done, the recent successes are so encouraging that further steps along the road to a slow antihydrogen beam are now planned.

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The antiparticles – mainly positrons – that are detected in cosmic rays on Earth or in the atmosphere are almost certainly the by-products of interactions. By going above the atmosphere, AMS should detect any antimatter among the primary cosmic rays. Detection of a significant quantity of antimatter on the ISS would constitute irrefutable proof that there is still an active source of antimatter in the cosmos. AMS will also look for dark matter by trying to detect the annihilation products of the hypothesized supersymmetric particles, and measure more precisely the composition of cosmic rays.

The central tracker was constructed at the University of Geneva and will soon be surrounded by a powerful cryogenic magnet and other high-precision detectors. The assembly and construction of the whole experiment, which will weigh more than 7 tonnes, will be finalized next spring.

AMS must be ready and delivered to the Kennedy Space Centre in Cape Canaveral, Florida, by the end of 2008 at the latest. It will be launched on a space shuttle and will remain on board the ISS for several years.

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According to the preface to the popular novel Angels and Demons (2000), author Dan Brown was apparently inspired by the imminent commissioning of CERN’s “antimatter factory”, the Antiproton Decelerator (AD). The real-life AD has now been fully operational for about five years, and the experiments there have produced some notable physics results. One of the big stories along the way was the synthesis in 2002 of antihydrogen atoms by the ATHENA and ATRAP collaborations.

This feat was an important step towards one of the ultimate goals of everyday antimatter science: precision comparisons of the spectra of hydrogen and antihydrogen. According to the CPT theorem, these spectra should be identical. To get an idea of what precision means in this context, take a look at the website of 2005 Nobel Laureate Theodor Hänsch, which has the following cryptic headline: f(1S–2S) = 2 466 061 102 474 851(34) Hz. This may look like a cryptic puzzle appearing in Brown’s fiction, but it simply means that the frequency of one of the n = 1 to n = 2 transitions in hydrogen has been measured with an absolute precision of about 1 part in 10¹⁴. This is impressive, but where do we stand with antihydrogen?

Storing antihydrogen

Both ATHENA and ATRAP produced antihydrogen by mixing antiprotons and positrons in electromagnetic “bottles” called Penning traps. Penning traps feature strong solenoidal magnetic fields and longitudinal electrostatic wells that confine charged particles. The antiprotons come from CERN’s AD, and the positrons come from the radioactive isotope ²²Na. The whole process involves cleverly slowing, trapping, and cooling both species of particles (Amoretti et al.. 2002 and Gabrielse et al.. 2002). But here’s the rub: when the charged antiproton and positron combine, the neutral antihydrogen is no longer confined by the fields of the Penning trap, and the precious anti-atom is lost. The ATHENA experiment demonstrated antihydrogen production because it could detect the annihilation of the anti-atoms when they escaped the Penning trap volume and annihilated on the walls.

To study antihydrogen using laser spectroscopy, anti-atoms need to be sustained for longer. In the 1s–2s transition mentioned above, the excited state (2s) has a lifetime of about a seventh of a second; while in ATHENA, an anti-atom would annihilate on the walls of the Penning trap within a few microseconds of its creation. Thus, the next-generation antihydrogen experiments include the provision for trapping the neutral anti-atoms that are produced in a mixture of charged constituents.

The Antihydrogen Laser Physics Apparatus (ALPHA) collaboration has recently commissioned a new device designed to trap the neutral anti-atoms. ALPHA takes the place of ATHENA at the AD and features five of the original groups from ATHENA (Aarhus, Swansea, Tokyo, RIKEN and Rio de Janeiro) plus new contributors from Canada (TRIUMF, Calgary, UBC and Simon Fraser), the US (Berkeley and Auburn), the UK (Liverpool) and Israel (Nuclear Research Center, Negev).

Neutral atoms – or anti-atoms – can be trapped because they have a magnetic moment, which can interact with an external magnetic field. If we build a field configuration that has a minimum magnetic field strength, from which the field grows in all directions, some quantum states of the atom will be attracted to the field minimum. This is how hydrogen atoms are trapped for studies in Bose–Einstein condensation (BEC). The usual geometry is known as an Ioffe–Pritchard trap. A quadrupole winding and two solenoidal “mirror coils” produce the field to provide transverse and longitudinal confinement, respectively. The image above also shows the electrodes that provide the axial confinement in the Penning trap for the charged antiprotons and positrons. The idea is that the antihydrogen produced in the Penning trap is “born” trapped within the Ioffe–Pritchard trap – if its kinetic energy does not exceed the depth of the trapping potential.

This is a big “if”. A ground-state hydrogen atom has a magnetic moment that gives us a trap depth of only about 0.7 K for a magnetic well depth of 1 T. The superconducting magnetic traps that we can build and squeeze into our experiments will give 1–2 T of well-depth for neutral atoms. All antihydrogen experiments to date occur in devices cooled by liquid helium at 4.2 K, but there are strong indications that the antihydrogen produced by direct mixing of antiprotons and positrons is warmer than this, with temperatures of at least hundreds of kelvin. ATRAP has devised a laser-assisted method of producing antihydrogen that May give colder atoms, but their temperature has not yet been measured. (Note that the highly excited antihydrogen atoms produced in both experiments can have significantly larger magnetic moments, thus experiencing higher trapping potentials. The trick, then, is to keep them around while they decay to the ground state.) Both groups are investigating new ways to produce colder anti-atoms, and the 2007 run at the AD (June–October) promises to be revealing.

Designer magnets

A second important issue facing both collaborations is the effect on the charged particles of adding the highly asymmetric Ioffe–Pritchard field to the Penning trap. Penning traps depend on the rotational symmetry of the solenoidal field for their stability. As ALPHA collaborator Joel Fajans of Berkeley initially pointed out, the addition of transverse magnetic fields to a Penning trap can be a recipe for disaster, leading either to immediate particle loss, or to a slower, but equally fatal, loss due to diffusion. Fajans’ solution, adopted by the ALPHA collaboration, is to use a higher-order magnetic multi-pole field for the transverse confinement. A higher-order field can, in principle, provide the same well-depth as a quadrupole while generating significantly less field at the axis of the trap, where the charged particles are confined.

To construct such a magnet, the ALPHA collaboration surveyed the experts in fabrication of superconducting magnets for accelerator applications. It turns out that the Superconducting Magnet Division at the Brookhaven National Laboratory (BNL) had previously developed a technique that is almost tailor-made to our needs. The key here is to use the proper materials in the construction of the magnet. To detect antiproton annihilations, ALPHA incorporates a three-layer silicon vertex detector similar to those used in high-energy experiments. However, the annihilation products (pions) must travel through the magnets of the atom trap before reaching the silicon. Therefore, it is highly desirable to minimize the amount of material used in the magnet construction to minimize multiple scattering between the vertex and the detector. So bulky stainless-steel collars for containing the magnetic forces, as used in the Tevatron or the LHC, cannot be used.

The Brookhaven process uses composite materials to constrain the superconducting cable that forms the basis of the magnet. Using a specially developed 3D winding machine, the team at BNL was able to wind an eight-layer octupole and the mirror coils directly onto the outside of the ALPHA vacuum chamber. The mechanical strength is provided by pre-tensioned glass fibres in an epoxy substrate. Only the superconducting cable is metal.

The new ALPHA device was designed and constructed during the AD shutdown of November 2004 to July 2006 and commissioned during the physics run at the AD in July–November 2006. The Brookhaven magnets performed beautifully, demonstrating that charged antiprotons and positrons can be stored in the full octupole field for times far exceeding those necessary to synthesize antihydrogen. We even made the first preliminary attempt to produce and trap antihydrogen in the full field configuration; but we have yet to observe evidence for trapping.

Meanwhile, the ATRAP collaboration worked hard to commission a new quadrupole trap for antihydrogen and succeeded in storing clouds of antiprotons and electrons in their new device. The 2007 physics run at the AD promises to be an exciting one for antihydrogen physics. Both ALPHA and ATRAP should have operational devices that are capable – in theory – of trapping neutral antimatter for the first time.

Back to Dan Brown

So let’s look at what is possible in experiments with antimatter today, leaving the speculation to aficionados of sci-fi and NASA. If you wanted to take antimatter to the offices of your national funding agency, you might consider taking some antiprotons, since most of the mass-energy of an antihydrogen atom is in the nucleus. This might be tempting, since our charged-particle traps are certainly deeper than those for neutral matter or antimatter. ATRAP and ALPHA initially capture antiprotons in traps with depths of a few kilo-electron-volts, corresponding to tens of millions of kelvin. But, density is an issue. A good charged-particle trap for cold positrons has a particle density of about 10⁹ cm^–3. Antiproton density is much smaller, but we’ll be optimistic and use this number. So to transport a milligram of antiprotons – of the order of 10²¹ particles – you would need a trap volume of 10¹² cm³, or 10⁶ m³. That means a cube 100 m wide, which will not fit in your luggage. Incidentally, a milligram of antimatter, annihilating on matter, would yield an energy equivalent to about 50 tonnes of TNT.

So, what about transporting some neutral antimatter? Neutral atom traps certainly have higher densities. The first BEC result for hydrogen at MIT reported a density in the order of 10¹⁵ cm^–3 for about 10⁹ atoms in the condensate. This is better, but still far less than a milligram, even if you can get the atoms from a gas bottle. The size of the trap is now down to 10⁵ cm³, which is more manageable. Note, however, that the BEC transition in this experiment was at 50 μK – far below the 4.2 K that we hope to achieve with antihydrogen. Unfortunately, to get really cold and dense atomic hydrogen requires using evaporative cooling – throwing hot atoms away to cool the remaining ones in the trap. This implies damaging your lab before you send the surviving, trapped anti-atoms to their final, cataclysmic fate. And don’t forget that the total history of antiproton production here on Earth amounts to perhaps a few tens of nanograms in the past 25 years or so. Unfortunately, the antiproton production cross-section is unlikely to change.

How many anti-atoms can we trap? The Japanese-led ASACUSA experiment, using an extra stage of deceleration after the AD, can trap around a million of the 30 million decelerated anti-protons that the AD delivers every 100 s or so. Suppose we could make all of these into antihydrogen (in comparison, ATHENA achieved about 15%). The trapping efficiency for neutral anti-hydrogen is anybody’s guess at this point – we would be grateful for 1%. This is why the very notion of having a dense cloud of interacting antihydrogen atoms will bring a weary smile to the face of anyone working in the AD zone. Using the above figures, it would take us 10¹⁹ s – about 300 billion years – to accumulate just one milligram. One might also question if anyone could engineer a device reliable enough to safely contain an explosive quantity of anti-matter – not in my lab, thanks.

Back down to the sober reality here at CERN, we would be happy just to demonstrate trapping of antihydrogen in principle. This means initially trapping just a few anti-atoms – not making a BEC or antihydrogen ice. The future of our emerging field seems to depend on this, although ASACUSA is developing a plan to do spectroscopy on antihydrogen in flight. Time will tell which approach proves more promising. Two things are certain: the real technology of antimatter production and trapping lags far behind Dan Brown’s imagination; and the Vatican is safe from us.

The post Keeping antihydrogen: the ALPHA trap appeared first on CERN Courier.

The ATHENA collaboration, whose primary is to study antihydrogen spectroscopically, has reported evidence that metastable protonium atoms (i.e. antiprotonic hydrogen, pbarp) can be created in binary antiproton reactions with H₂⁺ ions. These ions were produced when the positrons in the trap collided with H₂ molecules, inevitably present as “dirt in the test tube”. This serendipitous method of making protonium turns out to be interesting because it seems to produce it in states with principal quantum number (n) near 68 and angular momentum quantum number l < 10.

Ground-state n = 1, l = 0 protonium can be produced easily and has been known for many years. However, it annihilates almost instantaneously owing to the marked overlap of the p and pbar wave functions. In high-n protonium, however, there is little overlap, since the Bohr-model orbit radius is proportional to n². The p and pbar can then come into contact only by de-exciting radiatively to l ∼ 0 via a chain of transitions that the ATHENA team estimates to take about 1 ms. This extreme longevity should enable detailed laser-spectroscopy experiments on the protonium atom, leading to values of the antiproton’s properties relative to those of the proton, and so to a new class of CPT-invariance tests (N Zurlo et al. 2006). Two-body atoms are especially valuable in this respect since their transition frequencies can be calculated analytically

Another experiment at the AD, ASACUSA, has been exploiting longevity against annihilation for some years with the (neutral) antiprotonic helium atom, pbarHe⁺. Although this is a three-body atom, its high-n, high-l, pbarHe states have microsecond annihilation lifetimes and are easily produced when antiprotons with electron-volt energies collide with ordinary helium atoms. As in the antihydrogen experiment, H₂ impurities are always present in the “test tube” at some level and have long been known to reduce, or quench, the pbarHe⁺ lifetime, even at very low molecular concentrations, via binary collisions between H₂ and pbarHe⁺.

To understand this fully, the ASACUSA team introduced H₂ and D₂ molecules into the helium target at various temperatures and concentrations and then deduced the quenching cross-section from the annihilation lifetime of the antiproton in the (n,l) = (37,34) and (n,l) = (39,35) states, as a function of these variables (B Juhász et al. 2006). Below 30 K the cross-section levelled off in the first case, revealing a tunnelling effect with a small activation barrier, while the (39,35) state had a 1/v “Wigner”-type dependence. Such results can perhaps serendipitously fill some gaps in our understanding of astrophysics, since the measured cross-sections should be similar to those for binary reactions of hydrogen and deuterium, which play an important role in cold interstellar and protostellar clouds, but have not been well studied at low temperatures.

A final unsought discovery has resulted from ASACUSA’s quest for ever lower systematic errors in the laser-spectroscopy experiments on antiprotonic helium. This forced the team to go to extremely low helium target pressures. At helium densities less than 3 × 10¹⁶ cm^-3 they noticed a lengthening of the tail of the spectrum of time intervals between the formation of the pbarHe⁺ atom and the subsequent annihilation of the antiproton. This could only be explained by longevity of the pbarHe⁺⁺ two-body, doubly charged ion, which in higher-pressure gas is a short-lived intermediate stage between the formation of the neutral pbarHe⁺ atom and the “contact” ppbar annihilation (Hori et al. 2005). Once again, a two-body atom promises to become serendipitously available as a test bench for CPT tests. Following up this possibility is an important part of the ASACUSA experimental programme.

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Cancer therapy is about collateral damage: destroying the tumour while avoiding the healthy tissue around it. Unwanted exposure of healthy tissue could cause side effects and result in a reduced quality of life. It is also believed to increase the chances of secondary cancers developing. In radiation therapy there is an ongoing quest to reduce the radiation level to tissue outside the primary tumour volume.

In hadron therapy, which began in 1946 with Robert Wilson’s seminal paper, “Radiological Use of Fast Protons”, the dose profile of heavy charged particles (hadrons) does not irradiate healthy tissue because most of the energy is deposited at the end of the flight path of the particles – the Bragg peak – with little before and none beyond. However, the question remains of how to maximize the concentration of energy onto the target.

The first speculations that antiprotons could offer a significant gain in targeting tumours through the extra energy released by annihilation date back more than 20 years (Gray and Kalogeropoulos 1984). Now the ACE collaboration has tested this idea by directly comparing the effectiveness of cell irradiation using protons and antiprotons.

To simulate a cross-section of tissue inside a body, the experiment uses tubes filled with live hamster cells suspended in gelatine. These are irradiated with beams of protons or antiprotons at a variety of intensities with about a 2 cm range in water. After irradiation the gelatine is extruded from the tubes and cut into 1 mm slices. These are then dissolved in growth medium and the cells are placed in Petri dishes in an incubator. After a few days the naked eye can see that some of the cells have produced healthy offspring. This gives a measure of the survival of cells along the beam path for the different dose levels. Cell survival is plotted for the entrance and the Bragg-peak regions as a function of particle fluencies, and the ratio of dose for a 20% survival in these two regions is extracted.

Comparing beams of protons and antiprotons that cause identical damage at the entrance to the target, the results of the experiment show that the damage to cells inflicted at the end of the beam is four times higher for antiprotons (Holzscheiter et al. 2006.) The method directly samples the total effect of the beams on the cells, combining the enhanced energy deposition in the vicinity of the annihilation point and the higher biological effectiveness of this extra energy (delivered by nuclear fragments). The experiment demonstrates a significant reduction of the damage to the healthy cells along the entrance channel of a beam for antiprotons compared with protons.

While antiprotons may seem unlikely candidates for cancer therapy, the initial results from ACE indicate that these antimatter particles could lead to more effective radiation therapy. There is no doubt, however, that the first clinical application is still at least a decade away.

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In the ASACUSA experiment, samples of antiprotonic helium – an atom with an antiproton and an electron orbiting a normal helium nucleus – were produced using CERN’s Antiproton Decelerator facility, and irradiated with a tunable laser beam, the frequency of which could be measured very precisely with the Hall-Hänsch frequency-comb technique. The laser beam could be tuned to one of several characteristic frequencies of the antiprotonic atoms, each frequency corresponding to an atomic transition of the antiproton. Since these frequencies were determined by the properties of the antiproton, the ratio of the antiproton mass to the electron mass could then be calculated from the measured values.

The results can also be combined with an earlier high-precision measurement of the antiproton’s cyclotron frequency (which determines the curvature of its path in a magnetic field). This shows that there is no difference in the proton and antiproton charges either, apart from the sign. Still more precise experiments are planned with the optical comb, and may soon give an even smaller margin of error for the antiproton than the best one obtained for the proton itself (currently about five times smaller). Surprisingly, the antiproton may soon be known better than the proton.

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I have even read it. Indeed the author has me killed at the very beginning.

Correct. You die and the antimatter stolen from CERN is used to blackmail the Vatican. CERN does produce antimatter, and the contact of antimatter with ordinary matter results in annihilation where large quantities of energy appear. Aren’t you scared that one day Brown’s scenario may become real?

No, since there is no way to produce and store a large quantity of antimatter.

What does “a large quantity” mean? Are we talking about kilograms?

No, not even about nanograms. I am talking about single atoms. We are not able to produce and store amounts of antimatter that would cause damage of any kind, e.g. that could be used as an explosive, as in the book.

You mean we are not able to now – or ever? Is that a problem of technology or perhaps a result of the laws of physics?

Both. Let us start with technological reasons, which are probably less convincing. Even if somebody could produce lots of antimatter, their main headache would be how to store it. First, they must place it in a vacuum – any other “container” would immediately annihilate, that is disappear! So antimatter must be kept in the very middle of a vacuum by a magnetic field. This is possible, we hope to do it at CERN, but for a few or a few tens of atoms only.

The vacuum must be of the best quality. What we call a vacuum in daily life is far from the ideal. An electric light bulb is not empty but contains a very, very diluted gas. In the CERN “antimatter trap” the gas pressure is 10⁻¹⁷ mbar. This means that on average there are a few tens of thousands of atoms per cubic metre. So even here we have annihilation with “stray” atoms. While it is possible to guard a few hundred antimatter atoms, protecting, say, 1 mg of antimatter from annihilation is practically impossible. And every act of annihilation results in the freeing of a certain amount of energy and degrading of the vacuum. This is a chain reaction.

The technical limit is not a real one. What is impossible today may very well be possible tomorrow. Surely we will learn how to get a better and better vacuum?

I agree that technical arguments may not be convincing. However, in one day CERN produces about 10¹² antiprotons. Renovations, equipment maintenance and upgrading, holidays and other interruptions limit the antiproton production to about 200 days a year. In 50 years of operation at CERN about 10¹⁶ antiprotons would be produced. Even if all of them made anti-atoms, we would arrive at about one millionth of a milligram of antihydrogen – I repeat, in 50 years!

I must add that in the process of antihydrogen production only a tiny percentage of antiprotons make anti-atoms. Once, I calculated that even if all the natural energy resources of our planet – coal, petrol, gas – were used to produce antimatter, it would be enough to drive about 15,000 km by car. This is physics. It does not depend on our technological development.

So we can forget about antimatter as a future energy source?

Of course! Until we find “natural resources” of antimatter (and I would not count on that), the production of antimatter on Earth for energy or, as in the book, for terrorism will never pay off. Much more energy would be used for its production than we could ever get back from annihilation.

Would you agree that more people learned about CERN from Angels and Demons than from reading scientific information? Is CERN correctly described in Brown’s book?

This question is a trap so my answer will be diplomatic. In my opinion antimatter, and thus CERN as the only place where we are able to produce it, came into the book by accident. They were just a background. An atomic bomb at the Vatican would have done as well. I do not want to speak on behalf of the author but I have the impression that he wanted to touch on the conflict between science and faith. History shows that sometimes such a conflict has indeed been seen by the church and the scientific community.

And what about CERN? Is CERN really working on a proof that God does not exist – that scientific knowledge is the real god?

A difficult question. For sure there are many people working at CERN who believe in God and their work actually confirms their convictions. There are also those who do not believe in God but believe in science. For them every discovery may be proof that God does not exist, but it is not true that we are working to prove that.

“Soon all gods will be proven to be false idols. Science has now provided answers to almost every question man can ask.” This statement is made in the book by Maximilian Kohler, the [fictional] director-general of CERN. Do you agree?

No, I do not agree. I am sure that science does not contradict faith. One person may say that he studies the laws of nature, another one that he wishes to understand how God initiated or created our world. In my opinion it is the same. Both are doing the same even if they believe in different things. The point of view represented by the head of CERN in the book was very popular in the 1950s and 60s. Not for the first time people believed then that science was close to completion; that technology would save the world. It seemed that building a sufficiently large number of nuclear reactors would solve the energy problem on Earth and so all other problems would disappear. However, people have not become happier and the old problems are still here. We continue to be dependent on nature, which dictates the conditions. I believe that despite more and more knowledge, ultimately it is nature that wins.

Production of the first antimatter atom on Earth brought you great recognition…

Antihydrogen production indeed led to extraordinary publicity. That is probably the reason that this work is considered to be one of 16 very important discoveries made at CERN. In my opinion, and from the scientific point of view, producing the first antihydrogen atom does not deserve such honour; the very production of antihydrogen is not a revolution in physics. It did not bring anything new and we do not care about the production itself but about studies of the antihydrogen atom. This is not at all simple. The first atoms produced moved with almost the speed of light. Indeed, one has to be fast to study such an object. Antihydrogen thus has to be cooled down and locked in a bottle; the slower it is, the better we can watch it. So the real goal is not the production of, but studies of, antimatter. I am sure that at some time physicists will manage to measure its gravitational interactions. That would really be something.

Why are antimatter studies so interesting to the public? Usually it is difficult to sell what physicists do in their large laboratories.

This is not completely true. There are at least a few problems that may be sold easily and in an interesting way even when drinking a good wine at a garden party. One example is relativistic physics – everybody is interested in the fact that the faster you move the younger you are. Another subject is astrophysics or the surrounding universe. It is fascinating to many, probably because we can make certain observations ourselves on a cloudless night. Besides, the astrophysical photographs are so impressive they are printed on the front pages of the daily papers. The curvature of time and space is also an extremely interesting problem. The shortest path between two points is not at all a straight line.

And what about antimatter?

When it comes to particle physics, the problem is complicated. People do not know what we really do. Antimatter is an exception, which is surely due to science-fiction films, where antimatter is very often a subject. Serials gather an audience. If every Monday evening we watch the adventures of the same heroes who conquer the universe in space vessels powered by antimatter, then television characters are quickly treated as one’s own family. In this way antimatter has become a family member.

Is your interest in antimatter also a result of those films?

No. I must admit that I have not seen many of them. Discussions on antimatter began much earlier than when the first episode of Star Trek was produced. The ancient Greeks had already discussed it – albeit under different names. One can read about it in the writings of Aristotle or Plato – writings that are rather philosophical according to our modern views. But 19th-century physicists also wrote about it, not yet knowing about the existence of its components.

I was always fascinated by the idea of symmetry, especially between the world and the antiworld. Does it exist at all? I think that studies of antimatter are so interesting because even in our everyday life we like asymmetry. Just have a look at an ancient Greek temple or a medieval church. But not only buildings – look at a Persian carpet. Only those that are factory-made display a full symmetry; the really expensive ones are handmade. Most appreciated are the very small breaks in symmetry, the subtle “faults” of the carpet weaver.

It is said that as a young man you considered being an actor. Would you accept the role of Leonard Vetra, the creator of antimatter at CERN, if a film based on Angels and Demons was produced?

Yes, but only on the condition that they do not take my eye or burn “Illuminati” across my chest with a hot iron. I think that from the acting point of view I would manage – after all Vetra is murdered on the first page of the novel. Does he say anything at all?

Oh yes, but only a little. Exactly four sentences.

Then I am sure I would manage.

The post Setting the record straight appeared first on CERN Courier.

Dirac’s equation was unique for its time because it took into consideration both Albert Einstein’s special theory of relativity and the effects of quantum physics proposed by Edwin Schrödinger and Werner Heisenberg. While it worked well on paper, Dirac’s rather straightforward equation carried with it a most provocative implication: it permitted negative as well as positive values for the energy E. Initially few physicists seriously considered Dirac’s idea because no-one had ever observed particles of negative energy. From the standpoint of both physics and common sense, the energy of a particle could only be positive.

Attitudes towards Dirac’s equation changed dramatically in 1932, when Carl David Anderson reported the observation of a negatively charged electron in a project at the California Institute of Technology that originated with his mentor, Robert Millikan. Anderson named the new particle the “positron”. Both Dirac and Anderson would win Nobel Prizes for Physics for their discoveries. Dirac shared the 1933 Nobel prize with Schrödinger, and Anderson shared the 1936 Nobel prize with Victor Hess. However, the existence of the positron, the antimatter counterpart of the electron, raised the question of an antimatter counterpart to the proton.

As Dirac’s theory continued to explain successfully phenomena associated with electrons and positrons, it followed – from the revised standpoints of both physics and common sense – that it should also successfully explain protons. This would then demand the existence of an antimatter counterpart. The search for the antiproton was under way, but it would get off to a very slow start, as it would be another two decades before a machine capable of producing such a particle became available.

Enter the Bevatron

Anderson discovered the positron with a cloud chamber during investigations of cosmic rays, but it was extremely difficult, if not impossible, to use the same approach for finding the antiproton. If physicists were going to find the antiproton, they were first going to have to make one.

However, even with the invention of the cyclotron in 1931 by Ernest Lawrence, earthbound accelerators were not up to the task. Physicists knew that creating an antiproton would require the simultaneous creation of a proton or a neutron. Since the energy required to produce a particle is proportional to its mass, creating a proton-antiproton pair would require twice the proton rest energy, or about 2 billion eV. Given the fixed-target collision technology of the times, the best approach for making 2 billion eV available would be to strike a stationary target of neutrons with a beam of protons accelerated to an energy of about 6 billion eV.

In 1954, Lawrence commissioned the Bevatron accelerator to reach energies of several billion electron-volts – then designated as BeV (now universally known as GeV) – to be built at his Radiation Laboratory in Berkeley. (Upon Lawrence’s death in 1958, the laboratory was renamed the Lawrence Berkeley National Laboratory.) This weak-focusing proton synchrotron was designed to accelerate protons up to 6.5 GeV. Though never its officially stated purpose, the Bevatron was built to go after the antiproton. As Chamberlain noted in his Nobel laureate lecture, Lawrence and his close colleague, Edwin McMillan, who co-discovered the principle behind synchronized acceleration and coined the term “synchrotron”, were well aware of the 6 GeV needed to produce antiprotons and made certain the Bevatron would be able to get there.

Armed with a machine that had the energetic muscle to make antiprotons, Lawrence and McMillan put together two teams to go after the elusive particle. One team was led by Edward Lofgren, who managed operations of the Bevatron. The other was led by Segrè and Chamberlain. Segrè had been the first student to earn his physics degree at the University of Rome under Enrico Fermi. He had, with the aid of one of Lawrence’s cyclotrons, discovered technetium, the first artificially produced chemical element. He was also one of the scientists who determined that a plutonium-based bomb was feasible, and his experiments on the scattering of neutrons and protons and proton polarization broke new ground in understanding nuclear forces. Chamberlain had also studied under Fermi, and under Segrè as well. He was Segrè’s assistant on the Manhattan Project at Los Alamos while still a graduate student, and later joined Segrè at Berkeley to collaborate on the nuclear-forces studies.

Making an antiproton was only half the task; no less formidable a challenge was to devise a means of identifying the beast once it had been spawned. For every antiproton created, 40,000 other particles would be created. The time to cull the antiproton from the surrounding herd would be brief: within about 10^-7 s after it appears, an antiproton comes into contact with a proton and both particles are annihilated.

According to Chamberlain, again from his Nobel lecture, it was understood from the start that at least two independent quantities would have to be measured for the same particle to identify it as an antiproton. After considering several possibilities, it was decided that they should be momentum and velocity.

Measuring momentum

To measure momentum, the research team used a system of magnetic quadrupole lenses, which was suggested to them by Oreste Piccioni, an expert on quadrupole magnets and beam extraction, who was then at Brookhaven National Laboratory. The idea was to set up the system so that only particles of a certain momentum interval could pass through. As the Bevatron’s proton beam struck a target in the form of a copper block, fragments of nuclear collisions would emerge in all directions. While most of these fragments were lost, some would pass through the system. For specifically defined values of momentum, the negative particles among the captured fragments would be deflected by the magnetic lenses into and through collimator apertures.

To measure velocity, which was used to separate antiprotons from negative pions, the researchers deployed a combination of scintillation counters and a pair of Cherenkov detectors. The scintillation counters were used to time the flight of particles between two sheets of scintillator, 12 m apart. Under the specific momentum defined by Segrè, Chamberlain and their collaborators, relativistic pions traversed this distance 11 ns faster than the 51 ns it took for the more ponderous antiprotons. Signals from the two scintillators were set up to coincide only if they came from an antiproton. However, because it is possible for two pions to have exactly the right spacing to imitate the signal from an antiproton, the researchers also used the Cherenkov detectors.

One Cherenkov detector was somewhat conventional in that it used a liquid fluorocarbon medium. It was dubbed the “guard counter” because it could measure the velocity of particles moving faster than an antiproton. The second detector, which was designed by Chamberlain and Wiegand, used a quartz medium, and only particles moving at the speed predicted for antiprotons set it off.

In conjunction with the momentum and velocity experiments, Berkeley physicist Gerson Goldhaber and Edoardo Amaldi from Rome led a related experiment using photographic-emulsion stacks. If a suspect particle was truly an antiproton, the Berkeley researchers expected to see the signature star image of an annihilation event. Here the antiproton and a proton or neutron from an ordinary nucleus, presumably that of a silver or bromine atom in the photographic emulsion, would die simultaneously.

Success!

The antiproton experiments of Segrè and Chamberlain and their collaborators began in the first week of August, 1955. Their first run on the Bevatron lasted five consecutive days. Lofgren and his collaborators ran their experiments for the following two weeks. The Segrè and Chamberlain group returned on 29 August and ran experiments until the Bevatron broke down on 5 September. On 21 September, a week after operating crews had revived the Bevatron, Lofgren’s group was to begin a four-day run, but instead it ceded its time to Segrè and Chamberlain. That day, the future Nobel laureates and their team found their first evidence of the antiproton based on momentum and velocity. Subsequent analysis of the emulsion-stack images revealed the signature annihilation star that confirmed the discovery. In all, Segrè, Chamberlain and their group counted a total of 60 antiprotons produced during a run that lasted approximately 7 h.

The public announcement of the antiproton’s discovery received a mixed response. The New York Times enthusiastically proclaimed “New Atom Particle Found; Termed a Negative Proton”, while the particle’s hometown newspaper, the Berkeley Gazette, sombrely announced “Grim new find at UC”. The Berkeley reporter had been told that should an antiproton come in contact with a person, that person would blow up. Today, 50 years on, antiprotons have become a staple of high-energy physics experiments, with trillions being produced at CERN and Fermilab, and no known human fatalities.

The post Fifty years of antiprotons appeared first on CERN Courier.

Antiprotons at work

The AD began operating in 2000 and its experiments have reported spectacular production rates of antihydrogen atoms as well as topical observations of antiprotonic helium atoms. Though limited to low-energy antiproton research with only a pulsed extracted beam, the AD is regarded as the successor of LEAR after it closed down at the end of 1996, together with the Antiproton Accumulator (AA) and Antiproton Collector (AC). The AC machine was modified to become the AD – a decelerator to slow down the antiproton beam from a momentum of 3.57 GeV/c to 100 MeV/c. During deceleration, the beam undergoes stochastic and electron cooling. The extracted beam intensity is about 3 × 10⁷ antiprotons in a pulse of 90 ns, repeated every 86 s.

The AD delivers antiprotons only at the lowest energy that was available at LEAR, i.e. 5 MeV. For one experiment – ASACUSA – the antiproton beam is further slowed down to about 60 keV with a radio-frequency quadrupole decelerator (RFQD). A possible additional decelerator ring, ELENA, to serve all experiments, would have a cooled beam and would bring a major improvement if installed.

Plans are being realized for a new antiproton facility at GSI, where antiprotons with energy high enough for physics with strange and, especially, charm mesons will be available, in addition to very-low-energy antiprotons. An accelerator complex for research with both ion and antiproton beams is planned. This would provide an outstanding new experimental facility for studying matter at the level of atoms, protons and neutrons and their sub-nuclear constituents: quarks and gluons.

Research on fundamental symmetries was a very important part of the scientific programme at LEAP-05. Over the past 50 years, experimental tests have made physicists discard certain assumptions about symmetry: first, that physics is invariant under parity (P); and second, that it is invariant under the charge-parity (CP) transformation. Direct CP violation has been established in the decays of K-mesons and, recently, B-mesons. On the other hand, CP plus time-reversal (T) invariance, CPT, is believed to hold – and is partly experimentally verified – to a high degree of accuracy.

The symmetries under T, CP and CPT transformations are connected, and the CPT theorem demands that for each particle or element the equivalent antiparticle has the same mass, lifetime, spin and isospin – but an opposite value for all of the additive quantum numbers. The proof (or disproof) of the validity of this basic symmetry may be the key to such fundamental aspects as the universe’s matter-antimatter asymmetry. Physics is still in a phase where it is important to accumulate highly precise experimental data from different leptonic and/or hadronic systems. In this respect, the role of matter-antimatter asymmetry – especially baryonic proton-antiproton physics – is significant.

Probing how antiprotons interact with matter at very low energies is still a topical field for precise studies of the electromagnetic and strong forces and their interplay. High-precision spectroscopy of meta-stable antiprotonic atoms has produced very interesting and unique results. With the accuracy achieved in investigations of antiprotonic helium atoms, the CPT theorem can be tested to a level comparable to the existing bounds from other systems.

An alternative approach is the production and comparison of hydrogen and antihydrogen. A reasonable requirement for a new and unique CPT test of this kind is that it is eventually more stringent than existing tests with leptons and baryons. To make the required high-precision spectroscopic measurements, the hydrogen and antihydrogen atoms have to be at such low temperatures that laser cooling of trapped atoms, which is possible owing to the development of a continuous Lyman-alpha laser, appears to be necessary. Once all the basic technical requirements to produce antihydrogen atoms have been explored and optimized, tests of the gravitational force on antimatter will also be possible, free from the problems associated with charged particles.

When describing the nucleon-antinucleon (N-Nbar) interaction, it is implicitly assumed that the six-quark N-Nbar system can be regarded as a product of quark-antiquark nucleon wave functions with a complex potential that is dominated by the distance between the nucleons. Such a potential predicts a spectrum of many states, if the annihilation part is ignored. There is a rich dynamics of resonances or bound states around thresholds, where the annihilation effects are less dominant, since the phase space for the decay into meson resonances is more restricted. However, the transition from an N-Nbar system to a multiquark state, where quarks and antiquarks interact directly by gluon exchange, must be fully understood before invoking exotic mechanisms based on details of the interaction. New dedicated experiments could determine the energy and the quantum numbers of an N-Nbar system, clarifying the long-range interaction.

Probing the strong interaction

Antiproton beams are an excellent tool for addressing the regime of strong coupling. In antiproton-proton annihilations, particles with gluonic degrees of freedom as well as particle-antiparticle pairs are copiously produced, allowing spectroscopic studies with unprecedented statistics and precision. Phenomena arise that represent open problems in quantum chromodynamics (QCD) as they have their origin in the specific properties of the strong interaction and represent a major intellectual challenge. Quarks are confined within hadrons; the hadron mass does not balance with the summed mass of the quarks contained; and the characteristic self-interaction among gluons should allow for the existence of glue-balls and hybrids, consisting mainly of gluons and/or glue plus a quark-antiquark pair, respectively.

The charmonium system has turned out to be a powerful tool for understanding the strong interaction. The spectroscopy of the charm-anticharm system helped in tuning potential models of mesons in which the gluon condensate is determined. The gluon condensate is closely related to the charmonium masses since it is the gluon and quark-antiquark condensates that represent the energy density of the QCD vacuum. The QCD spectrum is much richer than in the simple quark model, as the gluons, which mediate the strong force between quarks, can also act as the principle components of entirely new types of hadronic matter: glueballs and hybrids.

The additional degrees of freedom carried by gluons allow glueballs and hybrids to have exotic quantum numbers that are forbidden for normal mesons and other fermion-antifermion systems. Such exotic systems can be identified by observing an overpopulation in the experimental meson spectrum, and by comparing their properties with predictions from models for lattice QCD considerations. Antiproton annihilation experiments have produced very promising results for gluonic hadrons.
A special session spiritedly discussed applications of antimatter, radiation and particle detection, covering well established medical treatments, diagnostic routines, plans for future developments and using nuclear physics to locate land-mines to reduce injuries, especially to children.

One highlight of the conference was a public presentation in the overcrowded Wolfgang Paul Lecture Hall at the University of Bonn, where more than 600 people listened to presentations on modern, high-quality physics and its excitements. At least some listeners were disappointed when these lectures stopped after four hours! The entire LEAP-05 was a brilliant preview of the physics to come from using antiprotons as a special and very effective tool.

• The conference was sponsored by Forschungszentrum Jülich; Deutsche Forschungsgemeinschaft; HiEnergy Technologies, Inc; Deutsche Telekom Stiftung; iseg Spezialelektronik GmbH; Bicron; W-IE-NER, Plein & Baus GmbH; and Pfeiffer Vacuum.

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The workshop was motivated by the recent progress in manipulating large numbers of ultra-slow antiprotons that has been made by the antihydrogen and antiprotonic-helium collaborations working at CERN’s Antiproton Decelerator (AD). The latest of these developments was in summer 2004. That was when the Monoenergetic Ultra-Slow Antiproton Source for High-Precision Investigations (MUSASHI) group of the ASACUSA collaboration first slowed the 5.3 MeV pulsed AD beam in a radio-frequency quadrupole decelerator (RFQD) to some tens of kilo-electron-volts, then confined and cooled more than 1 million antiprotons in a large multi-ring Penning trap. The trapping efficiency of about 4% is approximately 100 times higher than any previously achieved. The group also succeeded in extracting antiprotons from the trap as an ultra-slow DC beam of 10-500 eV. The fact that this unique beam can, in principle, be transported for some distance without serious loss makes beam sharing for a variety of experiments a real possibility.

Although the workshop was announced only two months beforehand, it attracted some 70 participants from all the related fields, and covered subjects ranging from fundamental questions about charge-parity-time-reversal (CPT) symmetry and gravitation, to the structure of exotic nuclei, atomic collisions and atomic physics. This report relates just a few of these topics; a full account will soon be published in the Proceedings series of the American Institute of Physics.

The early days of antiproton research were reviewed by John Eades of the University of Tokyo. Eades turned back the pages of scientific history in a talk entitled “The Antiproton and How It Was Discovered”, quoting the thoughts and opinions of some of the main participants, made both at the time and in retrospect. He underlined the initial doubts and inconsistencies that surrounded Paul Dirac’s relativistic-wave equation of 1930, and its final triumph as the positron, antiproton and other antiparticles were discovered.

Klaus Jungmann of the Kernfysisch Versneller Instituut (KVI), Groningen, gave a comprehensive overview of the current status of low-energy antiprotons and other exotic particles, and the experimental opportunities they offer as windows on fundamental forces and symmetries in nature. On the theoretical side, Ralf Lehnert of Vanderbilt University pointed to the large gap that will remain in our understanding of nature at the smallest scales until a consistent quantum theory is developed that underlies both the Standard Model and general relativity. He discussed the so-called Standard Model Extension (SME) as a theoretical framework that may bridge this gap, and which incorporates all Lorentz- and CPT-violating corrections compatible with key principles of physics . The SME predicts diurnal variations in spectroscopic measurements of matter and antimatter atoms, and could therefore be a guiding principle in designing future antihydrogen experiments.

Antihydrogen atoms and antihydrogen ions

The past three years have seen important progress by both the ATHENA and the Antihydrogen Trap (ATRAP) collaborations in synthesizing and experimenting with antihydrogen atoms at the AD. Some of the main results concern the accumulation of large numbers of positrons and antiprotons in “nested” Penning traps of various geometrical designs, leading to the observation of high formation rates for antihydrogen atoms. An unexpected consequence is that these antihydrogen atoms seem to be created before their constituent antiprotons have been fully cooled, with the result that they are themselves too hot to be easily stored and manipulated with existing techniques. Moreover, they are primarily formed in highly excited Rydberg states, while it is the ground and first-excited states that are of most interest for testing CPT invariance.

These obstacles to preparing usable antihydrogen atoms for physics experiments demand new ideas in trap design, going beyond the configuration of the nested electrostatic potential well used so far. Thus, Jeff Hangst of Aarhus described the present status of the high-field-gradient magnetic multipole trap proposed by the newly formed Antihydrogen Laser Physics Apparatus (ALPHA) collaboration; and Dieter Grzonka of Jülich reported on tests made on long-term electron storage in the ATRAP collaboration’s quadrupole magnet, which has a more moderate field gradient.

The storage of neutral atoms of antihydrogen requires the presence of magnetic field gradients to drive the so-called low-field-seeking atomic-spin states towards field minima, and will be essential to carry out high-precision antihydrogen spectroscopy. Since it appears that the atoms are produced in highly excited Rydberg states, they must be stored for long enough to allow them to relax to the ground state. Discussions at the workshop centred on various multipole and quadrupole trap designs that are likely to be useful in preparing such ground-state antihydrogen atoms.

Further new designs involve the so-called “cusp trap”, consisting of a potential well formed by two oppositely directed Helmholtz-coil fields, and a high-Q RF trap resonating at two frequencies, which can store positively and negatively charged particles simultaneously.

Ryugo Hayano of the University of Tokyo summarized both the present status of precision spectroscopy of antiprotonic helium and the development of the two-frequency RF trap for antihydrogen synthesis. In the latter, positrons and antiprotons may recombine within a volume of around 1 mm³, and thus form a source for an antihydrogen atomic beam. Sextupole magnets installed in such a beam could select and analyse specific antihydrogen spin alignments to measure the hyperfine structure of the antihydrogen ground state, much as was done with ordinary hydrogen atoms several decades ago.

Because of their larger mass, muons probe CPT-violation effects at a distance 200 time closer to the antiproton nucleus than positrons and electrons do.

Akihiro Mohri of RIKEN, Japan, showed that stable long-term storage of an electron plasma has been achieved at finite temperature in a cusp trap and that this can also trap synthesized antihydrogen atoms in low-field-seeking states. When the temperature of antihydrogen atoms and the magnetic field of the cusp trap are properly set, antihydrogen atoms in the ground state are selectively guided and focused along the magnetic axis, enabling an intensity-enhanced spin-polarized antihydrogen beam to be prepared.

A new path towards gravitational experiments with antihydrogen was proposed by Patrice Perez of CEA/Saclay, who discussed synthesis of antihydrogen ions (Hbar⁺). These could be formed via two-step reactions (pbar →Hbar →Hbar ⁺) when a 13 keV antiproton beam passes through a dense cloud of positronium atoms. The resulting Hbar⁺ ions would then be trapped, sympathetically cooled with laser-cooled alkali-earth ions, and finally ionized to the neutral state by a laser-detachment process to create the ultra-cold Hbar atoms necessary for detecting the extremely weak gravitational interaction.

Kanetada Nagamine of KEK proposed studying muonic antihydrogen (μ⁺pbar), the antimatter equivalent of muonic hydrogen (μ^–p), as an alternative to antihydrogen. The advantage of comparisons between μ^–p and μ⁺pbar is that because of their larger mass, muons probe CPT-violation effects at a distance 200 times closer to the antiproton nucleus than positrons and electrons do.

Further studies

Collision dynamics with antiprotons is also a potentially important subject, in which the antiproton behaves like a heavy electron. Although the Coulomb force is understood, its collision dynamics are not well known when more than three particles are involved. A familiar, puzzling example is the double ionization of helium by fast antiprotons, the cross-section for which is about twice as large as that for protons having the same velocity. Almost 20 years have passed since this observation, but it is not yet fully understood theoretically. This contrasts with the case of bound systems such as antiprotonic helium (pbarHe⁺⁺), where the observed transition levels have been theoretically accounted for at the level of one part per billion.

Joachim Ullrich of the Max Planck Institute, Heidelberg, discussed the importance of studying collision dynamics with antiproton energies in the range of 100 keV for which the time required to traverse atoms or molecules is of the order of 100 attoseconds (as). Since this is comparable to the orbital period of outer-shell electrons in atoms or molecules, crucial information on collision dynamics involving electron-electron correlation can be extracted.

Antiprotonic atoms have long been used to probe neutron density distributions in stable nuclei through studies of antiprotonic X-ray spectra, radiochemistry of the residual nuclei, and the charged pions emitted when the antiprotons annihilate. An antiproton captured in an electronic orbit de-excites to successively lower atomic levels until its overlap with the nucleus becomes appreciable. At this point annihilation takes place with a proton or a neutron near the “surface” of the nucleus (atomic number A), the actual charge state being identifiable from the charge balance of emerging pions; a nucleus of atomic number A-1 results.

Michiharu Wada of RIKEN proposed extending the pion-detection method by storing antiprotons and unstable nuclei in a nested trap. The charge-balance method can be applied to various nuclei including those for which the A-1 nuclei have no bound states. Slawomir Wycech of the Soltan Institute, Warsaw, emphasized that all these measurements test neutron density distributions in different regions of nuclei and yield complementary information on the rms and higher moments of density profiles as low as 0.001 of the central neutron density.

Looking to future antiproton facilities Paul Kienle of the Technischen Universität München discussed the possibility of an antiproton-ion collider at GSI’s Facility for Antiproton and Ion Research (FAIR), with energies of 30 MeV and 740 AMeV for protons and ions respectively. Cross-sections for antiproton absorption on protons and neutrons would be measured by detecting residual nuclei with A-1, using Schottky and recoil detectors respectively. This would permit rms radii for protons and neutrons to be determined separately in stable and short-lived nuclei by means of antiproton absorption at medium energies. A general discussion around the subject of ultra-slow antiproton physics ended this extremely fruitful workshop.

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Atoms, including exotic ones like these, can be efficiently synthesized only at chemical-energy scales (a few electron-volts or lower). This is many orders of magnitude below the energy scales needed for the production of antiprotons using an accelerator (a few giga-electron-volts). The AD reduces this gap by decelerating the 3 GeV antiprotons generated when the proton beam from the Proton Synchrotron hits an iridium target down to an energy of 5.3 MeV. This is still too high, however, for electromagnetic traps that can only capture antiprotons at the 10 keV range. Until now, thin “degrader” foils were used to slow antiprotons further, but the efficiency of such a system is low as many antiprotons stop and annihilate within the foils. Indeed, out of the 3 x 10⁷ antiprotons ejected every 2 min in a 90 ns pulse (or “shot”) of the AD, only about 25,000 were retained.

Now a team from the Atomic Spectroscopy And Collisions Using Slow Antiprotons (ASACUSA) experiment and CERN have replaced these foils by a radio-frequency quadrupole decelerator (RFQD). This 4 m-long device can decelerate antiprotons to 10-120 keV. In the tests, the antiprotons passed from the RFQD into a standard multi-ring trap (MRT), as was the case with the earlier work with degrader foils. The trap is filled with an electron gas that helps to cool the antiprotons through thermal exchange, as the electron gas dissipates energy through the emission of synchrotron radiation.

During the tests, around 1.2 x 10⁶ antiprotons per AD shot were stored in the MRT for 10 min or more. This is 50 times higher than the previous best values obtained with degrader foils and corresponds to an antiproton trapping efficiency of about 4%.

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The 2 t detector was carried by a 1,000,000 m³ balloon from Williams Field near the US McMurdo Station. It flew to altitudes of 37-39 km for a period of 8 days and 17 hours. Flight operations were carried out by the National Scientific Balloon Facility (NSBF) as part of the United States Antarctic Program, supported by NASA and by the National Science Foundation (NSF).

With this new detector the BESS group is continuing the systematic study of low-energy antiprotons in cosmic radiation. These rare particles are a unique probe for understanding elementary particle phenomena in the early universe.

Most cosmic-ray antiprotons are produced in collisions of primary cosmic-ray nuclei with the interstellar gas. However, if an excess of low-energy antiprotons beyond those expected from standard processes is observed, measurements from BESS may provide evidence for the primary origin of some cosmic-ray antiprotons through processes such as the evaporation of primordial black holes or the decay of possible forms of dark matter.

BESS has detected more than 2000 low-energy antiprotons in eight flights from northern Canada over the past 11 years. Most of the antiprotons measured by BESS are clearly secondary products of primary cosmic rays. However, the data obtained during the last solar minimum in the sunspot cycle (which occurred in 1996) suggest a spectrum flatter than expected in the low-energy region, and hence the exciting possibility of novel origins for cosmic antiprotons.

BESS also searches for antihelium in the cosmic radiation, the detection of which would have profound significance for both cosmology and particle physics. Unlike antiprotons, antihelium has a vanishingly small probability of creation by cosmic rays. Furthermore, our current understanding is that the universe is baryon-asymmetric, with an overwhelming dominance of matter over antimatter, and that antimatter stars or galaxies do not exist. The discovery of a single antihelium event would change this view.

The analysis of the BESS data has found no evidence for antihelium while recording more than 7 million helium nuclei, establishing the most stringent upper limit to the existence of antihelium and supporting baryon asymmetry.

In 2001 the BESS group started a project to improve the statistics and to lower the energy threshold of the detector. They developed a new instrument with a much thinner superconducting solenoid magnet and detector system and without an outer pressure vessel. The cryogen lifetime of the new magnet has also been greatly improved, and at polar latitudes a solar-power system increases flight times by more than an order of magnitude compared with typical one-day flights in Canada.

During the 2004 BESS-Polar flight, the data from some 900 million cosmic rays, totalling about 2 Tb, was recorded on an array of on-board hard disks. Following the flight around Antarctica, BESS-Polar descended by parachute to a landing site on the Ross Ice Shelf approximately 900 km from its launch point. A recovery crew was flown to the area, and in a series of flights from the remote Siple Dome Camp to the landing location the data disks and the remainder of the instrument and payload were recovered successfully.

• BESS is a collaboration between KEK, the NASA Goddard Space Flight Center, the University of Tokyo, Kobe University, the Institute of Space and Astronautical Science of JAXA, and the University of Maryland.

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One goal of future studies with antihydrogen will be to compare its spectroscopy with hydrogen’s. This will require the antihydrogen atoms to be trapped long enough for precise measurements to be made, which in turn will need very low antihydrogen temperatures, well below 0.5 K. The ATRAP collaboration at the AD has been experimenting with a new way of producing antihydrogen that might result in suitably low temperatures.

Until now, antihydrogen production has been achieved by bringing cooled antiprotons and positrons together in a nested Penning trap structure. The new method consists of exciting caesium atoms from an oven with two lasers, and then introducing the caesium into a positron trap. Excited positronium, a bound state of an electron and a positron, is then formed when a positron collides with a caesium atom and captures an electron. These positronium atoms carry virtually all the 10 meV or so binding energy of the caesium atoms. Finally, a fraction of the excited positronium atoms collide with trapped antiprotons to produce excited antihydrogen atoms with a probability that is expected to be much higher than for ground-state positronium.

The velocity distribution of the resulting excited antihydrogen is expected to be the same as that of the trapped antiprotons from which the antihydrogen forms, which can be made arbitrarily low in principle. Verifying this by directly measuring the antihydrogen velocity has not yet been possible, but if the low antihydrogen energy is confirmed, and if the highly excited states can be de-excited, this technique could become the method of choice for producing cold antihydrogen for precise spectroscopic analysis.

Further reading

C H Storry et al. 2004 Phys. Rev. Lett. 93 263401.

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The ATRAP experiment at CERN has made the first measurement of the velocity of slow antihydrogen atoms. This is an important step towards the goal of producing antihydrogen atoms cold enough – that is, slow enough – for precision spectroscopy.

Both the ATRAP and ATHENA experiments at CERN’s Antiproton Decelerator announced the production of large numbers of “cold” antihydrogen atoms in 2002. While these atoms were certainly much colder than those first observed at CERN in 1995, their actual energy was not known. Now the ATRAP collaboration has demonstrated a technique for determining the velocity of those antihydrogen atoms that pass through an oscillating electric field without ionizing.

In ATRAP, antihydrogen atoms form in a nested Penning trap and then move through an electric-field region prior to detection; only those atoms not ionized in the field are detected. The measurement of the atoms’ velocity depends on observing how the number of atoms detected varies with the oscillation frequency of a time varying field superimposed on a static field. The slowest atoms will be ionized and never reach the detector, while faster atoms may pass through unaffected depending on the phase of the field they encounter; as the frequency of the oscillating field is increased, fewer atoms will move fast enough to remain unionized. The team found that the most weakly bound atoms to make it through to detection have an energy of 200 meV. This corresponds to a velocity about 20 times higher than the average thermal velocity at a temperature of 4.2 K (Gabrielse et al. 2004). The speed of more tightly bound states, which could have lower velocities, could be measured by the same method with a higher static field, but this would require more time.

Further reading

G Gabrielse et al. 2004 Phys. Rev. Lett. 93 073401.

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For some years now, the Japanese-European ASACUSA collaboration at CERN has been tightening the limit on the antiproton charge (Q) and mass (M) relative to the values for the proton. Any difference, however small, would indicate that the CPT-theorem, which under certain axiomatic conditions guarantees identical properties and behaviour for matter and antimatter, is in some way deficient. Such an eventuality would be of earth-shattering importance for our understanding of the physical world.

The latest result from ASACUSA is that any proton-antiproton charge or mass difference must be smaller than one part in 10⁸ (Hori et al. 2003). As in the past, this limit was obtained by combining the ratio of the proton and antiproton Q/M values with their Q²M values. The former was obtained from previous measurements by the Harvard group of the proton and antiproton cyclotron frequencies in a Penning trap (Gabrielse et al. 1999), and the latter from the frequencies of laser-stimulated transitions in antiprotonic helium – that is, atomic helium in which an antiproton replaces one of the two orbiting electrons.

The improved precision was made possible through the use of a radiofrequency quadrupole decelerator to reduce the kinetic energy of antiprotons from the Antiproton Decelerator (AD) from 5.3 MeV to 65 keV. The lower energy ensured a much smaller variation in the position at which the antiprotons came to rest in low pressure (1 mb), low temperature (10 K) helium gas. This in turn allowed an adequate number of antiprotonic helium atoms to be formed in a volume small enough to be irradiated by the laser beams. Much of the art of high-precision experimentation lies in accounting for minute systematic errors, and in such a low-density environment systematic shifts in the resonant laser frequencies associated with collisions between “antiprotonic” and “ordinary” helium atoms could be better estimated and corrected for.

Further experiments completed in 2003 are about to bring another exciting new prospect into sight. These experiments showed that it is possible to create antiprotonic helium ions – with a single antiproton rather than a single electron orbiting the helium nucleus – in a state suitable for the kind of laser spectroscopy described above. The important thing here is that this pbarHe⁺⁺ ion is a two-body system, while the neutral atom pbarHe⁺⁺e^– comprises three bodies. The properties of two-body systems are in principle exactly calculable mathematically, while those of three-body systems can only be solved approximately using extremely complex calculations with powerful computers. The results of these calculations are consequently subject to their own errors, which beyond a certain level of precision can exceed those of the experimentally determined resonant laser frequencies. This sets a practical limit to the precision that is available with the neutral antiprotonic helium atom – which may indeed be reached after ASACUSA returns to the fray in 2004 with a new, higher precision laser system.

By performing experiments on the two-body ion instead of the neutral atom, this calculational roadblock can be circumvented. The experiments will be difficult, not least because the frequencies involved lie in the vacuum ultraviolet spectral region. What makes the game worth playing is that the pbarHe⁺⁺ ion is the nearest thing to the standard Bohr atom that is used to introduce undergraduate students to the concepts of atomic physics that physicists have ever had at their disposal. In many respects it is even more like hydrogen than the hydrogen atom itself. This is because the antiproton is non-relativistic and its de Broglie wavelength is some 40 times smaller than the Bohr radius, so that the semiclassical approximation, which is rather poor for normal, ground-state hydrogen atoms, turns out to be excellent for antiprotonic helium ions. All this means that the spin-independent contributions to the energy levels can literally be calculated on the back of an envelope to a few parts in 10⁹. For comparison, in the 2p-state of atomic hydrogen, relativistic corrections appear at the 10^-5 level and quantum electrodynamic corrections at 10^-6.

At the same time, even experiments with the “conventional” neutral antiprotonic atom may lead ASACUSA into a fascinating new regime in 2004. If we assume CPT-invariance, we can relate the mass of the antiproton to the electron mass instead of the proton mass. If ASACUSA is able to achieve a precision some 40 times better than the current 10 ppb, then the antiproton will become an even better known fundamental particle than the proton itself. This seemingly paradoxical situation comes about because no “protonic antihelium” counterpart to the antiprotonic helium atom is available with which the corresponding proton experiments could be made.

In the proton case, the limit on the charge neutrality of bulk matter has to be combined with the ratio of proton and electron cyclotron frequencies, ω_P and ω_e measured in the same Penning trap. While the charge neutrality limit is phenomenally precise (parts in 10²⁰), small corrections have to be applied to the measured ratio ω_P/ω_e because the two frequencies differ in magnitude by the large factor of the proton/electron mass ratio. One important consequence of this is that relativistic corrections, negligible for the proton, must be applied to the electron value. Taking this into account severely limits the precision obtained for the proton mass to about 0.5 ppb.

At this point we are confronted by some rather deep questions concerning the meaning of experimental results. In measuring any given quantity, we are really making a comparison with some arbitrarily chosen prototype object. The significance of that measurement, however, depends on the question we are trying to get nature to answer. It should be no surprise that when asking questions about fundamental particles, neither a block of platinum-iridium in Paris (the standard kg) nor a current-carrying wire loop (defining the MeV/c²) is a particularly useful prototype object. Of much greater interest are the values of particle properties with respect to those of other particles, these being the prototypes chosen, in a sense, by nature herself. Thus, choosing the proton as the prototype for the antiproton is clearly meaningful when asking questions about CPT invariance. Choosing the basic leptonic constituent of stable matter, the electron, as a prototype for the mass of the proton – the basic hadronic constituent – evidently has some fundamental significance in the larger picture of particle physics. Plausible though this choice may be, no theoretical basis yet exists within the Standard Model to predict what the hadron/lepton mass ratio should be.

If ASACUSA achieves the expected new precision for this ratio with the antiproton, it will then still be necessary to wait until some theoretical prediction arrives to put the result in the wider general context. We might also suggest that experimentalists find some way of doing a better job on the proton mass!

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Given the apparent absence of antimatter at the cosmic scale, it might seem strange that a recent paper from the ASACUSA collaboration on quantum tunneling effects in collisions between antiprotonic helium atoms and H₂ and D₂ molecules may be relevant to astrophysics. This is because antiprotonic helium consists of a one-electron “cloud” surrounding a composite, singly charged “nucleus” made up of an alpha particle (two positive charges) and an antiproton (one negative charge), so that it looks rather like a hydrogen atom.

No data exist on the reactions between H and D atoms and their molecules H₂ and D₂ at the low temperatures, around 30 K, that are characteristic of cold interstellar clouds and cold pre-stellar cores. These are exactly the astrophysical environments where more complex molecules may eventually be formed. For example, in certain regions the abundances of molecules such as H₂O, H₂S, CH₃OH and C₂H₅OH are so enhanced that surface ice chemistry must be occurring, while the reactants remain in close proximity to one another for 10⁵ years or more!

However, such reactions may not take place at all at these temperatures without tunnelling effects, so anything that provides a greater understanding of quantum tunnelling at low temperatures is of importance in answering the outstanding questions about ice chemistry. This is where the data from the ASACUSA experiment, reported in the paper “Quantum tunnelling effects revealed in collisions of antiprotonic helium with hydrogenic molecules at low temperatures” (Juhasz et al. 2003), may play an important role. The ASACUSA results provide a promising benchmark for theoretical models of such collisions, which could be generalized to more complex systems and may lead to a better understanding of astrophysical ice chemistry.

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LEAP’03, the latest in the series of biennial low-energy antiproton-physics conferences, could not fail to be topical this year. Running from 3-7 March, it began with the latest news on the production of antihydrogen atoms at energies low enough to permit them to be studied by laser spectroscopy, and ended with progress reports on two new antiproton facilities (support for one of which had been announced by the German government just days before).

This somewhat unusual time of the year for LEAP – previous conferences in the series have always taken place in the Autumn of even years – is attributable to the frenzied activity at CERN’s Antiproton Decelerator (AD), which now keeps many likely experimental participants busy between May and October. This year the meeting moved to Yokahama, where its packed programme more than compensated for the uncharacteristically wet and windy March weather. Some 60 talks reflected the recent surge of activity in what has become an exceedingly dynamic field.

An accelerator-produced antiproton normally spends only a brief instant in the world of matter. However, in this short time it can play many parts, from probing fundamental symmetry principles, to the study of atomic collisions, atomic bound states and nuclear physics. The initial session on discrete symmetries and antihydrogen was devoted to the current status of CPT invariance, the testing of which is perhaps the most powerful driving force behind current experimental activity in this field. Many particle physicists accept CPT invariance almost as an article of faith. They forget that, like Euclidean geometry, it is indeed a theorem, based on cherished but not entirely indispensable axioms such as Lorentz invariance (LIV), a feature the gravitational field only possesses locally. In his theoretical review, Nick Mavromatos of King’s College, London, concentrated on the fact that gravitation has shown itself to be particularly resistant to quantization under the terms of the CPT theorem. Pointing out that it is not difficult to construct models containing parameters that violate LIV and other CPT axioms, he neatly connected ultra-high, Planck-mass scale energies with ultra-low ones, by suggesting that experiments on neutral mesons, slow neutrons and in particular antihydrogen atoms, can place bounds on these parameters. In a backward look at data from the KTeV, NA48 and CPLEAR collaborations, Yoshiro Takeuchi of Nihon University, Tokyo, analysed these in terms of CP- and T-violation parameters and limits on CPT violation, concluding that relative to the level of K⁰-K^0bar mixing, CPT violation is currently constrained in the meson sector to a few parts in 10⁵ at best.

A starring role

The spotlight then turned on the experimentalists and the antiproton’s most recent starring role in the synthesis of large numbers of antihydrogen atoms. Nathaniel Bowden and Joseph Tan of Harvard brought participants up to date on the latest developments in antihydrogen synthesis from the ATRAP experiment. They reported on ATRAP’s use of positrons in a nested Penning trap to cool antiprotons to the cryogenic energies necessary for the recombination reaction between the different particles to take place, and on the field ionization method used for detection. The latter makes it possible to observe antihydrogen atoms under background-free conditions and to measure, for the first time, the distribution of principal quantum numbers for the synthesized atomic states. Makoto Fujiwara of Tokyo and Germano Bonomi of CERN described the production, detection and temperature dependence of antihydrogen atoms in the ATHENA experiment, which uses a similar Penning trap but with a distinctive open and modular design. This allows, among other things, a buffer gas to be introduced on the positron side, in which continuously introduced positrons from ²²Na dissipate enough energy to prevent them re-emerging from the trap. Differential pumping then maintains a good-enough vacuum to ensure the survival of antiprotons for many hours on their side of the trap. ATHENA identifies antihydrogen events without ambiguity by detecting the simultaneous annihilation of their component positrons and antiprotons. New techniques for probing the positron plasma that rule out alternative but unlikely interpretations of the data have recently been introduced.

Window on the world

Antiproton beams have long provided a window on the shadowy world of glueballs, hybrids and quarkonia. In his review of this rich source of information on hadron physics, Ted Barnes from Oak Ridge looked both backward to LEAR and forward to future antiproton machines. Surviving glueball candidates from the era of LEAR, which ended in 1996, include the f₀(1500) and, with less confidence, the f₀(1710), while exotics include the π₁(1400) and π₁(1600). The advent of new antiproton sources at GSI and the Japan Proton Accelerator Research Complex (J-PARC; previously the Japan Hadron facility) now promises to open this window once again. Several more specific talks reviewed topics such as charmonium states from proton-antiproton annihilations in the Fermilab experiment E835, and the future Proton Antiproton Detector Array (PANDA) at GSI.

Low-energy antiproton beams can readily be stopped in matter targets. Before fully coming to rest, the antiprotons eject electrons from nearby target atoms and remain bound in their place. Once installed in this antiprotonic atom, they undergo complex cascades through electromagnetism-dominated states before coming within the range of strong interactions. However, it is only in antiprotonic helium that this cascade is known to last more than a few picoseconds. In this case, the microsecond-scale annihilation lifetimes of some atomic states fortuitously makes them accessible to laser spectroscopy, thus ensuring that antiprotonic helium can, in some respects, rival antihydrogen as a benchmark for studies of CPT invariance.

Attention on the second day, therefore, turned once again to the AD, with a session on the experimental and theoretical studies of antiprotonic helium. Masaki Hori of CERN reported on the limit on the antiproton charge and mass that can now be deduced from measurements of transition frequencies in the antiprotonic atom to a few parts in 10⁷. The new limit results in part from the recent addition of a decelerating radio-frequency quadrupole (RFQD) to the ASACUSA beamline. This reduces the beam’s kinetic energy from the MeV to the keV scale, and so allows the antiprotons to be stopped in very-low-density helium, with concomitantly smaller systematic corrections to the measured frequencies. Further impetus, expected from two-photon laser techniques and more advanced laser systems, may soon improve the precision of the frequency measurement to several parts in 10⁹, and so also permit spectroscopy of the two-body antiprotonic helium ion (pbar He⁺⁺). Jun Sakaguchi of Tokyo described the ASACUSA microwave-spectroscopy experiment on antiprotonic helium, which has allowed the antiproton orbital magnetic moment to be determined from the hyperfine splitting of atomic levels to a few parts in 10⁵. The QED calculations that make all the above interpretations possible, were described by Vladimir Korobov of JINR Dubna, and further talks dealt with the physical chemistry of the antiprotonic helium atom.

Antihydrogen experiments dominated again in the following session on the future programme for the AD. Cody Storry of Harvard introduced a novel antihydrogen production mechanism that is being considered by ATRAP. A beam of caesium atoms previously excited into Rydberg states by a laser beam passes through the positron cloud confined in a Penning trap, where they produce positronium atoms that are also in Rydberg states. These have a much higher recombination cross-section with trapped antiprotons than is the case for ground-state positronium. The next major goal for both ATHENA and ATRAP is to begin laser spectroscopy of cold antihydrogen atoms, and several different schemes, including laser-stimulated recombination and ionization, are being investigated. From ASACUSA, Yasunori Yamazaki of RIKEN and Tokyo presented the idea that positrons and antiprotons may be confined in the same region of a trap incorporating a magnetic field “cusp”, while Eberhard Widmann, also from Tokyo, showed that a precision measurement of the ground-state hyperfine splitting in antihydrogen must now be seriously considered to fall within the AD’s “line of sight”. Throughout the history of modern physics, experiments with atomic beams have proved extremely fruitful in studying the hydrogen atom with high precision, and these latter two topics opened up the idea that the same can be true for antihydrogen.

The research arena then moved from earthbound laboratories to the atmosphere and space, where the search for cosmic antimatter has been under way for many years. Catherine Leluc of Geneva reviewed the status of the ALPHA Magnetic Spectrometer (AMS02). This is due for launch to the International Space Station in October 2005, but a pilot version (AMS01) has already been flown on the shuttle mission STS-91 in 1998. The AMS02 detector now incorporates improved acceptance and redundancy into its search for antimatter and dark matter in cosmic rays. The third-generation high-altitude balloon experiment BESS-Polar was discussed by Mitsuaki Nozaki of Kobe. This will be used to study low-energy cosmic-ray antiprotons in detail in a superconducting magnetic spectrometer, and is expected to have a 10-20 day flight through the top of the polar atmosphere in 2004. Finally, Piero Spillantini of Firenze described PAMELA, a successor to several balloon-borne experiments, which will be launched later this year into quasi-polar orbit on the Russian Resurs-DK1 satellite from the Baikonur Cosmodrome.

Before they can be captured into atomic states, antiprotons produced at accelerators must lose some nine orders of magnitude of kinetic energy, and as is the case for other particles, their interaction with matter over this range is of crucial importance. The fact that antiproton projectiles are both heavy and negatively charged has far-reaching consequences for their behaviour when passing through matter. Capture/loss processes and the excitation of target electrons drastically modify the Bethe-Bloch formula at the velocity scale of the electron orbitals in the target material. The RFQD installed in ASACUSA’s beamline has shed new light on this experimentally dark area, and Ulrik Uggerhoj from Aarhus was able to report on the latest results on stopping-power measurements made with antiproton beams from 1 to 100 keV in C, Al, Ni, Au and LiF foils. The results of theoretical approaches to the understanding of collisions of antiprotons with hydrogen and helium atoms, ions and molecules, and to the explanation of ionization phenomena in the low-velocity domain, were presented by John Reading of Texas A&M and several other speakers.

The final curtain

The life of an antiproton ends when it comes within range of the strong interaction, either after an atomic cascade or (occasionally) by a direct in-flight hit on the nucleus. It is then that it plays its final research role, as a nuclear probe. The AD as presently constituted is not easily adaptable for such studies, but with GSI and J-PARC now on the horizon, Josef Pochodzalla of Mainz was able to look forward to using antiprotons from these machines for the large-scale production of single and double Λ-hypernuclei. The weak decays and gamma-ray spectra of these hypernuclei can elucidate hyperon-hyperon and hyperon-nucleon interactions, measure fundamental properties of the hyperons themselves, and produce genuine hypernuclear states with symmetry properties unavailable to ordinary nuclei. These antiprotons that have suffered atomic capture can eventually de-excite to atomic ground states that “graze” the nucleus, so their annihilation constitutes an effective probe of the nuclear surface. This aspect occasioned both backward and forward glances in reports on new analyses of data from the PS209 experiment at LEAR and the possibility of similar studies at ASACUSA.

The final day of the LEAP’03 conference was appropriately devoted to the current and future antiproton facilities. The morning session opened with a review by Tommy Eriksson from CERN of the present status of and future prospects for the AD machine, where the name of the game is “ever lower energies”. The AD is now operating close to its design specifications, with pulses containing 10⁷ antiprotons being reliably delivered at an energy of 5.3 MeV every 100 seconds. Research at even lower (keV) beam energies has now been strongly boosted by the RFQD, which has permitted several million antiprotons to be captured in the Tokyo Penning trap and cooled to cryogenic energies. Naofumi Kuroda of Tokyo discussed their extraction in the form of a beam of antiprotons with kinetic energies on the eV scale. A new feature of the AD programme, which was described by Carl Maggiore of Pbar Medical, is an investigation with a 300 MeV/c (25 MeV) beam of a possible therapeutic role for antiprotons. This beam, astronomically high in energy for most other physicists, will soon be used to investigate the relative biological effect of antiprotons on biological cell samples.

In the final session on the topic of future antiproton facilities, Walter Henning, GSI’s director, described the laboratory’s new project and its potential for antiproton physics. The large-scale expansion of the GSI-Darmstadt laboratory, the funding of which has only very recently been agreed by the German federal government, will be carried out in two stages, with a 25% external contribution. One of its key elements will be the provision of antiproton beams below 15 GeV. Shoji Nagamiya, director of the J-PARC project, outlined progress on this new Japanese accelerator complex, centred on a 50 GeV proton synchrotron at Tokai, 150 km north-east of Tokyo. Planning has been under way since 2001 and is now rapidly gathering momentum. With financing amounting to some ¥134 billion (€980 million), Phase 1 is expected to produce its first beams in 2008. An opening ceremony was held in October 2002, and 30 letters of intent had been received by the end of December 2002, one-third each from Japan, Europe and North America. Both GSI and J-PARC now actively encourage the voice of antiproton users to be heard in the planning of their experimental programmes.

The best yet

The smooth organization of LEAP’03 largely resulted from the efforts of Eberhard Widmann and Ryugo Hayano of Tokyo University, with financial assistance being provided by RIKEN, KEK and the Antimatter Science Project at the University of Tokyo. Viewing the busy and spectacular Yokohama bay through the Sangyo Boeki Centre’s windows during the coffee breaks, the 100 participants could all agree that the form and content of the conference programme was the best yet.

The post The antiproton: a subatomic actor with many roles appeared first on CERN Courier.

Recent months have seen the much-awaited synthesis of cold antihydrogen atoms by two groups working at CERN’s Antiproton Decelerator (AD; Amoretti et al.; Gabrielse et al.). The aim of these collaborations is to compare spectral features of hydrogen and antihydrogen as a test of the CPT invariance principle, which states that under certain realistic assumptions about the quantum fields that represent them, matter and antimatter will always behave in the same way. However, it is not just in antihydrogen atoms that CPT symmetry can be tested, as the ASACUSA collaboration is demonstrating.

If CPT violation occurs anywhere in nature, it must be very small, and experimental searches for it have usually been done with kaon beams. These beams are coherent superpositions of particle and antiparticle waves, and since slightly different masses imply slightly different de Broglie wavelengths, a limit of a few parts in 10¹⁹ can be placed on any kaon particle and antiparticle mass difference by a detailed study of the interference effects observed in them. However, kaons are mesons, containing both a matter and an antimatter quark, and CPT violation might not show up in conjugate pairs of this kind. Protons (p) and antiprotons (p-) are made only of quarks and antiquarks respectively; hydrogen (H) and antihydrogen (H-) atoms are made only of quarks and leptons, and of antiquarks and antileptons. In such systems, CPT violation at some small but crucially important level can certainly not be excluded with equal rigour.

Although we have no quantum interferometer for the CPT conjugate H-H- pair, we do have powerful laser beams, which we can use to probe its members with extremely high precision. Since no other assumption than CPT invariance need be made in interpreting what happens when one of them is removed from a spectrometer and replaced by the other, the H-H- pair is in many ways the ideal CPT test-bench. However, it is very difficult to produce antihydrogen atoms moving so slowly that they do not drift out of a laser beam before it can stimulate one of their spectroscopic transitions. Solutions to this problem are now evidently in sight, but many difficulties remain before the extreme sensitivity afforded by laser techniques (and indeed necessary for meaningful tests of CPT invariance) can be reached.

ASACUSA’s alternative approach involves the much easier task of replacing an electron in an ordinary atom by an antiproton and measuring the spectroscopic frequencies of the resulting “antiprotonic atom”. However, we do not have the CPT conjugate “protonic antiatom” with which to compare it, and must calculate its transition frequencies from quantum electrodynamics, assuming the known proton values for the antiproton (and also that the calculations were done properly). In this way, the ASACUSA collaboration has determined the relative charge and mass of the proton-antiproton pair to six parts in 10⁸ (Hori et al.) by laser-stimulating optical-frequency transitions in antiprotonic helium (figure 1) – the only variety of antiprotonic atom known to live long enough to permit such quantum gymnastics.

How might we use this atom to investigate the antiproton’s magnetic properties? Unknown large-scale fields are sometimes measured by determining the energy required to turn over a magnetic dipole of known strength placed in them. At the atomic scale, this is the basis of classic experiments on magnetic effects like the ground-state hyperfine splitting in hydrogen. Likewise, by measuring the energy of the photon needed to flip the known magnetic dipole of the electron in the unknown magnetic field of the antiproton, we can measure the latter particle’s own dipole field.

ASACUSA has now carried out such an electron spin-flip experiment (Widmann et al.), in which two laser beams and a microwave beam were tuned to resonate with the antiprotonic helium atom (see figure 1 for an explanation of this “triple resonance” experiment). Microwave resonance peaks occurred at 12.89596 and 12.92467 GHz, corresponding to electron spin-flips in states of the atom with antiproton spin “up” and “down”. These values are consistent with calculated values assuming the proton’s orbital magnetic dipole moment for the antiproton, and limit any difference between them to less than six parts in 10⁵. The measured values also depend on the antiproton’s spin magnetic moment, but the corresponding limit for this (1.6%) is not yet as good as the value (0.3%) deduced from the fine structure of X-ray spectra in heavy antiprotonic atoms. A precision measurement of this latter quantity will require major improvements in the laser system. Therefore what will probably come next from ASACUSA are even tighter limits on the orbital moment, charge and mass.

The present result has an unusual feature. According to the equation for m_p (see figure 1), what is being measured is mainly the ratio g¹p- / g¹p of the factors defining the orbital current magnetism relationship for the members of the CPT conjugate pair. However, we have no atoms with orbiting protons in our matter world, and g¹p has always implicitly been taken by definition to be equal to 1. Thus while CPT invariance is respected within the six parts in 10⁵ limit given above, we do not know, in the empirical sense, that either g-factor really has the value unity.

The post ASACUSA measures microwave transition in antiprotonic helium appeared first on CERN Courier.

The ATRAP experiment at CERN’s Antiproton Decelerator has detected and measured large numbers of cold antihydrogen atoms. Relying on ionization of the cold antiatoms when they pass through a strong electric field gradient, the ATRAP measurement provides the first glimpse inside an antiatom, and the first information about the physics of antihydrogen.

ATRAP’s technique relies on trapping positrons between two bunches of antiprotons in a nested trap structure. The positrons are used to cool the antiprotons, and when they both reach a similar temperature, some combine to form antihydrogen atoms (a positron orbiting an antiproton nucleus). Being electrically neutral, these antiatoms drift out of the trap. Those that move along the axis of the apparatus soon find themselves traversing a strong electric field that strips off the positrons, thereby allowing the negatively charged antiprotons to be trapped and counted. “This measurement is completely background-free,” explains ATRAP spokesperson Jerry Gabrielse of Harvard University, “since the only way that a signal is detected is if antiprotons escape the nested trap in the form of neutral antihydrogen atoms.”

The ATRAP team has measured the field needed to ionize the antihydrogen atoms. The result shows that the antiatoms are formed in highly excited states (between n = 43 and n = 55). This is being interpreted as pointing to a three-body recombination scheme where a third body carries away the energy and momentum liberated by the antiatom’s formation. The ATRAP method has allowed the first measurement of the physics of antihydrogen, and is a step towards the precision measurements that will allow matter-antimatter comparisons to be made. The ultimate goal is to trap antihydrogen atoms and study their spectra with the same precision as for plain hydrogen (a few parts in 10¹⁴ for an analysis of the transition from the n = 2 to the n = 1 state).

The news comes shortly after another CERN experiment, ATHENA, announced its observation of cold antihydrogen. Using a completely different detection technique to ATHENA, and providing the first glimpse into the internal structure of antihydrogen, ATRAP has shown that CERN researchers are well on the way to understanding the first entry in the periodic table of the anti-elements. ATHENA and ATRAP use similar techniques for trapping the ingredients of antihydrogen, developed over many years by Gabrielse’s team. The fact that they use different detection methods reinforces the result, and is a good omen for future studies of antihydrogen at CERN.

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Physicists working at CERN’s Antiproton Decelerator (AD) have announced the first controlled production of large numbers of antihydrogen atoms at low energies. This is an important step on the way to testing the fundamental symmetry CPT through comparison of hydrogen with antihydrogen.

The hydrogen atom is the most completely understood atomic system, with its first excited state being pinned down to just 1.8 parts in 10¹⁴. Antihydrogen, on the other hand, is almost completely unknown. A comparison of the two systems would give a very precise test of CPT symmetry, which is assumed to be conserved in the Standard Model of particle physics. CPT is the combination of charge conjugation, parity and time reversal. The violation of the CP combination is well established in kaon and B-meson decays, but so far, no experiments have shown evidence that CPT is not conserved in nature.

This latest development at CERN follows the production of small numbers of fast-moving antihydrogen atoms at CERN and Fermilab in the US in the mid-1990s. It is the result of several years of development work into the antiparticle trapping and mixing systems needed to produce slow (cold) antihydrogen atoms that can themselves be trapped for further study. ATHENA, one of two CERN experiments that plan to study antihydrogen, has been the first to produce cold antihydrogen atoms.

Says CERN director-general, Luciano Maiani: “The controlled production of antihydrogen observed in ATHENA is a great technological and scientific event. Even more so because ATHENA has produced antihydrogen in unexpectedly abundant quantities.” Giving due credit to the ATRAP experiment (which also aims to study antihydrogen), he went on to say: “I’d like to recognize the contribution of ATRAP, which has pioneered the technology of trapping cold antiprotons and positrons, an essential step towards the present discovery.” Last year the ATRAP experiment was the first to use cold positrons to cool antiprotons.

The ATHENA collaboration of 39 scientists from nine institutions worldwide has built on these techniques with the addition of a high-yield positron accumulator and powerful particle detector. The abundant numbers of positrons from the accumulator, coupled with good granularity and background rejection from the detector, allowed the collaboration to see its first clear signals for antihydrogen in August – appropriately, the 100th anniversary of the birth of theorist Paul Dirac, who predicted the existence of antimatter in the late 1920s.

The ATHENA collaboration estimates that some 50 000 antihydrogen atoms were created in its apparatus before announcing their result. Antiprotons decelerated by the AD to a leisurely pace – by CERN’s standards – of a tenth of the speed of light were first trapped in an electromagnetic cage. From each AD pulse of 2 ¥ 1o⁷ antiprotons, some 10,000 were caught. The next stage was to mix them with about 75 million cold positrons collected from the decay of a radioactive isotope and caught within a second trap. Finally, the trap doors were opened, allowing the antiprotons and positrons to mix in a third trap. It is here that cold antihydrogen atoms formed.

ATHENA observes antihydrogen atoms when they annihilate with the walls of the mixing trap. Two photons from the positron annihilation are localized in space and time with charged particles coming from the antiproton annihilation. The next steps are to trap antihydrogen atoms and add a laser spectroscopy system. This will allow the CPT studies to begin.

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More information is available at http://www.arxiv.org/abs/hep-ex/0208028.

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A new laboratory inaugurated in May at the University of Sussex, UK, aims to shed light on matter-antimatter asymmetry by measuring the electric dipole moment of the neutron. The new Centre for the Measurement of Particle Electric Dipole Moments has been created thanks to a £1.7 million (€2.6 million) award from the UK’s Joint Infrastructure Fund.

The post New dipole moment centre inaugurated appeared first on CERN Courier.

Former CERN Courier editor Gordon Fraser has added brand-new material for the paperback edition of his fast-paced account of the story of antimatter.

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When the space shuttle Endeavour blasted off from Cape Kennedy on 5 December 2001, its main mission was to switch crew and transport supplies to the International Space Station, but it was also the focus of a range of scientific studies. As part of the AMS development programme, the Endeavour carried as a “hitch-hiker” a Prototype Synchrotron Radiation Detector (PSRD). On the shuttle’s return to Earth on 17 December, the PSRD was shipped back to ETH Zurich. The data-storage disks were then read out and analysis of the results collected during the 110 h deployment window began.

The product of a major international collaboration masterminded by the 1976 Nobel prize winner Samuel C C Ting, many functions familiar from collider experiments will be incorporated into the AMS detector. It is the first major particle-physics detector to be sent into space; the prototype AMS-01 first flew on a shuttle in June 1998. Findings from last year’s flight will have a vital bearing on the final configuration of the full detector, which is being prepared for a mission aboard the International Space Station. The 3 tonne prototype developed in 1998 was impressive enough, but the final configuration will weigh approximately 6 tonnes.

A fundamental goal of the AMS experiment is to look for antiparticles in the primary cosmic radiation of outer space. Other objectives include searching for otherwise invisible “dark matter” and carefully analysing details of the cosmic-ray spectrum. The detector will therefore be equipped with a powerful superconducting magnet and sophisticated tracking capability.

When high-energy charged particles are bent by a magnetic field, they emit a “screech” of electromagnetic synchrotron radiation. The characteristic wavelength of this radiation can be used to identify the charged particles.

The Synchrotron Radiation Detector (SRD) will consist of an array of yttrium aluminium perovskite (YAP) crystals. Not available for 1998’s prototype mission, the new SRD technology is seen as an integral part of the final AMS configuration, mounted as the outermost layer of the complete detector. The energy- resolving power of the array depends on its size, but it is hoped that ultimately the SRD will be able to detect multi-TeV (10¹² eV) electrons and positrons.

As well as monitoring the constant flux of cosmic rays in deep space, the AMS detector will be sensitive to special cosmic events such as the gamma-ray bursts now known to occur almost daily. These cataclysmic events are the largest explosions in the universe other than the Big Bang itself, but their origins are still a mystery. The extreme energies released in the bursts could provide new insights into the creation of matter and open up novel physics possibilities.

An imaginative theory proposed by John Ellis, Dimitri Nanopoulos and colleagues is the possibility that at this energy, the velocity of light could show a dispersion owing to quantum gravity effects. One objective of the SRD programme is to make careful measurements of the velocity of light under these conditions.

The post Helping the AMS experiment to detect antiparticles in space appeared first on CERN Courier.

Knowing the exact charges and masses of the protons, electrons and neutrons that constitute matter is evidently of fundamental importance for the entire edifice of particle physics. Furthermore, once we know these, we know, according to CPT-symmetry, the values for the corresponding components of the antiworld. Given the fact that we already know the particle values very precisely, should we even bother making the measurements for antiparticles?

Such an omission could hardly be more risky. The CPT theorem – which says that all observed phenomena will remain unchanged if we replace particles by antiparticles, invert their motions, and reflect everything in a mirror – is based on falsifiable assumptions. In a universe as old and as big as the one we see, there is time and space enough for even minute deviations from perfect CPT symmetry to become observable, perhaps even to predominate.

Warning messages

Nature even seems to be sending us warning messages in this respect: having provided a full kit of parts for making a large scale universe that is matter-antimatter symmetric, the one that it in fact assembles from these parts is completely asymmetric. Of course we have explanations for this imbalance, but they are not beyond question. Even worse, the CPT theorem itself rests, as T D Lee notes, “on a foundation which has to be unsound, at least at the Planck length (10^-33 cm – the measurement precision when quantum fluctuations begin to have gravitational implications), and maybe at a much larger distance” (T D Lee 1995 The Discovery of Nuclear Antimatter Italian Physical Society Conf. Proceedings vol. 53 eds L Maiani and R A Ricci). The symmetry between matter and antimatter, he concludes, “must rest on experimental evidence”.

How then can we measure the antiproton charge, Q, and mass, M, with very high precision? For the proton, as few particle physicists realize, the charge is measured by taking an acoustic cavity containing sulphur hexafluoride and trying to make it “sing” in tune with an oscillating electric field. The extent to which it does not can then be interpreted as a limit on the net charge of bulk matter: if the proton’s and the electron’s charges were not very close, sulphur hexafluoride would sing louder than it does.

In the case of electrons the charge, e, is no longer obtained, as one might think, from a Millikan-type oil drop experiment, but by combining measurements of the Josephson constant, e/h, and the fine structure constant, a, which appears as a scale factor for all energy levels in the hydrogen atom and is proportional to e²/h. That way, not only e but also h, the Planck constant, can be determined.

As antisulphur hexafluoride is not exactly common on Earth (and also for want of a suitable container for it), we must look elsewhere for a value of the antiproton’s charge. The electron e/h and e²//h experiments give us a clue as to how this can be done.

Some years ago the antiproton’s Q/M value was measured relative to that of the proton’s by the Harvard group at CERN’s LEAR low-energy antiproton ring (R A Ricci 1999 Phys. Rev. Lett. 82 3198) to the staggering precision of 9 parts in 10¹¹. This value was not deduced from measurements of the curvature of its trajectory in a magnetic field (such a measurement could never be made with a better error margin than a few parts per thousand) but by tickling it with microwaves to determine its cyclotron frequency, Q/M ¥ B, in the field B.

In physics we build on what we know, not on what we don’t know, so we are not allowed to assume that unknown CPT violations do not scale Q and M proportionately, leaving Q/M unchanged. The question of values for the charge and mass individually was therefore still open. An independent measure of some other combination of Q and M was needed, just as the fine structure constant gives a different combination of e and h above to the one given by the Josephson constant. What better than the Rydberg constant of the antiproton, which is proportional to Q²M?

For this we need an atom in which the antiproton orbits a nucleus, as the electron does in hydrogen. A good candidate (indeed the only one available at present) is the antiprotonic helium atom (an antiproton and an electron orbiting an alpha particle nucleus), easily created by stopping antiprotons from CERN’s antiproton decelerator in helium gas.

By probing this atom with laser beams, ASACUSA can measure a number of optical transition frequencies between pairs of antiproton orbital states with principal and angular momentum quantum numbers (n, L) differing by one (see “Thinking about antiprotonic helium” below). In sharp contrast with the case of electrons in ordinary helium and of hydrogen, these take values around 35-40 when the almost stationary antiproton is captured into an atomic orbit by a helium nucleus (see “Thinking about antiprotonic helium” below). Every such transition of the antiproton in this atom has the antiproton Rydberg constant as a common scale factor.

Now, of course, much of the art of high-precision experimentation lies in accounting for small systematic errors. Stopping the antiprotons in very cold (6 K) helium reduced those due to the Doppler effect severely. The major remaining systematic effect was the so-called density shift arising from the buffeting that the antiprotonic atom suffers from neighbouring ordinary helium atoms. On the theoretical side, the difficulty is that antiprotonic helium is a three-body system, not a two-body system like hydrogen, requiring sophisticated computer calculations.

Among the many transition frequencies measured, the experimenters therefore selected those with the most favourable experimental and theoretical conditions. Instead of trying to work out a value for the Rydberg constant, and combining it with the Q/M value, the equivalent procedure was adopted, asking the theorists to estimate how much the proton values for Q and M used in their calculations had to be changed to give the experimental frequencies, under the 9 parts in 10¹¹ constraint given by the Harvard Q/M ratio. This could be interpreted as a confidence limit on the charge and mass relative to those of the proton.

The result was that if there is any difference between the antiproton Q or M and the proton’s value, it is, with a 90% confidence level, less than 6 parts in 10⁸. This constraint is about 10 times as tight as that obtained by the same procedure at LEAR and a thousand times as tight as that obtained without using antiprotonic helium.

Can we stop here? No. As has been often pointed out, in science we can never verify concepts (like CPT symmetry) with absolute finality – there is always the possibility that still more precise measurements, or measurements of new quantities, will falsify them. This is why the ASACUSA group is now planning further improvements to its laser system that will push the limit to a few parts in one billion, and why it is now also measuring the magnetism of the antiproton to a few parts in 10⁵ or better by flipping the electron’s spin in its orbital magnetic field. The first results are being displayed prominently in the photograph by Hiroyuke Torii of Tokyo.

Thinking about antiprotonic helium

A useful starting point for thinking about antiprotonic helium is the semiclassical picture of the Bohr hydrogen atom usually presented in undergraduate textbooks. It has now been established that if the antiproton approaches an ordinary helium atom slowly enough, it readily replaces one of its electrons, entering an orbit with the same semiclassical radius (some 10⁵ nuclear radii – well beyond the range of annihilation – producing strong interactions) and therefore the same binding energy of about 39.5 eV.

As a first approximation we assume (as is also done in textbooks for ordinary helium) that it does not interact with the remaining electron, but that this nevertheless partially screens the nuclear charge to the value 1.7e instead of 2e. This approximation is adequate to reveal the general properties of the spectrum.

The total (kinetic plus potential) energy in such hydrogen-like atoms is quantized with energy levels E_n = -E_R/n², where E_R is the energy equivalent of the antiproton_helium Rydberg constant. Doing the calculations we then easily find that n is about 38 for E_n = 39.5 eV, and that the de Broglie wavelength of the antiproton is n times smaller than that of the electron (0.05 nm), justifying our semiclassical assumption. The electron itself is fully quantum mechanical as it was before the antiproton approached.

The figure “Energy levels” shows a schematic energy level diagram for the atom. Evidently the antiproton is in a very highly excited state. Hold the figure vertically in front of you and the n = 1, L = 0 ground state energy of about -60 keV will be found way off the page to the left and a few hundred metres underground!

Now one might have thought that if, during its nanosecond-long approach to the atom, the antiproton had been able to displace the first electron so easily, it would within a few more nanoseconds just as easily have ejected the second one and that the atom (right thumbnail sketch) would quickly become a positive ion (left thumbnail sketch).

The most important characteristic of this atom is that energy and angular momentum conservation prevents this from happening, at least immediately. Instead, the antiproton de-excites spontaneously (black arrows) through a chain of metastable states, emitting optical-frequency (2 eV or 600 nm) photons as it goes with microsecond-scale lifetimes.

Only when it arrives at one of the red states do the energy and angular momentum transfers become favourable for removing the second electron and so for changing the neutral atom into a positive ion. Once this happens the antiproton’s fate is sealed – such ions are very unstable in collisions with ordinary helium atoms, and these soon send the antiproton into the nucleus, where it annihilates.

ASACUSA’s trick is to stimulate transitions to green states with a tunable laser beam,  and to detect the resonance condition between the laser beam and the atom by the ensuing annihilation.

The post Weighing the antiproton appeared first on CERN Courier.

After coming into operation last year, CERN’s Antiproton Decelerator (AD) has got up to speed for physics this year. Changes to the AD since its debut mean that the three experiments – ASACUSA, ATHENA and ATRAP – now enjoy more intense antiproton beams.

The AD is a unique machine. Its job is to decelerate, not accelerate, antiparticle beams, and it has to handle energies that decrease by an unprecedented factor of 35 from the injection ceiling at 3.57 GeV to the ejection floor at 100 MeV.

In its first incarnation in 1987 as a collector of antiprotons, precooling the particles before they passed to the accumulator ring for CERN’s proton-antiproton collider, it was designed to operate at fixed energy, so this factor of 35 presented a big challenge.

The AD team’s design goal was to hang onto a quarter of the injected antiprotons through their vertiginous fall in energy, and to repeat the deceleration cycle once a minute. Recent AD improvements have put the team well on the way to reaching this target.

One important new feature is in the electron cooling system, adapted from that used for CERN’s LEAR low energy antiproton ring, which closed in 1996. Electron cooling gives the antiproton beam a final “cold shower” after the initial stochastic cooling, keeping the antiprotons tightly bunched at the lowest energies. Improvements have also been made to the radiofrequency deceleration system. This summer the AD succeeded in decelerating an injected antiproton beam without losing a single precious particle.

Meanwhile the three AD experiments are getting to the heart of the antimatter (see Weighing the antiproton).

The post Getting up to speed in the year AD 2 appeared first on CERN Courier.

In colliding beam physics, the luminosity (collision rate) of a machine determines the yields of interesting high -energy events. Future linear colliders could significantly benefit if the colliding beams were additionally focused to a smaller transverse size beyond that which is possible with conventional or superconducting magnetic beam transport elements.

The theory of a self-focusing plasma lens was first proposed in 1987 by Pisin Chen who also leads the current experiment at the Stanford Linear Accelerator Center (SLAC). If such lenses were, for example, located at the interaction point of a collider, they could focus both the electron and positron beams, thereby reducing beam spot size and increasing the luminosity, perhaps by one order of magnitude or more.

The Experiment E-150 Plasma Lens Collaboration was formed to investigate this process and study the feasibility of its application at proposed future linear colliders like the Next Linear Collider. The collaboration contains members from four laboratories and three universities (see the complete list at the end of this article).

Gas

The process started with a positron beam from the SLAC PEP-II positron source. This was sent through a damping ring and then accelerated to 28.5 GeV in the SLAC linac with a bunch intensity of 1-2 x 10¹⁰. The beam was delivered to the Final Focus Test Beam Facility (FFTB) at a rate of 1 or 10 Hz. At the focal point of the FFTB transport, a special plasma chamber contains a 3 mm diameter pulsed gas nozzle (see diagram) through which either hydrogen or nitrogen gas is “puffed” into the ultrahigh vacuum system at plenum gas pressures up to 75 atm with a discharge time of 800 ms. The gas is pumped off by a Roots-type pump. On either side of the central chamber are differential pumping sections semi-isolated from each other by thin titanium windows with small (2-5 mm diameter) apertures for the positron beams to pass through. These sections are evacuated by turbomolecular pumps and allow operation of the plasma lens with ultrahigh vacuum systems on either side.

The plasma lens was generated by ionizing the gas using a pulsed YAG laser operating at 10 Hz in the infrared region (wavelength 1064 nm) and delivering a pulse energy of 1.5 J. The relativistic positron beam exhibits effects from both its charge and its current. For a beam propagating in a vacuum, the Lorentz force induced by the collective electric and magnetic fields is nearly cancelled, which is why a high-energy beam can travel over kilometres without much increase in spread (emittance).

Plasma response

Upon entering an initially quiescent plasma, the plasma response to the intruding charge and current is such that the plasma electrons are attracted into the positron beam so as to neutralize the space charge of the beam and thereby cancel its radial electric field, which tends to self-repel the positrons. However, if the beam radius is much smaller than the natural plasma wavelength, the neutralization of the intruding beam current by the plasma return current is ineffective. This leaves the azimuthal magnetic field unbalanced, and this field focuses the beam.

The plasma lens concept also works for electron beams. In that case, the plasma electrons are expelled from the beam volume. The result is a near uniform focusing of the beam due to the less mobile ions – the beam is “pinched”.

In the FFTB experiment, typical plasma densities were of the order of 10¹⁸/cm³, corresponding to a plasma wavelength of about 30 mm. This is indeed much larger than the incoming beam radius, which is about 5 mm.

The focusing strength of such a lens is equivalent to that of focusing magnets like quadrupoles with gradients of the order of 10⁶ T/m. For comparison, a conventional small aperture (1 cm diameter) iron core quadrupole can maximally be excited to about 250 T/m.

The focusing effect of the plasma lens was measured using proven wire scanner technology developed for the SLC and FFTB. The wires are 4 mm and 7 mm carbon fibres. The scanner is located just downbeam of the plasma lens and is adjustable to allow mapping of the transverse beam size and pinpointing the longitudinal location of the beam waist.

Measuring the beam

The bremsstrahlung photons emitted by the positron scattering off the wires are detected in a Cherenkov cell type detector located 33 m downbeam of the lens. The variation in photon yield as the beam “dithers” across the wire provides a measure of the transverse beam profile from which the beam size can be determined.

A second, independent method to measure the strength of the plasma lens is a segmented synchrotron radiation monitor some 35 m downbeam of the lens. The harder the beam is focused, the higher the energy of the emitted synchrotron radiation. As the plasma focusing effect is transient, the monitoring of this synchrotron radiation provides an “on-line” measurement of the plasma focusing gradient. Such a plasma focusing induced synchrotron radiation signal was also observed for the first time. The energy was determined to be a few mega-electron-volts, which confirms the gradients derived from the plasma density.

Smaller beams

Figure 1 shows a typical set of scans for hydrogen and nitrogen (Z is in the direction of the momentum vector of the beam), illustrating the beam’s convergence toward a waist.

The plasma lens concept also works if there is no pre-ionization by the laser, a process called self or impact ionization. Here, the head of the positron bunch ionizes the gas and the remainder of the bunch is focused. The head of the bunch is not focused, so the efficiency in spot size reduction is lower than for laser pre-ionization. For the latter, the beam size was approximately halved for nitrogen. The reduction in the orthogonal dimension was comparable, so that the reduction in spot size was approximately a factor of four. The maximum possible spot size decrease is much higher, but in the SLAC experiment, the beam current had to be lowered and the incoming beam size enlarged so the fragile carbon fibres in the wire scanner would not melt in the very-high-energy density of the focused beam.

Earlier, the experimental setup was used to focus a 30 GeV electron beam using the self-ionization method. The collaboration is excited about potential future applications of the plasma lens concept and is presently repeating this electron focusing experiment using also laser pre-ionization.

The members of the collaboration, by institution, are: Fermilab (C Crawford and R Noble); KEK-Japan (K Nakajima); Lawrence Livermore (H Baldis and P Bolton); SLAC (P Chen, W Craddock, F-J Decker, C Field, R Iverson, F King, RKirby, J Ng, P Raimondi and D Walz); Hiroshima University, Japan (A Ogata), UCLA (D Cline, Y Fukui and V Kumar); University of Tennessee (A Weidemann).

The post Focusing an antimatter beam with matter appeared first on CERN Courier.

Now coming into action for physics is CERN’s new Antiproton Decelerator (AD), opening another chapter of CERN’s tradition of physics with antiprotons. With the AD, the focus switches from exploiting beams of antiprotons to capturing the precious nuclear antiparticles.

When CERN’s low-energy antiproton ring (LEAR) was closed in 1996 after more than 10 years of operation, it had supplied 1.3 x 10¹⁴ antiprotons – enough to supply about 10 000 particles to everyone on the planet, but representing a theoretical accumulation of only 0.2 ng of antimatter.

Although LEAR slowed down the particle beams supplied by CERN’s antiproton factory from 26 GeV by a factor of about 10 (itself no mean feat), its antiprotons were nevertheless still moving extremely fast. A particle with 100 MeV momentum corresponds to a temperature of billions of degrees.

Of all LEAR’s antiprotons, just a few were privileged to be selected and eventually cooled down to temperatures approaching absolute zero. The techniques learned in this work opened up substantial economies for antiparticles – probably one of the rarest, and therefore most expensive, commodities in the world.

Cooling antiprotons is a tricky business. They quickly annihilate with ordinary matter such as liquid helium, the conventional ultra-refrigeration medium. Instead, antiprotons have to be supercooled by a gas of electrons (negatively charged antiprotons can peacefully coexist with electrons).

In this way the TRAP Bonn-Harvard-Seoul collaboration was able to stack several thousand ultracold antiprotons at a time. Antiprotons cooled to such a low energy by the electrons were locked in a shallow trap using electric and magnetic fields to contain the valuable antiparticles. Meanwhile a large electromagnetic well was opened alongside to receive a fresh batch of antiprotons, which were then similarly cooled. The energies of the individual antiparticles were then just one ten-millionth of what they were in LEAR.

Interesting antiproton physics thus became feasible using a less ambitious antiproton source. This is the motivation behind CERN’s new AD, which supplies antiprotons to several experiments – ATRAP (son of TRAP at LEAR), ATHENA and ASACUSA.

One ultimate physics objective at LEAR was to isolate a lone antiproton and study it carefully. Gradually reducing the electromagnetic “depth” of its snare, the TRAP team spilled out excess antiparticles until just a single antiproton survived.

Like any other captive electrically charged particle, an antiproton orbits in a magnetic field – the principle of the cyclotron. Comparing the frequencies of this rotation for an antiproton and a proton gives a direct comparison of the masses of the particle and its antiparticle.

The TRAP team at LEAR was able to ascertain that the proton and antiproton masses are equal with increasing precision, eventually to just one part in 10 billion. Making a measurement to such astonishing accuracy is equivalent to fixing the position of an obect on the surface of the Earth to within a few millimetres.

This is by far (a factor of a million) the most incisive comparison yet of proton and antiproton properties. According to the fundamental theorems of physics, a particle and an antiparticle should be exactly equal and opposite so that their scalar quantities, like mass, are the same, but quantum numbers, like electron charge, should have opposite signs.

The major objective of ATRAP and ATHENA at the new AD is to synthesize and study antihydrogen – the simplest electrically neutral atoms of antimatter, each made up of a positron orbiting a lone antiproton.

Antihydrogen was first produced by experiment PS210 at LEAR in 1995. Synthesizing atomic antimatter was a major achievement, but no measurements were made – the antihydrogen was too hot and dissociated quickly into its component positrons and antiprotons.

Using electromagnetic traps, ATRAP and ATHENA aim to collect supercold antihydrogen that can be stored for further study. Comparing the properties of this antihydrogen with hydrogen under the same conditions will provide a much more stringent test of whether matter and antimatter behave in exactly the same way.

ASACUSA uses antiprotons for collision and annihilation studies, particularly to form exotic atoms, in which the negatively charged antiproton is captured in a target atom, replacing the electron of everyday atoms.

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by Gordon Fraser, Cambridge University Press, 0 521 65252 9, £16.95/$24.95.

The correct prediction of antimatter by Paul Dirac is arguably the most astonishing intellectual achievement of the 20th century. By insisting that quantum theory and special relativity must be consistent, he was able to deduce the generalization of the Schrödinger equation to the Dirac equation. By doing that he was able to give a proximate explanation for spin, and to predict a whole new set of particles, antimatter. That the human mind can discover a previously unknown part of the world is a great achievement. (I largely agree with Antonino Zichichi who argued for Dirac as the most important physicist of the 20th century in Physics World in March.) Gordon Fraser’s lively and interesting book provides a broad treatment of this story, and the history, science and implications of antimatter.

This is a very nice book, totally accessible to any curious reader, yet with occasional thought-provoking pieces even for experts. Fraser keeps a fast pace, explaining the science well but taking care not to dwell too long on any difficult aspect. In a few places I didn’t fully agree with his viewpoint or arguments. I will mention some of these as a service to possible readers, but they do not detract from the value of a successful book.

Publishers are notorious for writing anything they please on book jackets and in publicity. Fraser is not responsible for then remark on the jacket that the book is about how science fiction became fact, which is, of course, the opposite of what happened (the remark is taken from the title of chapter 1, but its meaning is different there), or the charming reference to “Hans van der Meer” in the publicity, mixing up Hans Dehmelt (whose work with traps is described in chapter 11) and Simon van der Meer (who figured out how to get antiprotons in sufficient quantities to make a collider.)

Chapter 1 describes the public excitement about the 1995 discovery of antiatoms, and then begins the history. My impression of one bit of the history differs a little here. Fraser says that at first Dirac thought that the antielectron was the proton. He may be correct, but I have heard over the years that people pushed rather hard on Dirac about where the predicted antielectron was – after all, predicting new particles was not normal then. Dirac defensively remarked that perhaps it was the proton, though he knew that that didn’t make sense.

The next chapter introduces the relevant symmetries, charge conjugation, parity and time reversal, and then provides a quick history from Galileo through Newton to Einstein. It includes the Thornhill portrait of Newton without a wig, which I have seen in the Master’s Lodge of Trinity College, Cambridge – Newton looks much more like a physicist there than in his usual wigged appearances. Here and later the book has a nice way of giving brief descriptions that capture the essence of people.

Chapter 3 is a history of the acceptance of atoms, and the discoveries of the electron, nucleus, proton and neutron. Next is a more thorough biographical treatment of Dirac, with some of the many anecdotes, followed by the development of quantum theory and the Dirac equation. Chapter 5 describes the positron discovery, including the opposition of R A Millikan. That opposition helped to make European physicists more aware that Carl Anderson’s CalTech data could be the antielectron than were the US physicists. There is also a (delightful for a theorist) quote from Rutherford of a sentiment that we still encounter: “It seems to be to a certain degree regrettable that we had a theory of the positive electron before the experiments…I would be more pleased if the theory had appeared after the establishment of the experimental facts.”

Fraser then presents a quick discussion of infinities, renormalization and Richard Feynman, and interesting speculations on Dirac and Feynman’s distinctive personalities and the strong influences of their fathers as they were growing up. The story moves to the development of accelerators and the discovery of the antiproton, and then to quarks. (A minor point: the wording of a sentence on p108 suggests that quarks have a known size, but in fact there is only an upper limit and quarks are expected to be far too small to measure their size directly.) Next comes further discussion of parity violation and then CP violation, leading up to Andrei Sakharov’s statement of the conditions required for an explanation of the mysterious baryon asymmetry of the universe.

Particle colliders, which of course, require expertise in handling antimatter, are brought in and some of their discoveries presented. The only typographic error I found was on p175, where the ratio of the top quark mass to the b-quark mass is about 35, not 300. Chapter 13 is basically on antimatter technology, including PET scans and more. Fraser gets somewhat sensational here, beginning the chapter with a survey of the Reagan era “Star Wars” antimissile programme, and then unfairly relating that to the US plans to build the Superconducting SuperCollider, even seeing a connection to antimatter propulsion proposals and personnel for Star Wars. He also laments the loss of the LEAR antiproton beam at CERN, and perhaps misses an opportunity to discuss the difficulties of doing all science projects in times of limited resources, and of deciding which ones to pursue.

Why the universe is matter and not antimatter is still a mystery. The explanation of the evidence in chapter 14 is very clear. However, there are more approaches that could eventually explain this mystery than the book suggests. The problem is that the calculations are very difficult and the underlying theory is not established. Perhaps most fundamentally, we do not yet know the origin and size of the CP-violating effects that are essential to explain the matter asymmetry. One piece of progress is that we do know now that the Standard Model cannot explain the matter asymmetry of the universe, so new physics must enter. It is likely that the phases that lead to the CP violation needed to generate the matter asymmetry arise when string theories are compactified to three space dimensions and when supersymmetry is broken, but these subjects are not yet well understood. If you think these approaches are somewhat far out, you’ll enjoy Fraser’s speculations on this issue even more.

The post Antimatter – the Ultimate Mirror appeared first on CERN Courier.

Antiproton-nucleus annihilation was measured at LEAR by the OBELIX experiment at very low antiproton momenta, down to 40 MeV/c. This momentum seems quite large with respect to the characteristic momentum in particle-antiparticle systems bound by electromagnetic attraction (Coulomb force). For proton-antiproton, this is of the order of 4 MeV/c. Nevertheless, this attraction appears to be important and can even affect the annihilation rate.

In fact, in this energy range, Bethe’s usual 1/v law is replaced by a 1/v² one, where v is the relative velocity of the interacting particles.

This 1/v² regime was predicted in 1948 by Wigner and is well known in atomic physics. In nuclear physics, in contrast, one usually encounters electromagnetic repulsion between protons, which gives rise to an exponential decrease of the reaction rate at low energies, a phenomenon that is particularly important in nuclear astrophysics.

The OBELIX experiment, for the first time, investigated with very high precision the behaviour of the reaction rate in a system with Coulomb attraction. In the figure, the measured antiproton-proton, antiproton-deuteron and antiproton-helium annihilation cross-sections are presented as a function of antiproton momentum.

These cross-sections are multiplied by the square of the relative velocity. For the proton-antiproton system, the situation is very clear: one can see that the product tends to a constant value with decreasing antiproton momentum. For a 1/v behaviour, this product should tend to zero. For the deuteron and helium cases, the analysis is more complicated.

This change of regime is instructive but not really unexpected. The most interesting observation comes from the comparison of the values of these three cross-sections. At high energies they are quite different – the antiproton nucleus annihilation cross-sections are several times that for antiproton-proton. Surprisingly, at low antiproton momentum, the antiproton-deuteron and antiproton-helium annihilation cross-sections drop to the proton antiproton level or even below it.

An accurate analysis of these annihilations shows that this is not a kinematic effect; it is a direct result of the dynamics of the antiproton-nucleus interaction.

This was confirmed independently by another LEAR experiment – PS207 – which measured, for the first time, the shift and the broadening of the antiproton-deuteron atomic ground state. This extremely difficult experiment showed that the width of this level, entirely determined by the annihilation process, is approximately the same for antiproton-proton and antiproton-deuteron atoms.

A geometrical picture of annihilation would suggest that the probability of this process should increase with the number of possible annihilating partners – the number of nucleons in nuclei. However, these experiments demonstrate clearly that this is not the case.

To understand the mystery, these experiments should be continued at lower energies and with heavier nuclei, not only to understand the dynamics of the annihilation process but also to measure the cross-sections.

This knowledge would be important, in particular for astrophysicists, who search for antimatter in the universe and need to know about the properties of low-energy matter-antimatter interaction. CERN’s antiproton decelerator (AD), currently starting operations, will be a powerful tool in obtaining this precious antimatter information.

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This winter The Delphic Oracle,by Geneva’s Miméscope company in collaboration with CERN, ran for an extended season in the pit that houses the Delphi experiment at CERN’s LEP electron-positron collider. Using a matter-antimatter collider as the scene, the play focused on Paul Dirac’s mathematical discovery of antimatter symmetry.

Writing the script was a challenge – presenting the ideas of antimatter as entertainment, not as a scientific seminar. Renilde Vanden Broeck of CERN’s press office, following a diploma course in Science Communication at the University of London, chose to present the idea behind and the build-up to The Delphic Oraclefor her course dissertation.

In the following extract, Renilde describes some obstacles encountered on the way to presenting antimatter on stage. Just two weeks before opening night, Anne Gaud McKee of the Miméscope company and I are walking to CERN’s reception area. We are both very excited about the forthcoming play’s freshly printed posters and leaflets. She is picking them up to have them distributed all over Geneva. I tell her about the first interviews she will have to do tonight and that the press is really picking up. She is very excited and suddenly exclaims: “You haven’t heard the last yet: we changed the whole script!” I think I am going to faint! “I can’t believe it – just two weeks before the first night!” I shout.

Anne explains they had a crisis a couple of days ago. They had mainly been working on four set pieces and hadn’t really practised the actual lines that Markus Schmid (who plays Dirac) has to say. “What was wrong with it?” I demand. Anne explains that Markus and the others found her script too difficult and dry. So much abstract thinking out loud. What they disliked most of all was that it had become too focused on the science and less on the show. The script wouldn’t work with the acrobatics and dance the audience was to see, and that would kill their imagination. These dream scenes are so poetic…and then to revert suddenly to those dry sterile lines. “It would annihilate the whole atmosphere!” Anne objects, and we laugh at the word “annihilate” – after so long, physics terminology is seeping into our everyday speech. Anne explains that Markus refused to say lines he couldn’t feel. “I instinctively sense that he was right, that there was something awfully wrong with my scenario,” she continues, “and then suddenly it hit me – after all our discussions they hadn’t understood a key item of the play, the famous Scientific Process! And there was so little time left!”

“It all started when we were rehearsing the cosmos scene,” Anne explains. “That scene is all about the infinitely big. Dirac goes to the cosmos to look for antimatter because that is the natural result of his prediction. As much antimatter as matter should have been created by the Big Bang. Thus there should have been antigalaxies, antistars, etc.”

Antimatter or no antimatter?

But Dirac comes back from his dream empty-handed with no antimatter. So the cast concluded that antimatter doesn’t exist. I told them that this is simply not true. Scientists don’t know this yet for sure and are still searching with sophisticated detectors. “If antimatter is not up there, that doesn’t necessarily mean that it is down here instead,” I insist. “Maybe there’s another reason why nature preferred matter to antimatter. Perhaps Dirac thinks that there is a slight, almost undetectable, difference between the two. Maybe if he could study antimatter closely he could find this asymmetry.”

Later, putting these ideas to the cast, Markus objects immediately. “We can’t tell all that!” he shouts, “They’ll be totally confused!” “We have to,” I insist, “because we can’t say that there is no antimatter in space – we don’t know that yet, so Dirac thinks that there could be another explanation.”

Cast members Claire de Buren and Yasmina Krim agree, but point out “But then the ‘particle collision scene’ has nothing to do with his initial hypothesis.” “Exactly,” I reply. “That scene is there because he questions his first theory and follows another line – abandoning the idea of antimatter in space to instead explain the dominance of matter over antimatter. He thinks, if only I could have a close look at antimatter colliding with matter…that’s where the particle collision dream scene comes in.

That is how science works! You follow one road and when you find that it leads nowhere, you go back to the crossroads and choose another route. That’s what scientific research is all about,” I explain, feeling that they were beginning to see how science really operates. Anne agrees. “It would be good if we could make people understand that science is not a smooth road to a fixed objective, but full of twists and turns, doubts and questions.”

The cast just hadn’t seen that science could be so vulnerable and fragile. It was such a relief that they finally understood. “Better late than never,” Anne laughs.

Diary

When the penny drops, they bombard me with questions. Now they understand why CERN has such big machines. I tell them about CERN’s new antiproton decelerator and its quest to look for any subtle differences between matter and antimatter. “We can never explain this in one hour! What are we going to do now?” says a horrified Anne, realizing she still didn’t have the right formula to communicate the difficult antimatter message. The next day she starts over, calling in Claire and saying: “Tell me as soon as you don’t understand anything.”

“It’s all so abstract,” Claire objects immediately. “You should tie the ideas down to everyday things – Dirac’s gestures, for instance. Integrate his thoughts into the normal things that people do.”

This leads Anne to hit on a new formula for the script. Suppose Dirac writes letters expressing his feelings? She remembers learning about one important event in Dirac’s life, when his research supervisor at Cambridge, R H Fowler, received the draft of a key paper from quantum mechanics pioneer Werner Heisenberg. Fowler passed the paper to Dirac, who later said this was what got him started in quantum mechanics. Suppose there had been a mistake or misunderstanding in Heisenberg’s paper which Dirac spotted? Pure fiction, but that was the hook for the final script.

So Anne begins to write for Dirac: “My dear and respected colleague and friend, this night I stayed up until four o’clock in the morning, and could it be because of the exhaustion, that I have finally managed to solve the equation that you sent me two weeks ago.”

The ficticious letters make the difficult Dirac come alive on stage. While he goes about his everyday life, his mind struggles with strange equations and is bewildered by their implications. Reluctant to go against the scientific tide, he says: “No physicist has ever seen a positive electron…I hope you will not take me for a madman.”

From such bold predictions came antimatter.

The Miméscope company

Cast: Claire de Buren,

Anne Gaud McKee, Yasmina Krim, Markus

Schmid Scenario: Anne Gaud McKee Choreography:

Markus Schmid Music: Christian Denisart

The post Raising the curtain on antimatter appeared first on CERN Courier.

A hundred metres underground in the pit that houses the Delphi experiment in the world’s largest electron positron collider, LEP at CERN – what better stage for a play about antimatter?

The Delphic Oracleby Geneva’s’Miméscope’ company, in collaboration with CERN, ran for an extended season this winter. Each audience had to be limited to 60 because of the logistics of the LEP pit, and it was carefully divided into smaller groups – electrons, positrons, up quarks – each with its own CERN guide to usher them down, around and back up again.

The Delphic Oracle’saction focused on Paul Dirac’s mathematical discovery of antimatter symmetry, with Markus Schmid playing the role of the scientist gripped by the intensity of original thought and the profound implications of a discovery that went on to change our view of the universe. Such intellectual acrobatics were underlined by Yasmina Krim’s graceful aerial ballet. The light show and backdrops illustrated matter and antimatter at work. At the end of the show, the audience was guided round the Delphi detector – a memorable experience, with even the access lift disguised as a time capsule.

Underground setting, lights and acrobatics apart, the strict scientific focus and the stark table-and-chair props  were redolent of Michael Frayn’s play Copenhagen about Niels Bohr and Werner Heisenberg, which all goes to show that physics can make good theatre.

In The Delphic Oracle, Heisenberg did not appear but was the addressee of the letters that Dirac laboriously compiled on stage. Unlike Copenhagen, where Frayn had scrupulously done his homework, physics purists might react to the liberal interpretation of Dirac’s work in The Delphic Oracleand its scientific message. Although Dirac saw the need for symmetry between positive and negative charges in his famous 1928 relativistic treatment of the electron, for several years the proton was identified as the corresponding positive charge. Dirac himself said so in a letter to Naturein October 1930.

Others (notably Oppenheimer and Weyl) began to worry about a particle as heavy as the proton partnering a light electron in a theory that was supposed to be absolutely symmetrical. In May 1931, Dirac grasped the bull by the horns and finally proposed what his equations had been saying all along: “We may call such a [positively charged] particle an anti-electron. We should not expect to find any of them in nature, on account of their rapid rate of recombination with electrons, but if they could be produced experimentally in high vacuum, they would be quite stable and amenable to observation.” These would have been fine words with LEP only a few metres away.

As an introduction to antimatter and as a spectacle, The Delphic Oraclewas memorable, displaying Dirac’s torment at having constructed a theory so perfect that its implications were unthinkable. Dirac suspected that antimatter had to exist, but it took him three tortured years to summon the courage to say so.

Like champagne, antimatter is always stimulating, however it is served.

The post <i>The Delphic Oracle</i>and antimatter appeared first on CERN Courier.

As CERN’s Antiproton Decelerator comes into operation a new era begins for high-precision studies of antiprotonic atoms and antihydrogen, as well as for the antiproton itself and the way in which it interacts with ordinary atoms.

The imminent arrival of these high-quality antiproton beams of extremely low energy (5 MeV) was heralded at the International Workshop on Atomic Collisions and Spectroscopy with Slow Antiprotons, held in Tsurumi on 19-21 July.

The workshop included a Zen Buddhism course (Zazenkai). Perhaps for the first time in a physics workshop, jet-lagged attendees were woken up at 3.30 a.m. for meditation practice in the temple.

Progress on the AD programme

In the more conventional sessions, 55 participants from 25 institutions in Europe, Japan, Russia and the US discussed theoretical, technical and experimental progress on the Antiproton Decelerator (AD) experimental programme. At the moment it follows two tracks. One is the ASACUSA collaboration’s programme of antiproton­atom collisions and laser/microwave spectroscopy of antiprotonic helium (in which one of the two normal orbital electrons is replaced by an antiproton). The other track is the synthesis and spectroscopic study of antihydrogen atoms by the ATHENA and ATRAP collaborations.

One motivating force behind ASACUSA is that, in much the same way as the hydrogen atom spectrum revealed the properties of the proton and electron over the course of the 20th century, high-precision laser and microwave measurements of atomic transitions of the antiproton in antiprotonic helium can reveal, with commensurate precision, the properties of the antiproton itself. Such measurements constitute valuable tests of validity of underlying physics symmetries. Antiprotonic helium was chosen instead of the simpler two-body protonium (antiprotonic hydrogen ­ a proton and an antiproton in orbit round each other) atom because it is stable against annihilation for some microseconds after formation, while protonium, under normal conditions, is not.

ASACUSA experiments that are already approved include a microwave triple resonance experiment on hyperfine splitting caused by the interaction of the electron spin and antiproton orbital moments, and a search for laser-induced transitions between, so-far unobserved, atomic levels.

In addition, a new high-resolution laser system is expected to reduce the measurement precision of all transition frequencies to less than one part per million ­ the level at which quantum electrodynamic effects appear. This has already been achieved in experiments that were studying another transition at CERN’s LEAR low-energy antiproton ring, which closed in 1996.

The status of ASACUSA on these fronts was reported by K Komaki and M Hori (Tokyo). The interpretation of spectral features of antiprotonic helium in terms of the properties of the atomic constituents requires energy-level calculations with precision similar to that of the experimental values, and this was discussed by Y Kino (Sendai), D Bakalov (Sofia), V I Korobov (Dubna) and G Korenman (Moscow).

Another goal of ASACUSA is to extend to lower energies the Aarhus and Tokyo LEAR experiments on the atomic interactions of antiprotons. This has sparked considerable theoretical interest. Contributions came from H Knudsen (Aarhus), P Krstic (Oak Ridge) and A Igarashi (Miyazaki) on ionization and energy loss for antiprotons interacting with matter. Such experiments require 10­100 keV rather than 5 MeV antiprotons. They will be produced by inserting a decelerating radiofrequency quadrupole (RFQ) in the AD beam.

This is under construction in CERN’s Proton Synchrotron division and will soon be tested in Aarhus. Antiprotons from the RFQ may be used directly or, for certain ASACUSA experiments, collected and cooled in a multiring harmonic trap (T Itchioka, Tokyo), where they will be reaccelerated to electron-volt or kilo-electron-volt energies.

Traps for charged (as well as neutral) particles feature prominently in the plans of the ATHENA and ATRAP collaborations. Here the main aim is to use the laser probes to compare identical atomic transition frequencies in hydrogen and antihydrogen.

Such experiments have an important advantage over those using antiprotonic helium. They are direct comparisons, in which symmetry-conjugate systems are compared without any need for theoretical input. On the other hand, the technical problems associated with producing antihydrogen are much more complex than is the case for antiprotonic helium, which is created in abundance whenever antiprotons are slowed to electron-volt energies in helium gas. In particular, the antihydrogen must be synthesized at micro-electron-volt rather than electron-volt energies.

For this, AD antiprotons and positrons from radioactive sources must first be collected as plasmas in suitable containers and cooled to liquid helium temperatures. M Holzscheiter (Los Alamos) reported on the progress of the ATHENA experiment, which aims to synthesize antihydrogen atoms at sub-Kelvin temperatures. K Fine (CERN) and H Totsuji (Okayama) discussed the behaviour of these plasmas in Penning traps (with hyperboloidal electrodes) and in Penning-Malmberg traps (cylindrical ones), both of which are adequate as plasma bottles for these processes.

Future proposals

As befits the promise offered by the birth of a new machine, many ideas that go beyond the current AD programme were presented in Tsurumi. They include the possibility that atomic protonium, so far ignored because of its short lifetime, may live long enough under near-vacuum conditions to be the subject of ASACUSA-type experiments (R S Hayano, Tokyo). Another possibility is the existence of metastable antiprotonic lithium (K Ohtsuki, Chofu-shi) and of antiprotons in solution in liquid helium (T Azuma, Tsukuba). H Schmidt­Böcking (Frankfurt) presented a proposal for a table-sized antiproton storage ring.

The Tsurumi workshop was organized by Yasunori Yamazaki of the Tokyo University Komaba campus and RIKEN, and it was sponsored by the Antimatter Science Project of the University of Tokyo, the Danish Natural Science Research Council’s Centre for CERN-related Atomic and Nuclear Physics and the Japanese RIKEN (Rikagaku Kenkyuujo) Institute.

In his concluding remarks, Mitio Inokuti (Argonne) commented on the confidence and excitement with which this worldwide physics community awaits AD beams. Tantalizing hints of this physics were revealed during the era of LEAR, of which the AD is now a worthy successor.

The post Zen and the art of low-energy antiproton experiments appeared first on CERN Courier.

CERN is best known for pushing the high-energy frontier of physics, but, with its new Antiproton Decelerator, the low-energy frontier is about to resume its place at the heart of the laboratory’s experimental programme.

The Antiproton Decelerator (AD) is scheduled to switch on for physics this month, and an important milestone was reached this summer when, for the first time, the AD team decelerated a beam of protons to the AD’s target momentum of just 100 MeV/c.

It might seem strange that milestones for the AD are measured in terms of achievements with protons, but, as Flemming Pedersen of the AD team explained, “We already know how to make antiprotons. The real challenge is low fields.”

With an AD beam momentum of just 100 MeV/c, the magnetic bending fields, which hold the beam in orbit, are so low that even the magnetic field of the Earth must be taken into account. Protons are used instead of antiprotons in the setting-up phase because higher intensities can be achieved, which make for easier diagnostics.

Decelerating the beam

Beam particles enter the AD with a momentum of 3.5 GeV/c, which is about 35 times as high as when they leave it. The beam is rapidly decelerated to 2000 MeV/c before undergoing stochastic cooling. Reducing the energy further is a delicate task, owing to the origin of the AD’s components. The AD is not a purpose-built machine ­ it has been assembled using components from the Antiproton Collector, the job of which was to collect antiprotons at 3.5 GeV for CERN’s historic proton­antiproton collider project of the 1980s.

Bending magnets that are designed for constant 3.5 GeV operation are not ideal for the AD, where the field is constantly cycled. In particular,eddy currents, provoked by changing the magnetic field in the AD, can become large, and these can disturb the beam. To avoid this problem the momentum is reduced from 550 MeV very slowly.

When the beam reaches 300 MeV/c there is another pause. This time the technique of electron cooling, better adapted to very low-energy beams, is used. Like the magnets,the electron cooler is recycled. It came from CERN’s previous low-energy antiproton facility, LEAR, which was the world’s second application of the technique pioneered by Gersh Budker at Novosibirsk in the late 1960s.

For the final approach to 100 MeV/c, the beam is slowed down again. Here the Earth’s magnetic field has to be considered, along with remanent fields induced in the AD’s metallic components.

The main challenge in reaching 100 MeV/c was to produce very stable power supplies that would control eddy currents in the magnets. Soon after the 100 MeV/c challenge had been met, decelerating to 100 MeV/c had become routine and the AD was shut down to allow physicists to install the three experiments that will start taking data with the new machine.

When work resumed in September, the first task was to consolidate what had already been achieved with protons and to add a further electron-cooling stage at 100 MeV/c before the beams are extracted for delivery to experiments. Next on the agenda was reversing the polarity of the bending magnets to handle antiprotons. Some further setting up is expected because, as Pedersen pointed out, “We can’t reverse the polarity of the Earth’s magnetic field!”

The post CERN gears up for deceleration appeared first on CERN Courier.

Antiprotonic atoms, in which an antiparticle is bound to an ordinary nucleus, carry important messages about the antiworld and are much easier to make than anti-atoms. Among antiprotonic atoms, protonium (a “nuclear” proton and an “orbital” antiproton) is particularly interesting because it is the simplest two-body system consisting of a strongly interacting particle­antiparticle pair.

An isolated protonium atom will not be destroyed by collisions with atoms of the medium in which it was produced and can only de-excite by giving off radiation. The lifetime can then easily exceed microseconds. The difficulty will be to produce the atoms in isolation.

Antiprotonic helium is a special case. An experiment at CERN discovered that this exotic atom can survive a very large number of collisions and survive long enough to be studied by laser spectroscopy.

Isolated antiprotonic lithium would also be of great interest because its antiproton orbit should be far outside the residual pair of electrons. It should then be able to descend a ladder of these slow electromagnetic transitions, which ends only when it approaches the electrons.

In studying the interactions of antiprotons with matter, it is important to understand their ionization effects ­ how antiprotons strip electrons from ordinary atoms.

An experiment at LEAR by a collaboration involving Aarhus, PSI Villigen, University College London and St Patrick’s Maynooth measured the ionization of hydrogen by antiprotons within the 30-1000 keV energy range, where the antiprotons can be considered to be “fast and heavy” (see next article). The experimentally observed effects concur with theoretical calculations.

However, at lower energies, where there are as yet no data, theoretical analysis becomes more difficult and different calculations disagree, although they suggest an at the most weak energy dependence.

The study of the ionization of helium by antiprotons, with removal of one or both electrons, was pioneered at LEAR and is also ripe for further investigation.

These additional physics objectives form an integral part of the ASACUSA experimental programme, which involves some 50 researchers from 19 research institutes and in which Japanese physicists play a prominent role.

Antiprotons cannot do this, but when their energy drops still further (below a few tens of electron volts) they will readily be captured by the nucleus (see previous article) and form antiprotonic atoms.

These effects showed up clearly in the very-low-energy domain of antiproton physics opened up at CERN’s LEAR low-energy antiproton ring, and groups from Aarhus and Tokyo carried out many atomic interaction experiments as a guide to a better theoretical understanding of these many-body collisions (see previous article).

In the LEAR era, such experiments injected high-energy antiprotons into metallic foils or high-density gases, which degraded the antiprotons to electron volt energies and (in some experiments) provided the target atoms in which they were finally captured.

If the target density or thickness could be made so small that only one collision occured, much more precise and better-controlled experiments on the atomic interactions of antiprotons would be possible, and the dynamics of antiprotonic atom formation could be studied in detail. At such low target densities the absence of collisions after the capture process should also ensure that all antiprotonic atoms are stable enough to be brought under the penetrating eye of laser spectroscopy (see previous article).

The thin-target condition, where a beam particle enters a target and makes a single interaction, is, in a sense, “business as usual” for high-energy particle experiments, yet it constitutes one of ASACUSA’s more difficult longer-term goals. The solution is to separate the deceleration of the antiprotons from the atomic interaction (or antiprotonic atom formation) to be studied.

However, the electron volt antiprotons required for these experiments have a millionth of the energy that even the AD can provide. This energy gap will be crossed in two stages. First, the AD will be supplemented by a decelerating Radio Frequency Quadrupole (under construction in CERN PS division) to reduce the energy to tens of kilo electron volts. The antiprotons will then be confined in a Penning trap that is being constructed at Tokyo University, cooled to cryogenic temperatures, and reaccelerated to a given electron volt-scale energy.

Finally, the reaccelerated antiprotons will be introduced into low pressure gas targets or jets or ultrathin foils. These experiments should start in 2000, after the first round of experiments (on antiprotonic helium) is complete.

The post Telegrams from the antiworld appeared first on CERN Courier.

Most physicists learn early in their career that it is impossible to find exact solutions for problems with more than two interacting bodies. Unfortunately, nature’s arrangements do not include making life easy for physicists ­ most of the phenomena that they find interesting (including those mentioned above) turn out to involve three bodies or more. Often physicists can avoid this handicap, sometimes by taking advantage of the fact that the masses and/or energies of some bodies may be much larger or smaller than those of other bodies; sometimes by using approximation methods; and sometimes by employing both approaches.

The many-body problem of the interaction of charged particle projectiles, such as protons and antiprotons, with atoms has repeatedly engaged many of the most agile minds of 20th-century physics. If, in such collisions, the incident particle is much heavier than the electrons in the target atom and its encounter with the atom is short- lived enough to be treated as a small perturbation, it will follow a straight, charge-independent, constant-velocity path through the atom and will not be deflected by electric fields.

This approximation, together with a few additional assumptions (for example, that the nucleus is too small a target to play a significant role), leads to the familiar Bethe­Bloch formula for the cumulative energy loss from multiple atomic encounters of charged particles passing through matter ­ of everyday importance in every particle physics experiment.

The “fast and heavy” approximation can at best hold down to projectile velocities about equal to that of the target atom’s electrons: about 25 keV for nucleons approaching hydrogen atoms. At lower energies the charge independence assumption will also be lost, because the projectile stays in the atom long enough to feel the nucleus. Among the more dramatic ultralow-energy effects is that of projectile protons repeatedly capturing and losing electrons.

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This year should see the start of physics with CERN’s new Antiproton Decelerator ring, marking the return of antiparticle physics to the CERN research stage three years after the closure of the LEAR low-energy antiproton ring in 1996.

The Antiproton Decelerator (AD) was built from CERN’s former Antiproton Collector ring, which was commissioned in 1987 to supplement the original Antiproton Accumulator (AA; meanwhile, elements of the AA have been sent to the Japanese KEK laboratory).

The task of the AD will be to take the antiprotons, which are produced by 26 GeV/c momentum protons hitting a target and selected at the optimum 3.57 GeV/c momentum level, and, as its name implies, decelerate them to much lower energies, using electron and stochastic cooling to control the beams.

Late last year the AD had a foretaste of particles ­ the much more readily available protons, in this case. The antiproton debut is scheduled to take place soon after the restart of the CERN machines this spring, with the physics programme following in September.

On the menu are the ATHENA and ATRAP experiments, which will use magnetic trapping to manufacture atoms of antihydrogen. Following the first synthesis of chemical antimatter at LEAR in 1995, physicists have been eagerly awaiting a chance to revisit atomic antimatter country to see whether there is any difference between the behaviour of matter and antimatter.

Also on the menu is the ASACUSA experiment by a Japanese-European collaboration, which aims to continue the exploration of antiprotonic atoms ­ atoms in which an orbital electron has been replaced by an antiproton.

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