In 2018 and 2019, the LHCb collaboration published surprising measurements of the Ξ_c⁰ and Ω_c⁰ baryon lifetimes, which were inconsistent with previous results and overturned the established hierarchy between the two. A new analysis of their hadronic decays now confirms this observation, promising insights into the dynamics of baryons.

The Λ_c⁺, Ξ_c⁺, Ξ_c⁰ and Ω_c⁰ baryons – each composed of one charm and two lighter up, down or strange quarks – are the only ground-state singly charmed baryons that decay predominantly via the weak interaction. The main contribution to this process comes from the charm quark transitioning into a strange quark, with the other constituents acting as passive spectators. Consequently, at leading order, their lifetimes should be the same. Differences arise from higher-order effects, such as W-boson exchange between the charm and spectator quarks and quantum interference between identical particles, known as “Pauli interference”. Charm hadron lifetimes are more sensitive to these effects than beauty hadrons because of the smaller charm quark mass compared to the bottom quark, making them a promising testing ground to study these effects.

Measurements of the Ξ_c⁰ and Ω_c⁰ lifetimes prior to the start of the LHCb experiment resulted in the PDG averages shown in figure 1. The first LHCb analysis, using charm baryons produced in semi-leptonic decays of beauty baryons, was in tension with the established values, giving a Ω_c⁰ lifetime four times larger than the previous average. The inconsistencies were later confirmed by another LHCb measurement, using an independent data set with charm baryons produced directly (prompt) in the pp collision (CERN Courier July/August 2021 p17). These results changed the ordering of the four single-charm baryons when arranged according to their lifetimes, triggering a scientific discussion on how to treat higher-order effects in decay rate calculations.

Using the full Run 1 and 2 datasets, LHCb has now measured the Ξ_c⁰ and Ω_c⁰ lifetimes with a third independent data sample, based on fully reconstructed Ξ_b^– → Ξ_c⁰ (→ pK^–K^–π⁺)π^– and Ω^–_b → Ω_c⁰ (→ pK^–K^–π⁺)π^– decays. The selection of these hadronic decay chains exploits the long lifetime of the beauty baryons, such that the selection efficiency is almost independent of the charm baryon decay time. To cancel out the small remaining acceptance effects, the measurement is normalised to the kinematically and topologically similar B^– → D⁰(→ K⁺K^–π⁺π^–)π^– channel, minimising the uncertainties with only a small additional correction from simulation.

The signal decays are separated from the remaining background by fits to the Ξ_c⁰ π^– and Ω_c⁰ π^– invariant mass spectra, providing 8260 ± 100 Ξ_c⁰ and 355 ± 26 Ω_c⁰ candidates. The decay time distributions are obtained with two independent methods: by determining the yield in each of a specific set of decay time intervals, and by employing a statistical technique that uses the covariance matrix from the fit to the mass spectra. The two methods give consistent results, confirming LHCb’s earlier measurements. Combining the three measurements from LHCb, while accounting for their correlated uncertainties, gives τ(Ξ_c⁰) = 150.7 ± 1.6 fs and τ(Ω_c⁰) = 274.8 ± 10.5 fs. These new results will serve as experimental guidance on how to treat higher-order effects in weak baryon decays, particularly regarding the approach-dependent sign and magnitude of Pauli interference terms.

The post Hadronic decays confirm long-lived Ω_c⁰ baryon appeared first on CERN Courier.

The highlight of the conference was new results on the muon magnetic anomaly. The Muon g-2 experiment at Fermilab released its final measurement of a_μ = (g-2)/2 on 3 June, while the conference was in progress, reaching a precision of 127 ppb on the published value. This uncertainty is more than four times smaller than that reported by the previous experiment. One week earlier, on 27 May, the Muon g-2 Theory Initiative published their second calculation of the same quantity, following that published in summer 2020. A major difference between the two calculations is that the earlier one used experimental data and the dispersion integral to evaluate the hadronic contribution to a_μ, whereas the update uses a purely theoretical approach based on lattice QCD. The strong tension with the experiment of the earlier calculation is no longer present, with the new calculation compatible with experimental results. Thus, no new physics discovery can be claimed, though the reason for the difference between the two approaches must be understood (see “Fermilab’s final word on muon g-2“). 

The MEG II collaboration presented an important update to their limit on the branching fraction for the lepton-flavour-violating decay μ → eγ. Their new upper bound of 1.5 × 10^–13 is determined from data collected in 2021 and 2022. The experiment recorded additional data from 2023 to 2024 and expects to continue data taking for two more years. These data will be sensitive to a branching fraction four to five times smaller than the current limit.

LHCb, Belle II, BESIII and NA62 all discussed recent results in quark flavour physics. Highlights include the first measurement of CP violation in a baryon decay by LHCb and improved limits on CP violation in D-meson decay to two pions by Belle II. With more data, the latter measurements could potentially show that the observed CP violation in charm is from a non-Standard-Model source. 

The Belle II collaboration now plans to collect a sample between 5 to 10 ab^–1 by the early 2030s before undergoing an upgrade to collect a 30 to 50 ab^–1 sample by the early 2040s. LHCb plan to run to the end of the High-Luminosity LHC and collect 300 fb^–1. LHCb recorded almost 10 fb^–1 of data last year – more than in all their previous running, and now with a fully software-based trigger with much higher efficiency than the previous hardware-based first-level trigger. Future results from Belle II and the LHCb upgrade are eagerly anticipated.

The 24th FPCP conference will be held from 18 to 22 May 2026 in Bad Honnef, Germany. 

The post Muons under the microscope in Cincinnati appeared first on CERN Courier.

Particle physics today benefits from a wealth of high-quality data at the same time as powerful new ideas are boosting the accuracy of theoretical predictions. These are particularly important while the international community discusses future projects, basing projections on current results and technology. The conference heard how theoretical investigations of specific models and “catch all” effective field theories are being sharpened to constrain a broader spectrum of possible extensions of the Standard Model. Theoretical parametric uncertainties are being greatly reduced by collider precision measurements and lattice QCD. Perturbative calculations of short-distance amplitudes are reaching to percent-level precision, while hadronic long-distance effects are being investigated both in B-, D- and K-meson decays, as well as in the modelling of collider events.

Comprehensive searches

Throughout Moriond 2025 we heard how a broad spectrum of experiments at the LHC, B factories, neutrino facilities, and astrophysical and cosmological observatories are planning upgrades to search for new physics at both low- and high-energy scales. Several fields promise qualitative progress in understanding nature in the coming years. Neutrino experiments will measure the neutrino mass hierarchy and CP violation in the neutrino sector. Flavour experiments will exclude or confirm flavour anomalies. Searches for QCD axions and axion-like particles will seek hints to the solution of the strong CP problem and possible dark-matter candidates.

The Standard Model has so far been confirmed to be the theory that describes physics at the electroweak scale (up to a few hundred GeV) to a remarkable level of precision. All the particles predicted by the theory have been discovered, and the consistency of the theory has been proven with high precision, including all calculable quantum effects. No direct evidence of new physics has been found so far. Still, big open questions remain that the Standard Model cannot answer, from understanding the origin of neutrino masses and their hierarchy, to identifying the origin and nature of dark matter and dark energy, and explaining the dynamics behind the baryon asymmetry of the universe.

Several fields promise qualitative progress in understanding nature in the coming years

The discovery of the Higgs boson has been crucial to confirming the Standard Model as the theory of particle physics at the electroweak scale, but it does not explain why the scalar Brout–Englert–Higgs (BEH) potential takes the form of a Mexican hat, why the electroweak scale is set by a Higgs vacuum expectation value of 246 GeV, or what the nature of the Yukawa force is that results in the bizarre hierarchy of masses coupling the BEH field to quarks and leptons. Gravity is also not a component of the Standard Model, and a unified theory escapes us.

At the LHC today, the ATLAS and CMS collaborations are delivering Run 1 and 2 results with beyond-expectation accuracies on Higgs-boson properties and electroweak precision measurements. Projections for the high-luminosity phase of the LHC are being updated and Run 3 analyses are in full swing. The LHCb collaboration presented another milestone in flavour physics for the first time at Moriond 2025: the first observation of CP violation in baryon decays. Its rebuilt Run 3 detector with triggerless readout and full software trigger reported its first results at this conference.

Several talks presented scenarios of new physics that could be revealed in today’s data given theoretical guidance of sufficient accuracy. These included models with light weakly interacting particles, vector-like fermions and additional scalar particles. Other talks discussed how revisiting established quantum properties such as entanglement with fresh eyes could offer unexplored avenues to new theoretical paradigms and overlooked new-physics effects.

The post Planning for precision at Moriond appeared first on CERN Courier.

A new result from the LHCb collaboration supports the hypothesis that the rare decays B^± → K^±e⁺e^– and B^± → K^±µ⁺µ^– occur at the same rate, further tightening constraints on the magnitude of lepton flavour universality (LFU) violation in rare B decays. The new measurement is the most precise to date in the high-q² region and the first of its kind at a hadron collider.

LFU is an accidental symmetry of the Standard Model (SM). Under LFU, each generation of lepton ℓ^± (electron, muon and tau lepton) is equally likely to interact with the W boson in decay processes such as B^± → K^±ℓ⁺ℓ^–. This symmetry leads to the prediction that the ratio of branching fractions for these decay channels should be unity except for kinematic effects due to the different masses of the charged leptons. The most straightforward ratio to measure is that between the muon and electron decay modes, known as R_K. Any significant deviation from R_K = 1 could only be explained by the existence of new physics (NP) particles that preferentially couple to one lepton generation over another, violating LFU.

B^± → K^±ℓ⁺ℓ^– decays are a powerful probe for virtual NP particles. These decays involve an underlying b–to–s quark transition – an example of a flavour-changing neutral current (FCNC). FCNC transitions are extremely rare in the SM, as they occur only through higher-order Feynman diagrams. This makes them particularly sensitive to contributions from NP particles, which could significantly alter the characteristics of the decays. In this case, the mass of the NP particles could be much larger than can be produced directly at the LHC. “Indirect” searches for NP, such as measuring the precisely predicted ratio R_K, can probe mass scales beyond the reach of direct-production searches with current experimental resources.

The new measurement is the most precise to date in the high-q² region

In the decay process B^± → K^±ℓ⁺ℓ^–, the final-state leptons can also originate from an intermediate resonant state, such as a J/ψ or ψ(2S). These resonant channels occur through tree-level Feynman diagrams. Their contributions significantly outnumber the non-resonant FCNC processes and are not expected to be affected by NP. R_K is therefore measured in ranges of dilepton invariant mass-squared (q²), which exclude these resonances, to preserve sensitivity to potential NP effects in FCNC processes.

The new result from the LHCb collaboration measures R_K in the high-q² region, above the ψ(2S) resonance. The high-q² region data has a different composition of backgrounds compared to the low-q² data, leading to different strategies for their rejection and modelling, and different systematic effects. With R_K expected to be unity in all domains in the SM, low-q² and high-q² measurements offer powerfully complementary constraints on the magnitude of LFU-violating NP in rare B decays.

The new measurement of R_K agrees with the SM prediction of unity and is the most precise to date in the high-q² region (figure 1). It complements a refined analysis below the J/ψ resonance published by LHCb in 2023, which also reported R_K consistent with unity. Both results use the complete proton–proton collision data collected by LHCb from 2011 to 2018. They lay the groundwork for even more precise measurements with data from Run 3 and beyond.

The post Breaking new ground in flavour universality appeared first on CERN Courier.

As direct searches for physics beyond the Standard Model continue to push frontiers at the LHC, the b-hadron physics sector remains a crucial source of insight for testing established theoretical models.

The ATLAS collaboration recently published a new measurement of the B⁰ lifetime using B⁰ → J/ψK^*0 decays from the entire Run-2 dataset it has recorded at 13 TeV. The result improves the precision of previous world-leading measurements by the CMS and LHCb collaborations by a factor of two.

Studies of b-hadron lifetimes probe our understanding of the weak interaction. The lifetimes of b-hadrons can be systematically computed within the heavy-quark expansion (HQE) framework, where b-hadron observables are expressed as a perturbative expansion in inverse powers of the b-quark mass.

ATLAS measures the “effective” B⁰ lifetime, which represents the average decay time incorporating effects from mixing and CP contributions, as τ(B⁰) = 1.5053 ± 0.0012 (stat.) ± 0.0035 (syst.) ps. The result is consistent with previous measurements published by ATLAS and other experiments, as summarised in figure 1. It also aligns with theoretical predictions from HQE and lattice QCD, as well as with the experimental world average.

The analysis benefitted from the large Run-2 dataset and a refined trigger selection, enabling the collection of an extensive sample of 2.5 million B⁰ → J/ψK^*0 decays. Events with a J/ψ meson decaying into two muons with sufficient transverse momentum are cleanly identified in the ATLAS Muon Spectrometer by the first-level hardware trigger. In the next-level software trigger, exploiting the full detector information, these muons are then combined with two tracks measured by the Inner Detector, ensuring they originate from the same vertex.

The B⁰-meson lifetime is determined through a two-dimensional unbinned maximum-likelihood fit, utilising the measured B⁰-candidate mass and decay time, and accounting for both signal and background components. The limited hadronic particle-identification capability of ATLAS requires careful modelling of the significant backgrounds from other processes that produce J/ψ mesons. The sensitivity of the fit is increased by estimating the uncertainty of the decay-time measurement provided by the ATLAS tracking and vertexing algorithms on a per-candidate basis. The resulting lifetime measurement is limited by systematic uncertainties, with the largest contributions arising from the correlation between B⁰ mass and lifetime, and ambiguities in modelling the mass distribution. 

ATLAS combined its measurement with the average decay width (Γ_s) of the light and heavy B_s-meson mass eigenstates, also measured by ATLAS, to determine the ratio of decay widths as Γ_d/Γ_s = 0.9905 ± 0.0022 (stat.) ± 0.0036 (syst.) ± 0.0057 (ext.). The result is consistent with unity and provides a stringent test of QCD predictions, which also support a value near unity.

The post A new record for precision on B-meson lifetimes appeared first on CERN Courier.

Schwinger lined us up in his office, and spent several hours assigning thesis subjects. It was a remarkable performance. I was the last in line. Having run out of well-defined thesis problems, he explained to me that weak and electromagnetic interactions share two remarkable features: both are vectorial and both display aspects of universality. Schwinger suggested that I create a unified theory of the two interactions – an electroweak synthesis. How I was to do this he did not say, aside from slyly hinting at the Yang–Mills gauge theory.

By the summer of 1958, I had convinced myself that weak and electromagnetic interactions might be described by a badly broken gauge theory, and Schwinger that I deserved a PhD. I had hoped to partly spend a postdoctoral fellowship in Moscow at the invitation of the recent Russian Nobel laureate Igor Tamm, and sought to visit Niels Bohr’s institute in Copenhagen while awaiting my Soviet visa. With Bohr’s enthusiastic consent, I boarded the SS Île de France with my friend Jack Schnepps. Following a memorable and luxurious crossing – one of the great ship’s last – Jack drove south to Padova to work with Milla Baldo-Ceolin’s emulsion group in Padova, and I took the slow train north to Copenhagen. Thankfully, my Soviet visa never arrived. I found the SU(2) × U(1) structure of the electroweak model in the spring of 1960 at Bohr’s famous institute at Blegsdamvej 19, and wrote the paper that would earn my share of the 1979 Nobel Prize.

We called the new quark flavour “charm”, completing two weak doublets of quarks to match two weak doublets of leptons, and establishing lepton–quark symmetry, which holds to this day

A year earlier, in 1959, Augusto Gamba, Bob Marshak and Susumo Okubo had proposed lepton–hadron symmetry, which regarded protons, neutrons and lambda hyperons as the building blocks of all hadrons, to match the three known leptons at the time: neutrinos, electrons and muons. The idea was falsified by the discovery of a second neutrino in 1962, and superseded in 1964 by the invention of fractionally charged hadron constituents, first by George Zweig and André Petermann, and then decisively by Murray Gell-Mann with his three flavours of quarks. Later in 1964, while on sabbatical in Copenhagen, James Bjorken and I realised that lepton–hadron symmetry could be revived simply by adding a fourth quark flavour to Gell-Mann’s three. We called the new quark flavour “charm”, completing two weak doublets of quarks to match two weak doublets of leptons, and establishing lepton–quark symmetry, which holds to this day.

Annus mirabilis

1964 was a remarkable year. In addition to the invention of quarks, Nick Samios spotted the triply strange Ω^– baryon, and Oscar Greenberg devised what became the critical notion of colour. Arno Penzias and Robert Wilson stumbled on the cosmic microwave background radiation. James Cronin, Val Fitch and others discovered CP violation. Robert Brout, François Englert, Peter Higgs and others invented spontaneously broken non-Abelian gauge theories. And to top off the year, Abdus Salam rediscovered and published my SU(2) × U(1) model, after I had more-or-less abandoned electroweak thoughts due to four seemingly intractable problems.

Much to my surprise and delight, all of them would be solved within just a few years, with the last theoretical obstacle removed by Makoto Kobayashi and Toshihide Maskawa in 1973 (see “Four intractable problems” panel). A few months later, Paul Musset announced that CERN’s Gargamelle detector had won the race to detect weak neutral-current interactions, giving the electroweak model the status of a predictive theory. Remarkably, the year had begun with Gell-Mann, Harald Fritzsch and Heinrich Leutwyler proposing QCD, and David Gross, Frank Wilczek and David Politzer showing it to be asymptotically free. The Standard Model of particle physics was born.

Charmed findings

But where were the charmed quarks? Early on Monday morning on 11 November, 1974, I was awakened by a phone call from Sam Ting, who asked me to come to his MIT office as soon as possible. He and Ulrich Becker were waiting for me impatiently. They showed me an amazingly sharp resonance. Could it be a vector meson like the ρ or ω and be so narrow, or was it something quite different? I hopped in my car and drove to Harvard, where my colleagues Alvaro de Rújula and Howard Georgi excitedly regaled me about the Californian side of the story. A few days later, experimenters in Frascati confirmed the BNL–SLAC discovery, and de Rújula and I submitted our paper “Is Bound Charm Found?” – one of two papers on the J/ψ discovery printed in Physical Review Letters on 5 July 1965 that would prove to be correct. Among five false papers was one written by my beloved mentor, Julian Schwinger.

The second correct paper was by Tom Appelquist and David Politzer. Well before that November, they had realised (without publishing) that bound states of a charmed quark and its antiquark lying below the charm threshold would be exceptionally narrow due the asymptotic freedom of QCD. De Rújula suggested to them that such a system be called charmonium in an analogy with positronium. His term made it into the dictionary. Shortly afterward, the 1976 Nobel Prize in Physics was jointly awarded to Burton Richter and Sam Ting for “their pioneering work in the discovery of a heavy elementary particle of a new kind” – evidence that charm was not yet a universally accepted explanation. Over the next few years, experimenters worked hard to confirm the predictions of theorists at Harvard and Cornell by detecting and measuring the masses, spins and transitions among the eight sub-threshold charmonium states. Later on, they would do the same for 14 relatively narrow states of bottomonium.

Other experimenters were searching for particles containing just one charmed quark or antiquark. In our 1975 paper “Hadron Masses in a Gauge Theory”, de Rújula, Georgi and I included predictions of the masses of several not-yet-discovered charmed mesons and baryons. The first claim to have detected charmed particles was made in 1975 by Robert Palmer and Nick Samios at Brookhaven, again with a bubble-chamber event. It seemed to show a cascade decay process in which one charmed baryon decays into another charmed baryon, which itself decays. The measured masses of both of the charmed baryons were in excellent agreement with our predictions. Though the claim was not widely accepted, I believe to this day that Samios and Palmer were the first to detect charmed particles.

The SLAC electron–positron collider, operating well above charm threshold, was certainly producing charmed particles copiously. Why were they not being detected? I recall attending a conference in Wisconsin that was largely dedicated to this question. On the flight home, I met my old friend Gerson Goldhaber, who had been struggling unsuccessfully to find them. I think I convinced him to try a bit harder. A couple of weeks later in 1976, Goldhaber and François Pierre succeeded. My role in charm physics had come to a happy ending. 

• This article is adapted from a presentation given at the Institute of High-Energy Physics in Beijing on 20 October 2024 to celebrate the 50th anniversary of the discovery of the J/ψ.

The post Charm and synthesis appeared first on CERN Courier.

The accidental symmetries observed between the three generations of leptons are poorly understood, with no compelling theoretical motivation in the framework of the Standard Model (SM). The b → cτ^–ν_τ transition has the potential to reveal new particles or forces that interact primarily with third-generation particles, which are subject to the less stringent experimental constraints at present. As a tree-level SM process mediated by W-boson exchange, its amplitude is large, resulting in large branching fractions and significant data samples to analyse.

The observable under scrutiny is the ratio of decay rates between the signal mode involving τ and ν_τ leptons from the third generation of fermions and the normalisation mode containing μ and ν_μ leptons from the second generation. Within the SM, this lepton flavour universality (LFU) ratio deviates from unity only due to the different mass of the charged leptons – but new contributions could change the value of the ratios. A longstanding tension exists between the SM prediction and the experimental measurements, requiring further input to clarify the source of the discrepancy.

The LHCb collaboration analysed four decay modes: B⁰ → D^(*)+ℓ^–ν_ℓ, with ℓ representing τ or μ. Each is selected using the same visible final state of one muon and light hadrons from the decay of the charm meson. In the normalisation mode, the muon originates directly from the B-hadron decay, while in the signal mode, it arises from the decay of the τ lepton. The four contributions are analysed simultaneously, yielding two LFU ratios between taus and muons – one using the ground state of the D⁺ meson and one the excited state D^*+.

The control of the background contributions is particularly complicated in this analysis as the final state is not fully reconstructible, limiting the resolution on some of the discriminating variables. Instead, a three-dimensional template fit separates the signal and the normalisation from the background versus: the momentum transferred to the lepton pair (q²); the energy of the muon in the rest frame of the B meson (E_μ^*); and the invariant mass missing from the visible system. Each contribution is modelled using a template histogram derived either from simulation or from selected control samples in data.

This constitutes the world’s second most precise measurement of R(D)

To prevent the simulated data sample size from becoming a limiting factor in the precision of the measurement, a fast tracker-only simulation technique was exploited for the first time in LHCb. Another novel aspect of this work is the use of the HAMMER software tool during the minimisation procedure of the likelihood fit, which enables a fast, but exact, variation of a template as a function of the decay-model parameters. This variation is important to allow the form factors of both the signal and normalisation channels to vary as the constraints derived from the predictions that use precise lattice calculations can have larger uncertainties than those obtained from the fit.

The fit projection over one of the discriminating variables is shown in figure 1, illustrating the complexity of the analysed data sample but nonetheless showcasing LHCb’s ability to distinguish the signal modes (red and orange) from the normalisation modes (two shades of blue) and background contributions.

The measured LFU ratios are in good agreement with the current world average and the predictions of the SM: R(D⁺) = 0.249 ± 0.043 (stat.) ± 0.047 (syst.) and R(D^*+) = 0.402 ± 0.081(stat.) ± 0.085 (syst.). Under isospin symmetry assumptions, this constitutes the world’s second most precise measurement of R(D), following a 2019 measurement by the Belle collaboration. This analysis complements other ongoing efforts at LHCb and other experiments to test LFU across different decay channels. The precision of the measurements reported here is primarily limited by the size of the signal and control samples, so more precise measurements are expected with future LHCb datasets.

The post R(D) ratios in line at LHCb appeared first on CERN Courier.

Changing flavour

The discussion on rare B decays continued in the session on flavour-changing neutral-currents. With new lattice-QCD results pinning down short-distance local hadronic contributions, the discussion focused on understanding the long-distance contributions arising from hadronic resonances and charm rescattering. Arianna Tinari (Zurich) and Martin Hoferichter (Bern) judged the latter not to be dramatic in magnitude. Lakshan Madhan (Cambridge) presented a new amplitude analysis in which the long and short-distance contributions are separated via the kinematic dependence of the decay amplitudes. New theo­retical analyses of the nonlocal form factors for B → K^(*)μ⁺μ^– and B → K^(*)e⁺e^– were representative of the workshop as a whole: truly the bee’s knees.

Another challenge to accurate theory predictions for rare decays, the widths of vector final states, snuck its way into the flavour-changing charged-currents session, where Luka Leskovec (Ljubljana) presented a comprehensive overview of lattice methods for decays to resonances. Leskovec’s optimistic outlook for semileptonic decays with two mesons in the final state stood in contrast to prospects for applying lattice methods to D-D mixing: such studies are currently limited to the SU(3)-flavour symmetric point of equal light-quark masses, explained Felix Erben (CERN), though he offered a glimmer of hope in the form of spectral reconstruction methods currently under development.

LHCb’s beauty and charm physics programme reported substantial progress. Novel techniques have been implemented in the most recent CP-violation studies, potentially leading to an impressive uncertainty of just 1° in future measurements of the CKM angle gamma. LHCb has recently placed a special emphasis on beauty and charm baryons, where the experiment offers unique capabilities to perform many interesting measurements ranging from CP violation to searches for very rare decays and their form factors. Going from three quarks to four and five, the spectroscopy session illustrated the rich and complex debate around tetraquark and pentaquark states with a big open discussion on the underlying structure of the 20 or so discovered at LHCb: which are bound states of quarks and which are simply meson molecules? (CERN Courier November/December 2024 p26 and p33.)

LHCb’s ability to do unique physics was further highlighted in the QCD, electroweak (EW) and exotica session, where the collaboration has shown the most recent publicly available measurement of the weak-mixing angle in conjunction with W/Z-boson production cross-sections and other EW observables. LHCb have put an emphasis on combined QCD + QED and effective-field-theory calculations, and the interplay between EW precision observables and new-physics effects in couplings to the third generation. By studying phase space inaccessible to any other experiment, a study of hypothetical dark photons decaying to electrons showed the LHCb experiment to be a unique environment for direct searches for long-lived and low-mass particles.

Attendees left the workshop with a fresh perspective

Parallel to Implications 2024, the inaugural LHCb Open Data and Ntuple Wizard Workshop, took place on 22 October as a satellite event, providing theorists and phenomenologists with a first look at a novel software application for on-demand access to custom ntuples from the experiment’s open data. The LHCb Ntupling Service will offer a step-by-step wizard for requesting custom ntuples and a dashboard to monitor the status of requests, communicate with the LHCb open data team and retrieve data. The beta version was released at the workshop in advance of the anticipated public release of the application in 2025, which promises open access to LHCb’s Run 2 dataset for the first time.

A recurring satellite event features lectures by theorists on topics following LHCb’s scientific output. This year, Simon Kuberski (CERN) and Saša Prelovšek (Ljubljana) took the audience on a guided tour through lattice QCD and spectroscopy.

With LHCb’s integrated luminosity in 2024 exceeding all previous years combined, excitement was heightened. Attendees left the workshop with a fresh perspective on how to approach the challenges faced by our community.

The post The B’s Ke⁺e^–s appeared first on CERN Courier.

With the long shutdown on the horizon, the third run of the LHC is progressing in earnest. Its high-availability operation and mastery of operational risks were highly praised. Run 3 data is of immense importance as it will be the dataset that experiments will work with for the next decade. With the newly collected data at 13.6 TeV, the LHC experiments showed new measurements of Higgs and di-electroweak-boson production, though of course most of the LHC results were based on the Run 2 (2014 to 2018) dataset, which is by now impeccably well calibrated and understood. This also allowed ATLAS and CMS to bring in-depth improvements to reconstruction algorithms.

AI algorithms

A highlight of the conference was the improvements brought by state-of-the-art artificial-intelligence algorithms such as graph neural networks, both at the trigger and reconstruction level. A striking example of this is the ATLAS and CMS flavour-tagging algorithms, which have improved their rejection of light jets by a factor of up to four. This has important consequences. Two outstanding examples are: di-Higgs-boson production, which is fundamental for the measurement of the Higgs boson self-coupling (CERN Courier July/August 2024 p7); and the Higgs boson’s Yukawa coupling to charm quarks. Di-Higgs-boson production should be independently observable by both general-purpose experiments at the HL-LHC, and an observation of the Higgs boson’s coupling to charm quarks is getting closer to being within reach.

The LHC experiments continue to push the limits of precision at hadron colliders. CMS and LHCb presented new measurements of the weak mixing angle. The per-mille precision reached is close to that of LEP and SLD measurements (CERN Courier September/October 2024 p29). ATLAS presented the most precise measurement to date (0.8%) of the strong coupling constant extracted from the measurement of the transverse momentum differential cross section of Drell–Yan Z-boson production. LHCb provided a comprehensive analysis of the B⁰ → K^0* μ⁺μ^– angular distributions, which had previously presented discrepancies at the level of 3σ. Taking into account long-distance contributions significantly weakens the tension down to 2.1σ.

Pioneering the highest luminosities ever reached at colliders (setting a record at 4.7 × 10³⁴ cm^–2 s^–1), SuperKEKB has been facing challenging conditions with repeated sudden beam losses. This is currently an obstacle to further progress to higher luminosities. Possible causes have been identified and are currently under investigation. Meanwhile, with the already substantial data set collected so far, the Belle II experiment has produced a host of new results. In addition to improved CKM angle measurements (alongside LHCb), in particular of the γ angle, Belle II (alongside BaBar) presented interesting new insights in the long standing |V_cb| and |V_ub| inclusive versus exclusive measurements puzzle (CERN Courier July/August 2024 p30), with new |V_cb| exclusive measurements that significantly reduce the previous 3σ tension.

ATLAS and CMS furthered their systematic journey in the search for new phenomena to leave no stone unturned at the energy frontier, with 20 new results presented at the conference. This landmark outcome of the LHC puts further pressure on the naturalness paradigm.

A highlight of the conference was the overall progress in neutrino physics. Accelerator-based experiments NOvA and T2K presented a first combined measurement of the mass difference, neutrino mixing and CP parameters. Neutrino telescopes IceCube with DeepCore and KM3NeT with ORCA (Oscillation Research with Cosmics in the Abyss) also presented results with impressive precision. Neutrino physics is now at the dawn of a bright new era of precision with the next-generation accelerator-based long baseline experiments DUNE and Hyper Kamiokande, the upgrade of DeepCore, the completion of ORCA and the medium baseline JUNO experiment. These experiments will bring definitive conclusions on the measurement of the CP phase in the neutrino sector and the neutrino mass hierarchy – two of the outstanding goals in the field.

The KATRIN experiment presented a new upper limit on the effective electron–anti-neutrino mass of 0.45 eV, well en route towards their ultimate sensitivity of 0.2 eV. Neutrinoless double-beta-decay search experiments KamLAND-Zen and LEGEND-200 presented limits on the effective neutrino mass of approximately 100 meV; the sensitivity of the next-generation experiments LEGEND-1T, KamLAND-Zen-1T and nEXO should reach 20 meV and either fully exclude the inverted ordering hypothesis or discover this long-sought process. Progress on the reactor neutrino anomaly was reported, with recent fission data suggesting that the fluxes are overestimated, thus weakening the significance of the anti-neutrino deficits.

Neutrinos were also a highlight for direct-dark-matter experiments as Xenon announced the observation of nuclear recoil events from⁸B solar neutrino coherent elastic scattering on nuclei, thus signalling that experiments are now reaching the neutrino fog. The conference also highlighted the considerable progress across the board on the roadmap laid out by Kathryn Zurek at the conference to search for dark matter in an extraordinarily large range of possibilities, spanning 89 orders of magnitude in mass from 10^–23 eV to 10⁵⁷ GeV. The roadmap includes cosmological and astrophysical observations, broad searches at the energy and intensity frontier, direct searches at low masses to cover relic abundance motivated scenarios, building a suite of axion searches, and pursuing indirect-detection experiments.

Neutrinos also made the headlines in multi-messenger astrophysics experiments with the announcement by the KM3Net ARCA (Astroparticle Research with Cosmics in the Abyss) collaboration of a muon-neutrino event that could be the most energetic ever found. The energy of the muon from the interaction of the neutrino is compatible with having an energy of approximately 100 PeV, thus opening a fascinating window on astrophysical processes at energies well beyond the reach of colliders. The conference showed that we are now well within the era of multi-messenger astrophysics, via beautiful neutrinos, gamma rays and gravitational-wave results.

The conference saw new bridges across fields being built. The birth of collider-neutrino physics with the beautiful results from FASERν and SND fill the missing gap in neutrino–nucleon cross sections between accelerator neutrinos and neutrino astronomy. ALICE and LHCb presented new results on He³ production that complement the AMS results. Astrophysical He³ could signal the annihilation of dark matter. ALICE also presented a broad, comprehensive review of the progress in understanding strongly interacting matter at extreme energy densities.

The highlight in the field of observational cosmology was the recent data from DESI, the Dark Energy Spectroscopic Instrument in operation since 2021, which bring splendid new data on baryon acoustic oscillation measurements. These precious new data agree with previous indirect measurements of the Hubble constant, keeping the tension with direct measurements in excess of 2.5σ. In combination with CMB measurements, the DESI measurements also set an upper limit on the sum of neutrino masses at 0.072 eV, in tension with the inverted ordering of neutrino masses hypothesis. This limit is dependent on the cosmological model.

In everyone’s mind at the conference, and indeed across the domain of high-energy physics, it is clear that the field is at a defining moment in its history: we will soon have to decide what new flagship project to build. To this end, the conference organised a thrilling panel discussion featuring the directors of all the major laboratories in the world. “We need to continue to be bold and ambitious and dream big,” said Fermilab’s Lia Merminga, summarising the spirit of the discussion.

“As we have seen at this conference, the field is extremely vibrant and exciting,” said CERN’s Fabiola Gianotti at the conclusion of the panel. In these defining times for the future of our field, ICHEP 2024 was an important success. The progress in all areas is remarkable and manifest through the outstanding number of beautiful new results shown at the conference.

The post A rich harvest of results in Prague appeared first on CERN Courier.

The LHCb collaboration has undertaken a new study of B → DD decays using data from LHC Run 2. In the case of B⁰ → D⁺D^– decays, the analysis excludes CP-symmetry at a confidence level greater than six standard deviations – a first in the analysis of a single decay mode.

The study of differences between matter and antimatter (CP violation) is a core aspect of the physics programme at LHCb. Measurements of CP violation in decays of neutral B⁰ mesons play a crucial role in the search for physics beyond the Standard Model thanks to the ability of the B⁰ meson to oscillate into its antiparticle, the B⁰ meson. Given increases in experimental precision, improved control over the magnitude of hadronic effects becomes important, which is a major challenge in most decay modes. In this measurement, a neutral B meson decays to two charm D mesons – an interesting topology that offers a method to control these high-order hadronic contributions from the Standard Model via the concept of U-spin symmetry.

In the new analysis, B⁰ → D⁺D^– and B_s⁰ → D_s⁺D_s^– are studied simultaneously. U-spin symmetry exchanges the spectator down quarks in the first decay with strange quarks to form the second decay. A joint analysis therefore strongly constrains uncertainties related to hadronic matrix elements by relating CP-violation and branching-fraction measurements in the two decay channels.

In both decays, the same final state is accessible to both matter and antimatter states of the B⁰ or B_s⁰ meson, enabling interference between two decay paths: the direct decay of the meson to the final state; and a decay after the meson has oscillated into its antiparticle counterpart. The time-dependent decay rate of each flavour (matter or antimatter) of the meson depends on CP-violating effects and is parameterised in terms dependent on the fundamental properties of the B mesons and the fundamental CP-violating weak phases β and β_s, in the case of B⁰ and B_s⁰ decays, respectively. The tree-level and exchange Feynman diagrams participating to this decay process, which in turn depend on specific values of the terms in the Cabibbo–Kobayashi–Maskawa quark-mixing matrix, determine the expected value of the β_(s) phases. This matrix encodes our best understanding of the CP-violating effects within the Standard Model, and testing its expected properties is a crucial means to fully exploit closure tests of this theoretical framework.

The study of differences between matter and antimatter is a core aspect of the physics programme at LHCb

The analysis uses flavour tagging to identify the matter or antimatter flavour of the neutral B meson at its production and thus allows the determination of the decay path – a key task in time- dependent measurements of CP violation. The flavour-tagging algorithms exploit the fact that b and b quarks are almost exclusively produced in pairs in pp collisions. When the b quark forms a B meson (and similarly for its antimatter equivalent), additional particles are produced in the fragmentation process of the pp collision. From the charges and species of these particles, the flavour of the signal B meson at production can be inferred. This information is combined with the reconstructed position of the decay vertex of the meson, allowing the flavour-tagged decay-time distribution of each analysed flavour to be measured.

Figure 1 shows the asymmetry between the decay-time distributions of the B⁰ and the B⁰ mesons for the B⁰ → D⁺D^–decay mode. Alongside the B_s⁰ → D_s⁺D_s^– data, these results represent the most precise single measurements of the CP-violation parameters in their respective channels. Results from the two decay modes are used in combination with other B → DD measurements to precisely determine Standard Model parameters.

The post Using U-spin to squeeze CP violation appeared first on CERN Courier.

“Why the excitement over the new discoveries?” asked the Courier in December 1974 (see “The new particles“). “A brief answer is that the particles have been found in a mass region where they were completely un­expected, with stability properties which, at this stage of the game, are completely inexplicable.”

The J/ψ is now known to be made up of a charm quark and a charm antiquark. Unable to decay via the strong interaction, its width is just 92.6 keV, corresponding to an unexpectedly long lifetime of 7.1 × 10^–21 s. Charm quarks do not form ordinary matter like protons and neutrons, but J/ψ resonances and D mesons, which contain a charm quark and a less-massive up, down or strange antiquark.

Fifty years on from the November Revolution, charm physics is experiencing a renaissance. The LHCb, BESIII and Belle II experiments are producing a huge number of interesting and precise measurements in the charm system, with two crucial groundbreaking results on D⁰ mesons by LHCb holding particular significance: the observation that they violate CP symmetry when they decay; and the observation that they oscillate into their antiparticles. The rate of CP violation is particularly interesting – about 10 times larger than the most sophisticated Standard Model (SM) predictions, preliminary and uncertain though they are. Are these predictions naive, or is this the first glimpse of why there is more matter than antimatter in the universe?

Suppressed

Despite the initial confusion, the charm quark had already been indirectly discovered in 1970 by Sheldon Glashow, John Iliopoulos and Luciano Maiani (GIM), who introduced it to explain why K⁰ → μ⁺μ^– decays are suppressed. Their paper gained widespread recognition during the November Revolution, and the GIM mechanism they discovered impacts cutting-edge calculations in charm physics to this day.

Previously, only the three light quarks (up, down and strange) were known. Alongside electrons and electron neutrinos, up and down quarks make up the first generation of fermions. The detection of muons in cosmic rays in 1936 was the first evidence for a second generation, triggering Isidor Rabi’s famous exclamation “Who ordered that?” Strange particles were found in 1947, providing evidence for a second generation of quarks, though it took until 1964 for Murray Gell-Mann and George Zweig to discover this ordering principle of the subatomic world.

In a model of three quarks, the decay of a K⁰ meson (a down–antistrange system) into two muons can only proceed by briefly transforming the meson into a W⁺W^– pair – an infamous flavour-changing neutral current – linked in a loop by a virtual up quark and virtual muon neutrino. While the amplitude for this process is problematically large given observed rates, the GIM mechanism cancels it almost exactly by introducing destructive quantum interference with a process that replaces the up quark with a new charm quark. The remaining finite value of the amplitude stems from the difference in the masses of the virtual quarks compared to the W boson, m_u²/M_W² and m_c²/M_W². Since both mass ratios are close to zero, K⁰ → μ⁺μ^– is highly suppressed.

The interference is destructive because the Cabibbo matrix describing the coupling strength of the charged weak interaction is a rotation of the two generations of quarks. All four couplings in the matrix – up–down (cos θ_C), charm-strange (cos θ_C), charm-down (sin θ_C) and up-strange (–sin θ_C) – arise in the decay of a K⁰ meson, with the minus sign causing the cancellation.

Maybe the charm quark will in the end provide the ultimate clue to explain our existence

The direct experimental detection of the first particle containing charm is typically attributed to Ting and Richter in 1974, however, there was already some direct evidence for charmed mesons in Japan in 1971, though unfortunately in only one cosmic-ray event, and with no estimation of background (see “Cosmic charm” figure). Unnoticed by Western scientists, the measurements indicated a charm-quark mass of the order of 1.5 GeV, which is close to current estimates. In 1973, the quark-mixing formalism was extended by Makoto Kobayashi and Toshihide Maskawa to three generations of quarks, incorporating CP violation in the SM by allowing the couplings to be complex numbers with an imaginary part. The amount of CP violation contained in the resulting Cabibbo–Kobayashi–Maskawa (CKM) matrix does not appear to be sufficient to explain the observed matter–antimatter asymmetry in the universe.

The third generation of quarks began to be experimentally established in 1977 with the discovery of ϒ resonances (bottom–antibottom systems). In 1986, GIM cancellations in the matter–antimatter oscillations of neutral B mesons (B⁰–B⁰ mixing) indicated a large value of the top-quark mass, with m_t²/M_W² not negligible, in contrast to m_u²/M_W² and m_c²/M_W². The top quark was directly discovered at the Tevatron in 1995. With the discovery of the Higgs boson in 2012 at the LHC, the full particle spectrum of the SM has now been experimentally confirmed.

Charm renaissance

More recently, two crucial effects in the charm system have been experimentally confirmed. Both measurements present intriguing discrepancies by comparison with naive theoretical expectations.

First, in 2019, the LHCb collaboration at CERN observed the first definitive evidence for CP violation in charm. A difference in the behaviour of matter and antimatter particles, CP violation can be expressed directly in charm decays, indirectly in the matter–antimatter oscillations of charmed particles, or in a quantum admixture of both effects. To isolate direct CP violation, LHCb proved that the difference in matter–antimatter asymmetries seen in D⁰ → K⁺K^– and D⁰ → π⁺π^– decays (ΔA_CP) is nonzero. Though the observed CP violation is tiny, it is nevertheless approximately a factor 10 larger than the best available SM predictions. Currently the big question is whether these naive SM expectations can be enhanced by a factor of 10 due to non-perturbative effects, or whether the measurement of ΔA_CP is a first glimpse of physics beyond the SM, perhaps also answering the question of why there is more matter than antimatter in the universe.

Two years later, LHCb definitively demonstrated the transformation of neutral D⁰ mesons into their antiparticles (D⁰–D⁰mixing). These transitions only involve virtual down-type quarks (down, strange and bottom), causing extreme GIM cancellations as m_d²/M_W², m_s²/M_W²  and m_b²/M_W² are all negligible (see “Matter–antimatter mixing” figure). Theory calculations are preliminary here too, but naive SM predictions of the mass splitting between the mass eigenstates of the neutral D-meson system are at present several orders of magnitude below the experimental value.

The charm system has often proved to be more experimentally challenging than the bottom system, with matter–antimatter oscillations and direct and indirect CP violation all discovered first for the bottom quark, and indirect CP violation still awaiting confirmation in charm. The theoretical description of the charm system also presents several interesting features by comparison to the bottom system. They may be regarded as challenges, peculiarities, or even opportunities.

A challenge is the use of perturbation theory. The strong coupling at the scale of the charm-quark mass is quite large – α_s(m_c) ≈ 0.35 – and perturbative expansions in the strong coupling only converge as (1, 0.35, 0.12, …). The charm quark is also not particularly heavy, and perturbative expansions in Λ/m_c only converge as roughly (1, 0.33, 0.11, …), assuming Λ is an energy scale of the order of the hadronic scale of the strong interaction. If the coefficients being multiplied are of similar sizes, then these series may converge.

Numerical cancellations are a peculiarity, and often classified as strong or even crazy in cases such as D⁰–D⁰ mixing, where contributions cancel to one part in 10⁵.

The fact that CKM couplings involving the charm quark (V_cd, V_cs and V_cb) have almost vanishing imaginary parts is an opportunity. With CP-violating effects in charm systems expected to be tiny, any measurement of sizable CP violating effects would indicate the presence of physics beyond the SM (BSM).

A final peculiarity is that loop-induced charm decays and D-mixing both proceed exclusively via virtual down-type quarks, presenting opportunities to extend sensitivity to BSM physics via joint analyses with complementary bottom and strange decays.

At first sight, these effects complicate the theoretical treatment of the charm system. Many approaches are therefore based on approximations such as SU(3)_F flavour symmetry or U-spin symmetry (see “Using U-spin to squeeze CP violation”). On the other hand, these properties can also be a virtue, making some observables very sensitive to higher orders in our expansions and providing an ideal testing ground for QCD tools.

Thanks to many theoretical improvements, we are now in a position to start answering the question of whether perturbative expansions in the strong coupling and the inverse of the quark mass are applicable in the charm system. Recently, progress has been made with observables that are free from severe cancellations: a double expansion in Λ/m_cand α_s (the heavy-quark expansion) seems to be able to reproduce the D⁰ lifetime (see “Charmed life” figure); and theoretical calculations of branching fractions for non-leptonic two-body D⁰ decays seem to be in good agreement with experimental values (see “Two body” figure).

All these theory predictions still suffer from large uncertainties, but they can be systematically improved. Demonstrating the validity of these theory tools with higher precision could imply that the measured value of CP violation in the charm system (ΔA_CP) has a BSM origin.

The future

Charm physics therefore has a bright future. Many of the current theory approaches can be systematically improved with currently available technologies by adding higher-order perturbative corrections. A full lattice-QCD description of D-mixing and non-leptonic D-meson decays requires new ideas, but first steps have already been taken. These theory developments should give us deeper insights into the question of whether ΔA_CP and D⁰–D⁰ mixing can be described within the SM.

More precise experimental data can also help in answering these questions. The BESIII experiment at IHEP in China and the Belle II experiment at KEK in Japan can investigate inclusive semileptonic charm decays and measure parameters that are needed for the heavy-quark expansion. LHCb and Belle II can investigate CP-violating effects in D⁰–D⁰ mixing and in channels other than D⁰ → K⁺K^– and π⁺π^–. The super tau–charm factory proposed by China could contribute further precise data and a future e⁺e^– collider running as an ultimate Z factory could provide an independent experimental cross-check for ΔA_CP.

Another exciting field is that of rare charm decays such as D⁺ → π⁺μ⁺μ^– and D⁺ → π⁺ νν, which proceed via loop diagrams similar to those in K⁰ → μ⁺μ^– decays and D⁰–D⁰ oscillations. Here, null tests can be constructed using observables that vanish precisely in the SM, allowing future experimental data to unambiguously probe BSM effects.

Maybe the charm quark will in the end provide the ultimate clue to explain our existence. Wouldn’t that be charming?

The post Charming clues for existence appeared first on CERN Courier.

The key concept is the quantum fluctuation. Just because a collision doesn’t have enough energy to bring a new particle into existence does not mean that a very heavy new particle cannot inform us about its existence. Thanks to Heisenberg’s uncertainty principle, new particles could be virtually exchanged between the other particles involved in the collisions, modifying the probabilities for the processes we observe in our detectors. The effect of massive new particles could be unmistakable, giving physicists a powerful tool for exploring more deeply into the unknown than accelerator technology and economic considerations allow direct searches to go.

The effect of massive new particles could be unmistakable

The search for new particles and forces beyond those of the Standard Model is strongly motivated by the need to explain dark matter, the huge range of particle masses from the tiny neutrino to the massive top quark, and the asymmetry between matter and antimatter that is responsible for our very existence. As direct searches at the LHC have not yet provided any clue as to what these new particles and forces might be, indirect searches are growing in importance. Studying very rare processes could allow us to see imprints of new particles and forces acting at much shorter distance scales than it is possible to explore at current and future colliders.

Anticipating the November Revolution

The charm quark is a good example. The story of its direct discovery unfolded 50 years ago, in November 1974, when teams at SLAC and MIT simultaneously discovered a charm–anticharm meson in particle collisions. But four years earlier, Sheldon Glashow, John Iliopoulos and Luciano Maiani had already predicted the existence of the charm quark thanks to the surprising suppression of the neutral kaon’s decay into two muons.

Neutral kaons are made up of a strange quark and a down antiquark, or vice versa. In the Standard Model, their decay to two muons can proceed most simply through the virtual exchange of two W bosons, one virtual up quark and a virtual neutrino. The trouble was that the rate for the neutral kaon decay to two muons predicted in this  manner turned out to be many orders of magnitude larger than observed experimentally.

Glashow, Iliopoulos and Maiani (GIM) proposed a simple solution. With visionary insight, they hypothesised a new quark, the charm quark, which would totally cancel the contribution of the up quark to this decay if their masses were equal to each other. As the rate was non-vanishing and the charm quark had not yet been observed experimentally, they concluded that the mass of the charm quark must be significantly larger than that of the up quark.

Their hunch was correct. In early 1974, months before its direct discovery, Mary K Gaillard and Benjamin Lee predicted the charm quark’s mass by analysing another highly suppressed quantity, the mass difference in K⁰–K⁰ mixing.

As modifications to the GIM mechanism by new heavy particles are still a hot prospect for discovering new physics in the 2020s, the details merit a closer look. Years earlier, Nicola Cabibbo had correctly guessed that weak interactions act between up quarks and a mixture (d cos θ + s sin θ) of the down and strange quarks. We now know that charm quarks interact with the mixture (–d sin θ + s cos θ). This is just a rotation of the down and strange quarks through this Cabibbo angle. The minus sign causes the destructive interference observed in the GIM mechanism.

With the discovery of a third generation of quarks, quark mixing is now described by the Cabibbo–Kobayashi–Maskawa (CKM) matrix – a unitary three-dimensional rotation with complex phases that parameterise CP violation. Understanding its parameters may prove central to our ability to discover new physics this decade.

On to the 1980s

The story of indirect discoveries continued in the late 1980s, when the magnitude of B⁰_d – B⁰_d mixing implied the existence of a heavy top quark, which was confirmed in 1995, completing the third generation of quarks. The W, Z and Higgs bosons were also predicted well in advance of their discoveries. It’s only natural to expect that indirect searches for new physics will be successful at even shorter distance scales.

Rare weak decays of kaons and B mesons that are strongly suppressed by the GIM mechanism are expected to play a crucial role. Many channels of interest are predicted by the Standard Model to have branching ratios as low as 10^–11, often being further suppressed by small elements of the CKM matrix. If the GIM mechanism is violated by new-physics contributions, these branching ratios – the fraction of times a particle decays that way – could be much larger.

Measuring suppressed branching ratios with respectable precision this decade is therefore an exciting prospect. Correlations between different branching ratios can be particularly sensitive to new physics and could provide the first hints of physics beyond the Standard Model. A good example is the search for the violation of lepton-flavour universality (CERN Courier May/June 2019 p33). Though hints of departures from muon–electron universality seem to be receding, hints that muon–tau universality may be violated still remain, and the measured branching ratios for B → K(K^*)µ⁺µ^– differ visibly from Standard Model predictions.

The first step in this indirect strategy is to search for discrepancies between theoretical predictions and experimental observables. The main challenge for experimentalists is the low branching ratios for the rare decays in question. However, there are very good prospects for measuring many of these highly suppressed branching ratios in the coming years.

Six channels for the 2020s

Six channels stand out today for their superb potential to observe new physics this decade. If their decay rates defy expectations, the nature of any new physics could be identified by studying the correlations between these six decays and others.

The first two channels are kaon decays: the measurements of K⁺ → π⁺νν by the NA62 collaboration at CERN (see “Needle in a haystack” image), and the measurement of K_L → π⁰νν by the KOTO collaboration at J-PARC in Japan. The branching ratios for these decays are predicted to be in the ballpark of 8 × 10^–11 and 3 × 10^–11, respectively.

The second two are measurements of B → Kνν and B → K^*νν by the Belle II collaboration at KEK in Japan. Branching ratios for these decays are expected to be much higher, in the ballpark of 10^–5.

The final two channels, which are only accessible at the LHC, are measurements of the dimuon decays B_s → µ⁺µ^– and B_d → µ⁺µ^– by the LHCb, CMS and ATLAS collaborations. Their branching ratios are about 4 × 10^–9 and 10^–10 in the Standard Model. Though the decays B → K(K^*)µ⁺µ^–are also promising, they are less theoretically clean than these six.

The main challenge for theorists is to control quantum-chromodynamics (QCD) effects, both below 10^–16 m, where strong interactions weaken, and in the non-perturbative region at distance scales of about 10^–15 m, where quarks are confined in hadrons and calculations become particularly tricky. While satisfactory precision has been achieved at short-distance scales over the past three decades, the situation for non-perturbative computations is expected to improve significantly in the coming years, thanks to lattice QCD and analytic approaches such as dual QCD and chiral perturbation theory for kaon decays, and heavy-quark effective field theory for B decays.

Another challenge is that Standard Model predictions for the branching ratios require values for four CKM parameters that are not predicted by the Standard Model, and which must be measured using kaon and B-meson decays. These are the magnitude of the up-strange (V_us) and charm-bottom (V_cb) couplings and the CP-violating phases β and γ. The current precision on measurements of V_us and β is fully satisfactory, and the error on γ = (63.8 ± 3.5)° should be reduced to 1° by LHCb and Belle II in the coming years. The stumbling block is V_cb, where measurements currently disagree. Though experimental problems have not been excluded, the tension is thought to originate in QCD calculations. While measurements of exclusive decays to specific channels yield 39.21(62) × 10^–3, inclusive measurements integrated over final states yield 41.96(50) × 10^–3. This discrepancy makes the predicted branching ratios differ by 16% for the four B-meson decays, and by 25% and 35% for K⁺ → π⁺νν and K_L → π⁰νν. These discrepancies are a disaster for the theorists who had succeeded over many years of work to reduce QCD uncertainties in these decays to the level of a few percent.

One solution is to replace the CKM dependence of the branching ratios with observables where QCD uncertainties are under good control, for example: the mass differences in B⁰_s −  B⁰_s and B⁰_d −  B⁰_d mixing (∆M_s and ∆M_d); a parameter that measures CP violation in K⁰ – K⁰ mixing (ε_K); and the CP-asymmetry that yields the angle β. Fitting these observables to the experimental data avoids us being forced to choose between inclusive and exclusive values for the charm-bottom coupling, and avoids the 3.5° uncertainty on γ, which in this strategy is reduced to 1.6°. Uncertainty on the predicted branching ratios is thereby reduced to 6% and 9% for B → Kνν and B → K^*νν, to 5% for the two kaon decays, and to 4% for B_s → µ⁺µ^– and B_d → µ⁺µ^–.

So what is the current experimental situation for the six channels? The latest NA62 measurement of K⁺ → π⁺νν is 25% larger than the Standard Model prediction. Its 36% uncertainty signals full compatibility at present, and precludes any conclusions about the size of new physics contributing to this decay. Next year, when the full analysis has been completed, this could turn out to be possible. It is unfortunate that the HIKE proposal was not adopted (CERN Courier May/June 2024 p7), as NA62’s expected precision of 15% could have been reduced to 5%. This could turn out to be crucial for the discovery of new physics in this decay.

The present upper bound on K_L → π⁰νν from KOTO is still two orders of magnitude above the Standard Model prediction. This bound should be lowered by at least one order of magnitude in the coming years. As this decay is fully governed by CP violation, one may expect that new physics will impact it significantly more than CP-conserving decays such as K⁺ → π⁺νν.

Branching out from Belle

At present, the most interesting result concerns a 2023 update from Belle II to the measured branching ratio for B⁺ → K⁺νν (see “Interesting excess” image). The resulting central value from Belle II and BaBar is currently a factor of 2.6 above the Standard Model prediction. This has sparked many theoretical analyses around the world, but the experimental error of 30% once again does not allow for firm conclusions. Measurements of other charge and spin configurations of this decay are pending.

Finally, both dimuon B-meson decays are at present consistent with Standard Model predictions, but significant improvements in experimental precision could still reveal new physics at work, especially in the case of B_d.

It will take a few years to conclude if new physics contributions are evident in these six branching ratios, but the fact that all are now predicted accurately means that we can expect to observe or exclude new physics in them before the end of the decade. This would be much harder if measurements of the V_cb coupling were involved.

So far, so good. But what if the observables that replaced V_cb and γ are themselves affected by new physics? How can they be trusted to make predictions against which rare decay rates can be tested?

Here comes some surprisingly good news: new physics does not appear to be required to simultaneously fit them using our new basis of observables ΔM_d, ε_K and ΔM_s, as they intersect at a single point in the V_cb–γ plane (see “No new physics” figure). This analysis favours the inclusive determination of V_cb and yields a value for γ that is consistent with the experimental world average and a factor of two more accurate. It’s important to stress, though, that non-perturbative four-flavour lattice-QCD calculations of ∆M_s and ∆M_d by the HPQCD lattice collaboration played a key role here. It is crucial that another lattice QCD collaboration repeat these calculations, as the three curves cross at different points in three-flavour calculations that exclude charm.

Interesting years are ahead in the field of indirect searches for new physics

In this context, one realises the advantages of V_cb–γ plots compared to the usual unitarity-triangle plots, where V_cb is not seen and 1° improvements in the determination of γ are difficult to appreciate. In the late 2020s, determining V_cb and γ from tree-level decays will be a central issue, and a combination of V_cb-independent and V_cb-dependent approaches will be needed to identify any concrete model of new physics.

We should therefore hope that the tension between inclusive and exclusive determinations of V_cb will soon be conclusively resolved. Forthcoming measurements of our six rare decays may then reveal new physics at the energy frontier (see “New physics” figure). With a 1° precision measurement of γ on the horizon, and many V_cb-independent ratios available, interesting years are ahead in the field of indirect searches for new physics.

In 1676 Antonie van Leeuwenhoek discovered a microuniverse populated by bacteria, which he called animalcula, or little animals. Let us hope that we will, in this decade, discover new animalcula on our flavour expedition to the zeptouniverse.

The post Six rare decays at the energy frontier appeared first on CERN Courier.

In the Standard Model of particle physics, the three charged lepton flavours couple to the electroweak gauge bosons W and Z with the same strength – an idea known as lepton flavour universality (LFU). This implies that differences in the rates of processes involving W or Z bosons together with electrons, muons and tau leptons should arise only from differences in the leptons’ masses. Experimental results agree with LFU at the 0.1–0.2% level in the decays of tau leptons, kaons and pions, but hints of deviations have been seen in B-meson decays, for example in the combination of measurements of B → D^(*)τν and B → D^(*)μν decays at the BaBar, Belle and LHCb experiments.

The W and Z bosons are so heavy that the probabilities for them to decay to electrons, muons and tau leptons are expected to be equal to very high precision, if LFU holds. This implies that the ratios of these probabilities such as R(μ/e), which compares W → μν and W → eν, and R(τ/μ), which compares W → τν and W → μν, should be unity. Experiments at the LEP electron–positron collider measured a surprisingly large value of R(τ/μ) = 1.070 ± 0.026, but a more precise measurement from the ATLAS collaboration at the LHC found R(τ/μ) = 0.992 ± 0.013, in agreement with LFU. This measurement made use of the large sample of top-quark pair events produced at ATLAS during Run 2 of the LHC from 2015 to 2018. These top-quark events can be cleanly selected, with each event containing two W bosons and two b-quarks produced from the decays of the top quarks.

In a new measurement, ATLAS has turned its attention to the comparison of W decays to muons and electrons, via the ratio R(μ/e). The collaboration again used top-quark pair events as a clean and copious source of W bosons. Counting the number of events with one electron from W → eν, one muon from W → μν, and one or two b-tagged jets, provides the cleanest way to measure the rate of top-quark pair production. But this rate can also be measured from the number of top-quark pair events with two electrons or two muons. If R(μ/e) = 1 and W → eν and W → μν decays occur with equal probability, the rates of such ee and μμ events should be the same, after correcting for detector efficiencies. Any difference would suggest a violation of LFU.

Some measurement uncertainties have similar effects on the ee and μμ final states, so they largely cancel in the ratio R(μ/e). However, electrons and muons behave in very different ways in the ATLAS detector, giving different detection efficiencies with differing and uncorrelated uncertainties that do not cancel in the ratio. To reduce the sensitivity of the measured R(μ/e) to these effects, the double ratio R(μ/e)/√(R(μμ/ee) was measured first, where R(μμ/ee) corresponds to the comparison of Z → μμ and Z → ee decay probabilities, determined from the same dataset. The final R(μ/e) was then obtained by making use of the very precise measurement of R(μμ/ee) from the LEP experiments and the SLD experiment at SLAC, which has an uncertainty of only 0.0028. This latter ratio acts as a calibration of the relative detection efficiencies of electrons and muons in ATLAS, reducing the associated uncertainties in R(μ/e).

The final result from this new ATLAS analysis is R(μ/e) = 0.9995 ± 0.0045, perfectly compatible with unity. The measurement is compared to previous results from LHC and LEP experiments (see figure 1). Thanks to the large data sample and careful control of all systematic uncertainties, it improves on the uncertainty of 0.006 from all previous measurements combined. At least in W decays, LFU survives intact.

The post Zooming in on leptonic W decays appeared first on CERN Courier.

The weak force, unlike other fundamental forces, has a distinctive feature: its interactions slightly differ when involving quarks or antiquarks. This phenomenon, known as CP violation, allows for an asymmetry in the likelihood of a process occurring with matter compared to its antimatter counterpart, which is an essential requirement to explain the large dominance of matter in the universe. However, the size of CP violation predicted by the Standard Model (SM), and in accordance with experimental measurements so far, is not large enough to explain this cosmological imbalance. This is why physicists are actively searching for new sources of CP violation and striving to improve our understanding of the known ones. The phenomenology offered by the quantum-mechanical oscillations of neutral mesons into their antimatter counterparts, the antimesons, provides a particularly rich experimental ground for such studies.

The LHCb collaboration recently measured a set of parameters that determine the matter–antimatter oscillation of the neutral D⁰ meson into the D⁰ anti­meson with unprecedented precision. This enables the search for the predicted hitherto unobserved CP violation in this oscillation.

D⁰ mesons are composed of a charm quark and an up antiquark. Their oscillations are extremely slow, with an oscillation period over a thousand times longer than their lifetimes. As a result, only a very few D⁰ mesons transform before they decay. Oscillations are therefore identified as extremely small changes in the flavour mixture – matter or antimatter – as a function of the time at which the D⁰ or the D⁰ decays.

In LHCb’s analysis, the initial matter–antimatter flavour of the neutral meson is experimentally inferred from the charge of the accompanying pion in the CP-conserving decay chains D^*(2010)⁺ → D⁰π⁺ and D^*(2010)^– → D⁰π^–. The mixing effect (or oscillation) then appears as a decay-time dependence of the ratio, R, of the number of “suppressed” and “favoured” decay processes of the neutral meson. The suppressed decays can occur with or without a net oscillation of the D⁰ meson, while the favoured decays are largely dominated by the direct process. In the absence of mixing, this ratio is predicted to be constant as a function of the D⁰ decay time while, in the case of mixing, it approximately follows a parabolic behaviour, increasing with time. Figure 1 shows the ratio R, including data for both matter (R⁺ for D⁰ → K⁺π^–) and antimatter (R^– for D⁰ → K^–π⁺) processes, and corresponding model predictions. The variation depends not only on the oscillation parameters but also on the various observables of CP violation, which differentiate between matter and antimatter.

This analysis is the most precise measurement of these parameters to date, improving the uncertainty on both mixing and CP-violating observables by a factor of 1.6 compared to the previous best result, also by LHCb. This improvement is largely due to an unpre­cedentedly large sample of about 1.6 million suppressed decays and 421 million favoured decays collected during Run 2, making LHCb unique in probing up-type quark transitions. The results confirm the matter–antimatter oscillation of the D⁰ meson and show no evidence of CP violation in the oscillation.

These findings call for future analyses of this and other decays of the D⁰ meson using data from the third and fourth run of the LHC, exploiting the potential of the currently operating detector upgrade (Upgrade I). The detector upgrade proposed for the fifth and sixth runs of the LHC (Upgrade II) would provide a six-times-bigger sample, yielding the precision needed to definitively test the predictions of the SM.

The post LHCb squeezes D-meson mixing appeared first on CERN Courier.

Rare radiative b-hadron decays are powerful probes of the Standard Model (SM) sensitive to small deviations caused by potential new physics in virtual loops. One such process is the decay of B⁰_s → μ⁺μ^–γ. The dimuon decay of the B⁰_s meson is known to be extremely rare and has been measured with unprecedented precision by LHCb and CMS. While performing this measurement, LHCb also studied the B⁰_s → μ⁺μ^–γ decay, partially reconstructed due to the missing photon, as a background component of the B⁰_s → μ⁺μ^– process and set the first upper limit on its branching fraction to 2.0 × 10^–9 at 95% CL (red arrow in figure 1). However, this search was limited to the high-dimuon-mass region, whereas several theoretical extensions of the SM could manifest themselves in lower regions of the dimuon-mass spectrum. Reconstructing the photon is therefore essential to explore the spectrum thoroughly and probe a wide range of physics scenarios.

The LHCb collaboration now reports the first search for the B⁰_s → μ⁺μ^–γ decay with a reconstructed photon, exploring the full dimuon mass spectrum. Photon reconstruction poses additional experimental challenges, such as degrading the mass resolution of the B⁰_s candidate and introducing additional background contributions. To cope with this ambitious search, machine-learning algorithms and new variables have been specifically designed with the aim of discriminating the signal among background processes with similar signatures. The analysis is performed separately for three dimuon mass ranges to exploit any differences along the spectrum, such as the ϕ(1020) meson contribution in the low invariant mass region. The μ⁺μ^–γ invariant mass distributions of the selected candidates are fitted, including all background contributions and the B⁰_s → μ⁺μ^–γ signal component. Figure 2 shows the fit for the lowest dimuon mass region.

No significant signal of B⁰_s → μ⁺μ^–γ is found in any of the three dimuon mass regions, consistent with the background-only hypothesis. Upper bounds on the branching fraction are set and can be seen as the black arrows in figure 1. The mass fit is also performed for the combined candidates of the three dimuon mass regions to set a combined upper limit on the branching fraction to 2.8 × 10^–8 at 95% CL.

The SM theoretical predictions of b decays becomes particularly difficult to calculate when a photon is involved, and they have large uncertainties due to the B⁰_s → γ local form factors. The B⁰_s → μ⁺μ^–γ decay provides a unique opportunity to validate the different theoretical approaches, which do not agree with each other, as shown by the coloured bands in figure 1. Theoretical calculations of the branching fractions are currently below the experimental limits. The upgraded LHCb detector and the increased luminosity of the LHC’s Run 3 is currently providing conditions for studying rare radiative b-hadron decays with greater precision and, eventually, for finding evidence for the B⁰_s → μ⁺μ^–γ decay.

The post LHCb targets rare radiative decay appeared first on CERN Courier.

BESIII is an experiment situated on the BEPCII storage ring at IHEP in Beijing. It involves more than 600 physicists drawn not only from China but also other nations, including Germany, Italy, Poland, the Netherlands, Sweden and the UK from the CERN member states. The detector has collected data at a range of running points with centre-of-mass energies from 1.8 to 4.95 GeV, most of which are inaccessible to other operating colliders. This energy regime allows researchers to make largely unique studies of physics above and below the charm threshold, and has led to important discoveries and measurements in light-meson spectroscopy, non-perturbative QCD, and charm and tau physics.

The ψ(3770), discovered at SLAC in 1977, is the lightest charmonium state above the open-charm threshold. Charmonium consists of a bound charm quark and anti-charm quark, whereas open-charm states such as D⁰ and D⁺ mesons are systems in which the charm quark co-exists with a different anti-quark. The ψ(3770) can decay into D and anti-D mesons, whereas charmonium states below threshold, such as the J/ψ, are too light to do so, and must instead decay through annihilation of the charm and anticharm quarks.

The sample is more than 20 times larger than the world’s previous charm-threshold data set

Open-charm mesons are also produced in copious quantities at the LHC and at Belle II. However, in ψ(3770) decays at BESIII they are produced in pairs, with no accompanying particles. This makes the BESIII sample a uniquely clean laboratory in which to study the properties of D mesons. If one meson is reconstructed, or tagged, in a known charm decay, the other meson in the event can be analysed in an unbiased manner. When reconstructed in a decay of interest, the unbiased sample of mesons can be used to measure absolute branching fractions and the relative phases between any intermediate resonances in the D decay.

“Both sets of information are not only interesting in themselves, but also vital for studies with charm and beauty mesons at LHCb and Belle II,” explains Guy Wilkinson of the University of Oxford. “For example, measurements of phase information performed by BESIII with the first tranche of ψ(3770) data have been essential input in the world-leading determination of the CP-violating angle γ of the unitarity triangle by LHCb in events where a beauty meson decays into a D meson and an accompanying kaon.” Exploitation of the full 20 fb^–1 sample will be essential in helping LHCb and Belle II realise their full potential in CP-violation measurements with larger data sets in the future, he adds. “Hence BESIII is very complementary to the higher energy experiments, demonstrating the strong synergies that exist between particle-physics facilities worldwide.”

This summer, BEPCII will undergo an upgrade that will increase its luminosity. Over the rest of the decade more data will be taken above and below the charm threshold. In the longer term, there are plans, elsewhere in China, for a Super Tau Charm Facility – an accelerator that would build on the BEPCII and BESIII programme with datasets that are two orders of magnitude larger.

The post BESIII passes milestone at the charm threshold appeared first on CERN Courier.

The opening talk by Monica Pepe Altarelli underscored LHCb’s diverse physics programme, solidifying its role as a highly versatile forward detector. While celebrating successes, her talk candidly addressed setbacks, notably the new results in tests of lepton-flavour universality. LHCb detector and computing upgrades for Run 3 include a fully software-based trigger using graphics processing units. The collaboration is also working towards an Upgrade II programme for Long Shutdown 4 (2033–2034) that would position LHCb as a potentially unique global flavour facility.

On mixing and CP violation, the October workshop unveiled intriguing insights in both the beauty and charm sectors. In the beauty sector, notable highlights encompass measurements of the mixing parameter ΔΓ_s and of CP-violating phases such as ϕ_s,d, ϕ_s^sss and γ. CP asymmetries were further scrutinised in B → DD decays, accounting for SU(3) breaking and re-scattering effects. In the charm sector, the estimated CP asymmetries considering final-state interactions were found to be small compared to the experimental values related to D⁰ → π^–π⁺ and D⁰ → K^–K⁺ decays. Novel measurements of CP violation in three-body charm hadron decays were also presented.

Unique capabilities

On the theoretical front, discussions delved into the current status of bottom-baryon lifetimes. Recent lattice predictions on the ε_K parameter were also showcased, offering refined constraints on the unitarity triangle. The LHCb experiment’s unique capabilities were discussed in the heavy ions and fixed-target session. Operating in fixed-target mode, LHCb collected data pertaining to proton–ion and lead–ion interactions during LHC Run 2 using the SMOG system. Key highlights included measurements impacting theoretical models of charm hadronisation, global analyses of nuclear parton density functions, and the identification of helium nuclei and deuterons. The first Run 3 data with the SMOG2 upgrade showed promising results in proton–argon and proton–hydrogen collisions, opening a path to measurements with implications for heavy-ion physics and astrophysics.

The session on flavour-changing charged currents unveiled a recent measurement concerning the longitudinal polarisation of D^*− mesons in B⁰ → D^*−τ^–ν_τ decays, aligning with Standard Model (SM) expectations. Discussions delved into lepton-flavour-universality tests that showed a 3.3σ tension with predictions in the combined R(D^(*)) measurement. Noteworthy were new lattice-QCD predictions for charged current decays, especially R(D^(*)), showcasing disparities in the SM prediction across different lattice groups. Updates on the CKM matrix elements |V_ub| and |V_cb| lead to a reduced tension between inclusive and exclusive determinations. The session also discussed the impact of high-energy constraints of Wilson coefficients on charged-current decays and Bayesian inference of form-factor parameters, regulated by unitarity and analyticity. The QCD spectroscopy and exotics session also featured important findings, including the discovery of novel baryon states, notably Ξ_b(6087)⁰ and Ξ_b(6095)⁰. Pentaquark exploration involved diverse charm–hadron combinations, alongside precision measurements of the Ω⁰_c mass and first observations of b-hadron decays with potential exotic-state contributions. Charmonia-associated production provided fresh insights for testing QCD predictions, and an approach based on effective field theory (EFT) interpreting pentaquarks as hadronic molecules was presented. A new model-independent Born–Oppenheimer EFT framework for the interpretation of doubly heavy tetraquarks, utilising lattice QCD predictions, was introduced. Scrutinising charm–tetraquark decays and the interpretation of newly discovered hadron states at the LHC were also discussed.

During the flavour-changing neutral-current session a new analysis of B⁰ → K^*0μ⁺μ^– decays was presented, showing consistency with SM expectations. Stringent limits on branching fractions of rare charm decays and precise differential branching fraction measurements of b-baryon decays were also highlighted. Challenges in SM predictions for b → sℓℓ and rare charm decays were discussed, underscoring the imperative for a deeper comprehension of underlying hadronic processes, particularly leveraging LHCb data. Global analyses of b → dℓℓ and b → sℓℓ decays were presented, alongside future prospects for these decays in Run 3 and beyond. The session also explored strategies to enhance sensitivity to new physics in B^± → π^±μ⁺μ^– decays.

The keynote talk, delivered by Svjetlana Fajfer, offered a comprehensive summary and highlighted existing anomalies that demand further consideration. Tackling these challenges necessitates precise measurements at both low and high energies, with the collaborative efforts of LHCb, Belle II, CMS and ATLAS. Additionally, advancements in lattice QCD and other novel theoretical approaches are needed for precise theoretical predictions in tandem with experimental efforts.

The post Tango for two: LHCb and theory appeared first on CERN Courier.

Measurements using neutral B⁰ and B⁰_s mesons are particularly powerful because they resolve ambiguities that other measurements cannot. Due to the interference between B⁰_(s) – B⁰_(S) mixing and decay amplitudes, the physical CP-violating parameters in these decays are functions of a combination of γ and the relevant mixing phase, namely γ + 2β in the B⁰ system, where β = arg(–V_cd V^*_cb/ V_td V^*_tb), and γ–2β_s in the B⁰_s system, where β_s = arg(–V_ts V^*_tb/ V_td V^*_tb). Measurements of these physical quantities can therefore be interpreted in terms of the angles γ and β_(s), and γ can be derived using independent determinations of the other parameter as input.

The LHCb collaboration recently presented a new measurement of B⁰_s → D^∓_s K^± decays collected during Run 2. This is a challenging analysis, as it requires a decay time-dependent fit to extract the CP-violating observables expressed as amplitudes of the four different decay paths that arise from B⁰_s and – B⁰_s to D^∓_s K^± final states. Previously, LHCb measured γ in this decay using the Run 1 dataset, obtaining γ = 128 ⁺¹⁷_–22°. The B⁰_s – B⁰_s oscillation frequency ∆m_s must be precisely constrained in order to determine the phase differences between the amplitudes. In the Run 2 measurement, the established uncertainty on ∆m_s would have been a limiting systematic uncertainty, which motivated the recent LHCb measurement of ∆m_s using the flavour-specific B⁰_s → D^–_s π⁺ decays from the same dataset. Combined with Run 1 measurements of ∆m_s, this has led to the most precise contribution to the world average and has greatly improved the precision on γ in the B⁰_s → D^∓_s K^± analysis. Indeed, for the first time the four amplitudes are resolved with sufficient precision to show the decay rates separately (see figure 1).

The angle γ is determined using inputs from other LHCb measurements of the CP-violating weak phase –2β_s, along with measurements of the decay width and decay-width difference. The final result, γ = 74 ± 11°, is compatible with the SM and is the most precise determination of γ using B⁰_s meson decays to date.

The post Resolving asymmetries in B⁰ and B⁰_s oscillations appeared first on CERN Courier.

Kaons, which contain one strange and either a lighter up or down quark, have played a central role in the development of the Standard Model (SM). Augusto Ceccucci (CERN) pointed out that many of the SM’s salient features – including flavour mixing, parity violation, the charm quark and CP violation – were discovered through the study of kaons, leading to the Cabibbo-Kobayashi-Maskawa (CKM) quark mixing matrix. The full particle content of the SM was finally experimentally established at CERN with the Higgs-boson discovery in 2012, but many open questions remain.

The kaon’s special role in this context was the central topic of the workshop. The study of rare kaon decays provides a unique sensitivity to new physics, up to  scales higher than those at collider experiments. In the SM, the rare decay of a charged or neutral kaon into a pion plus a pair of charged or neutral leptons is strongly suppressed, even more so than the similar rare B-meson decays. This is due to the absence at tree-level of flavour-changing neutral current interactions (e.g. s → d) in the SM. Such a transition can only proceed at loop level involving the creation of at least one very heavy (virtual) electroweak gauge boson (figure “Decayed”, left). While experimentally this suppression constitutes a formidable challenge in identifying the decay products amongst a variety of background signals, new-physics contributions could leave a significantly measurable imprint through tree-level or virtual contributions. In contrast to rare B decays, the “gold-plated” rare kaon decay channels K⁺→π⁺νν and K_L→π⁰νν do not suffer from large hadronic uncertainties and are experimentally clean due to the limited number of possible decay channels.

The charged-kaon decay is currently being studied at NA62, and a measurement of its branching ratio with a precision of 15% is expected by 2025. However, as highlighted by NA62 physics coordinator Karim Massri (Lancaster University), to improve this measurement and thus significantly increase the  likelihood of a discovery, the experimental precision must be reduced to the level of the theoretical prediction, i.e. 5%. This can only be achieved with a next-generation experiment. The HIKE experiment, a proposed high-intensity kaon factory at CERN currently under approval, would reach the 5% precision goal on the measurement of K⁺→π⁺νν during its first phase of operation. experiment, a future high-intensity kaon factory at CERN currently under approval, will reach the 5% precision goal on the measurement of K⁺→π⁺νν during its first phase of operation. Afterwards, a second phase with a neutral K_L beam aiming at the first observation of the very rare decays K_L→π⁰ℓ⁺ℓ^– is foreseen. With a setup and detectors optimised for the measurement of the most challenging processes, the HIKE programme would be able to achieve unprecedented precision on most K⁺ and K_L decays.

For KOTO, Koji Shimi and Hashime Nanjo reported on the experimental progress on K_L→π⁰ℓ⁺ℓ^– and presented a new bound on its branching ratio. A planned phase two of KOTO, if funded, aims to measure the branching ratio with a precision of 20%. Although principally designed for the study of (rare) bottom-quark decays, LHCb can also provide information about the rare decay of the shorter-lived K_S.Radoslav Marchevski (EPFL Lausanne) presented the status and the prospects for a proposed LHCb-Phase II upgrade.

From the theory perspective, underpinned by impressive new perturbative, lattice QCD and effective-field-theory calculations presented at the workshop, the planned measurement of K⁺→π⁺νν at HIKE clearly has discovery potential, remarked Gino Isidori (University of Zurich). Together with other rare decay channels such as K_L→μ⁺μ^–, K_L→π⁰ℓ⁺ℓ^– and K⁺→π⁺ℓ⁺ℓ^– that would be measured by HIKE, added Giancarlo D’Ambrosio (INFN), the combined global theory analyses of experimental data will allow for discovering new physics if it exists within the reach of the experiment, and for providing solid constraints for new physics.

A decision on HIKE and other proposed experiments in CERN’s North Area will take place in early December.

The post Kaon physics at a turning point appeared first on CERN Courier.

At the European Physical Society Conference on High Energy Physics, held in Hamburg in August, the LHCb collaboration announced first results on the production of antihelium and antihypertriton nuclei in proton–proton (pp) collisions at the LHC. These promising results open a new research field, that up to now has been pioneered by ground-breaking work from the ALICE collaboration on the central rapidity interval |y| < 0.5. By extending the measurements into the so-far unexplored forward region 1.0 < y < 4.0, the LHCb results provide new experimental input to derive the production cross sections of antimatter particles formed in pp collisions, which are not calculable from first principles.

LHCb’s newly developed helium-identification technique mainly exploits information from energy losses through ionisation in the silicon sensors upstream (VELO and TT stations) and downstream (Inner Tracker) of the LHCb magnet. The amplitude measurements from up to ~50 silicon layers are combined for each subdetector into a log-likelihood estimator. In addition, timing information from the Outer Tracker and velocity measurements from the RICH detectors are used to improve the separation power between heavy helium nuclei (with charge Z = 2) and lighter, singly charged particles (mostly charged pions). With a signal efficiency of about 50%, a nearly background-free sample of 1.1 × 10⁵ helium and antihelium nuclei is identified in the data collected during LHC Run 2 from 2016 to 2018 (see figure, inset).

The helium identification method proves the feasibility of new research fields at LHCb

As a first step towards a light-nuclei physics programme in LHCb, hypertritons are reconstructed via their two-body decay into a now-identified helium nucleus and a charged pion. Hypertriton (³_ΛH) is a bound state of a proton, a neutron and a Λ hyperon that can be produced via coalescence in pp collisions. These states provide experimental access to the hyperon–nucleon interaction through the measurement of their lifetime and of their binding energy. Hyperon–nucleon interactions have significant implications for the understanding of astrophysical objects such as neutron stars. For example, the presence of hypernuclei in the dense inner core can significantly suppress the formation of high-mass neutron stars. As a result, there is some tension between the observation of neutron stars heavier than two solar masses and corresponding hypertriton results from the STAR collaboration at Brookhaven. ALICE seems to have resolved the tension between hypertriton measurements at colliders and neutron stars. An independent confirmation of the ALICE result has up to now been missing, and can be provided by LHCb.

The invariant-mass distribution of hypertriton and antihypertriton candidates is shown in figure 1. More than 100 signal decays are reconstructed, with a statistical uncertainty on the mass of 0.16 MeV, similar to that of STAR. In a next step, corrections for efficiencies and acceptance obtained from simulation, as well as systematic uncertainties on the mass scale and lifetime measurement, will be derived.

The new helium identification method from LHCb summarised here proves the feasibility of a rich programme of measurements in QCD and astrophysics involving light antinuclei in the coming years. The collaboration also plans to apply the method to other LHCb Run 2 datasets, such as proton–ion, ion–ion and SMOG collision data.

The post Antinuclei production in pp collisions appeared first on CERN Courier.

The key focus of the conference series is to review the latest results in heavy-flavour physics and discuss future directions. Heavy-flavour decays, in particular those of hadrons that contain b quarks, offer powerful probes of physics beyond the Standard Model (SM). Beauty 2023 took place 30 years after the opening meeting in the series. A dedicated session was devoted to reflections on the developments in flavour physics over this period, and also celebrating the life of Sheldon Stone, who passed away in October 2021. Sheldon was both an inspirational figure in flavour physics as a whole, a driving force behind the CLEO, BTeV and LHCb experiments, and a long-term supporter of the Beauty conference series.

LHC results
Many important results have emerged from the LHC since the last Beauty conference. One concerns the CP-violating parameter sin2β, for which measurements by the BaBar and Belle experiments at the start of the millennium marked the dawn of the modern flavour-physics era.  LHCb has now measured sin2β with a precision better than any other experiment, to match its achievement for ϕ_s, the analogous parameter in B_s⁰ decays, where ATLAS and CMS have also made a major contribution. Continued improvements in the knowledge of these fundamental parameters will be vital in probing for other sources of CP violation beyond the SM.

Over the past decade, the community has been intrigued by strong hints of the breakdown of lepton-flavour universality, one of the guiding tenets of the SM, in B decays. Following a recent update from LHCb, it seems that lepton universality may remain a good symmetry, at least in the class of electroweak-penguin decays such as B→K^(*)l⁺l^–, where much of the excitement was focused (CERN Courier January/February 2023 p7). Nonetheless, there remain puzzles to be understood in this sector of flavour physics, and anomalies are emerging elsewhere. For example, non-leptonic decays of the kind B_s→ D_s ⁺K^– show intriguing patterns through CP-violation and decay-rate information.

The July conference was noteworthy as being a showcase for the first major results to emerge from the Belle II experiment. Belle II has now collected 362 fb^-1 of integrated luminosity on the Υ(4S) resonance, which constitutes a dataset similar in size to that accumulated by BaBar and the original Belle experiment, and results were shown from early tranches of this sample.  In some cases, these results already match or exceed in sensitivity and precision what was achieved at the first generation of B-factory experiments, or indeed elsewhere. These advances can be attributed to improved instrumentation and analysis techniques. For example, world-leading measurements of the lifetimes of several charm hadrons were presented, including the D⁰, D⁺, D_s⁺ and Λ_c⁺. Belle II and its accelerator, SuperKEKB, will emerge from a year-long shutdown in December with the goal to increase the dataset by a factor of 10-20 in the coming half decade.

Full of promise
The future experimental programme of flavour physics is full of promise. In addition to the upcoming riches expected from Belle II, an upgraded LHCb detector is being commissioned in order to collect significantly larger event samples over the coming decade. Upgrades to ATLAS and CMS will enhance these experiments’ capabilities in flavour physics during the High-Luminosity LHC era, for which a second upgrade to LHCb is also foreseen. Conference participants also learned of the exciting possibilities for flavour physics at the proposed future collider FCC-ee, where samples of several 10¹² Z⁰ decays will open the door to ultra-precise measurements in an analysis environment much cleaner than at the LHC. These projects will be complemented by continued exploration of the kaon sector, and studies at the charm threshold for which a high-luminosity Super Tau Charm Factory is proposed in China.

The scientific programme of Beauty 2023 was complemented by outreach events in the city, including a `Pints of Science’ evening and a public lecture, as well as a variety of social events. These and the stimulating presentations made the conference a huge success, demonstrating that flavour remains a vibrant field and continues to be a key player in the search for new physics beyond the Standard Model.

The post Beauty in the Auvergne appeared first on CERN Courier.

Charge-parity (CP) violation parameters in tree-dominated b → c c s quark transitions are a powerful probe of physics beyond the Standard Model (SM). When B⁰_(s) and B⁰_(s) mesons decay through these transitions to the same final-state particles, an interference between mixing and decay amplitudes occurs, making these processes particularly sensitive to CP violation.

In the SM, B⁰_(s) – B⁰_(s) mixing is possible because the flavour eigenstates are not the (physical) mass eigenstates: a neutral B meson, once produced, evolves as a quantum superposition of B⁰_(s) and B⁰_(s) states. Due to this time-dependent mixing amplitude, an interference between mixing and decay amplitudes can lead to an observable time-dependent CP asymmetry in the decay rates. It was through the observation of this phenomenon in the “golden mode” B⁰ → J/ψ K⁰_s that, in 2001, the BaBar and Belle collaborations reported the first unequivocal evidence for CP violation in B decays, for which Kobayashi and Maskawa were awarded the 2008 Nobel Prize in Physics.

As the 3 × 3 Cabibbo–Kobayashi–Maskawa (CKM) matrix that describes quark mixing in the SM is expected to be unitary, it leads to relations among its complex elements. These can be represented as triangles in a complex plane, all of them with the same area (which is a measure of the amount of CP violation in the SM). The most famous of them, the so-called unitary triangle, has sides of roughly the same size and internal angles denoted as α, β and γ. Since individually none of the CKM parameters are predicted by theory, the search for new physics relies on over-constraining them by looking for any hint of internal inconsistency. For that, precision is the key.

LHCb has become a major actor in precision studies of CP violation

Having analysed the full proton–proton collision data set with 13 TeV, and adding it to previous measurements at 7 and 8 TeV, LHCb recently brought the CP-violating parameters in B⁰ → J/ψ K⁰_s  and in another golden channel, B⁰_s → J/ψ K⁺K^–, to a new level of precision. These parameters (sin2β and φ_s, respectively) are predicted with high accuracy through global CKM fits and, given their clean experimental signatures, are paramount for new-physics searches. The measured time-dependent CP asymmetry of B⁰ and B⁰ decay rates is shown in figure 1 with the resulting amplitude proportional to sin2β. Similarly, the update of the B⁰_s → J/ψ K⁺K^– analysis with the 13 TeV data resulted in the world’s most precise φ_s measurement. Both angles agree with SM expectations and with previous measurements.

These legacy results for sin2β and φ_s from the first LHC runs represent a new milestone in LHCb’s hunt for physics beyond the SM. Along with the world-leading determination of γ (with a current precision of less than four degrees), and the discovery of CP violation in charm in 2019, LHCb has fulfilled and exceeded its own goals of more than a decade ago, becoming the major actor in precision studies of CP violation. LHCb is taking data with a brand new detector at larger interaction rates than before, boosting the experimental sensitivity and tightening the grip around the Standard Model.

The post CP studies open windows on new physics appeared first on CERN Courier.

A crucial missing piece in our understanding of quantum chromodynamics (QCD) is a complete description of hadronisation in hard scattering processes with a large momentum transfer, which has now been investigated by the LHCb collaboration in proton–lead (pPb) collisions. While perturbative QCD describes reasonably well the transverse momentum (p_T) dependence of heavy-quark production in proton–proton (pp) collisions, the situation is different in heavy-ion collisions due to the formation of quark–gluon plasma (QGP), which affects the behaviour of particles traversing the medium. In particular, hadronisation can be affected, modifying the relative abundance of hadrons compared to pp collisions. Several models predict an enhanced strange-quark production. Thus an abundance of strange baryons is seen as a signature of QGP formation.

The role that QGP may play in pPb collisions is currently unclear. Some models predict the formation of “QGP droplets”, which could partially induce the same behaviour, albeit less pronounced, as in PbPb collisions. In addition, in pPb interactions, “cold nuclear matter” (CNM) effects are also present that can mimic the behaviour caused by QGP but via different mechanisms. For all these reasons, a strangeness enhancement in pPb collisions would strongly indicate the formation of a deconfined medium in small systems, providing crucial information about QGP properties and formation once the CNM effects are under control.

The LHCb collaboration recently analysed pPb data for QGP effects with the twofold purpose of searching for strangeness enhancement and providing a precise understanding of the CNM effects. This search was performed by measuring the production ratio of the strange baryon Ξ⁺_c, which has never been observed in pPb collisions before, to the strangeless baryon Λ⁺_c. Using an earlier pPb sample, LHCb has also studied the ratios of the D⁺_s, D⁺ and D⁰ , the first being measured for the first time down to zero p_T in the forward region, precisely addressing CNM effects. All measurements are performed differentially in p_T and the rapidity of the produced particle, and compared to the latest theory predictions. The Ξ⁺_c cross section has been measured for the first time in pPb collisions, giving strong indications on the factorisation scale μ₀ of the theory model. This result allows to set the absolute scale of the theoretical computations in terms of strangeness production, a trend confirmed with even higher precision by comparing the measurement to the Λ⁺_c production-cross section evaluated in the same decay mode. Moreover, the ratio is roughly constant as a function of p_T  and behaves in the same way at positive (pPb) and negative (Pbp) rapidities (see figure 1). The measurement is consistent with models incorporating initial-state effects due to gluon-shadowing in nuclei, suggesting that QGP formation and the resulting strangeness enhancement have little or no effect on Ξ⁺_c production in pPb collisions.

This interpretation is confirmed by the measurement of the D⁺_s, D⁺ and D⁰ cross sections and corresponding ratios in different rapidity regions. While the ratios show little enhancement within the statistical uncertainty, a large asymmetry is observed in the forward-backward production. This strongly indicates CNM effects and provides detailed constraints on models of nuclear parton distribution functions and hadron production in a very wide range of Bjorken-x (10^–2– 10^–5). A strong suppression is observed for the D mesons, giving insight into the nature of the CNM effects involved. An explanation via additional final-state effects is challenged by the Ξ⁺_c data that are well described by models not including them. The production ratios of Ξ⁺_c, D⁺_s, D⁺ and D⁰ measured as a function of p_T in pPb collisions confirm these findings. All these studies will profit from the increased statistics in pPb collisions that are expected from future LHC runs.

The post Charm production in proton–lead collisions appeared first on CERN Courier.

At a CERN seminar on 13 June, the LHCb collaboration presented the world’s most precise measurements of two key parameters relating to CP violation. Based on the full LHCb dataset collected during LHC Runs 1 and 2, the first concerns the observable sin2β while the second concerns the CP-violating phase φ_s – both of which are highly sensitive to potential new-physics contributions. 

CP violation was first observed in 1964 in kaon mixing, and confirmed among B mesons in 2001 by the e⁺e^– B-factory experiments BaBar and Belle. The latter enabled the first measurements of sin2β and were a vital confirmation of the Standard Model (SM). In the SM, CP violation arises due to a complex phase in the Cabibbo–Kobayashi–Maskawa mixing matrix, which, being unitary, defines a triangle in the complex plane: one side is defined to have unit length, while the other two sides and three angles must be inferred via measurements of certain hadron decays. If the measurements do not provide a consistent description of the triangle, it would hint that something is amiss in the SM. 

The measurement of sin2β, which determines the angle β in the unitarity triangle, is more difficult at a hadron collider than it is at an e⁺e^– collider. However, the large data samples available at the LHC and the optimised design of the LHCb experiment have enabled a measurement that is twice as precise as the previous best result from Belle. The LHCb team used decays of B⁰ mesons to J/ψ K⁰_S, which can proceed either directly or by first oscillating into their antimatter partners. The interference between the amplitudes for the two decay paths results in a time-dependent asymmetry between the decay-time distributions of the B⁰ and B⁰. The amplitude of the oscillation, and thus the magnitude of CP violation present, is a measurement of sin2β for which LHCb finds a value of 0.716 ± 0.013 ± 0.008, in agreement with predictions.

Based on an analysis of B⁰_S → J/ψ K⁺K^– decays, LHCb also presented the world’s best measurement of the CP-violating phase φ_s, which plays a similar role in B⁰_S meson decays as sin2β does in B⁰ decays. As for B⁰ mesons, a B⁰_S may decay directly or oscillate into a B⁰_S and then decay. CP violation causes these decays to proceed at slightly different rates, manifesting itself as a non-zero value of φ_s due to the interference between mixing and decay. The predicted value of φ_s is about –0.037 rad, but new-physics effects, even if also small, could change its value significantly.

A detailed study of the angular distribution of B⁰_S decay products using the Run 1 and 2 data samples enabled LHCb to measure this decay-time-dependent CP asymmetry φ_s = -0.039 ± 0.022 ± 0.006 rad. Representing the most precise single measurement to date, it is consistent with previous measurements and with the SM expectation. The precision measurement of φ_s is one of LHCb’s most important goals, said co-presenter Vukan Jevtic (TU Dortmund): “Together with sin2β, the new LHCb result marks an important advance in the quest to understand the nature and origin of CP violation.” 

With both results currently limited by statistics, the collaboration is looking forward to data from the current and future LHC runs. “In Run 3 LHCb will collect a larger data sample taking advantage of the new upgraded LHCb detector,” concluded co-presenter Peilian Li (CERN). “This will allow even higher precision and therefore the possibility to detect, through these key quantities, the manifestation of new-physics effects.”

The post LHCb sets record precision on CP violation appeared first on CERN Courier.

At the recent Moriond Electroweak conference, the LHCb collaboration presented a new, high-precision measurement of charge–parity (CP) violation using a large sample of B⁰_s → ϕϕ decays, where the ϕ mesons are reconstructed in the K⁺K^– final state. Proceeding via a loop transition (b → sss, such “penguin” decays are highly sensitive to possible contributions from unknown particles and therefore provide excellent probes for new sources of CP violation. To date, the only known source of CP violation, which is governed by the Cabibbo–Kobayashi–Maskawa matrix in the quark sector, is insufficient to account for the huge excess of matter over antimatter in the universe; extra sources of CP violation are required.

A B⁰_s or B⁰_s meson can change its flavour and oscillate into its antiparticle at a frequency Δm_s/2π, which has been precisely determined by the LHCb experiment. Thus a B⁰_s meson can decay either directly to the ϕϕ state or via changing its flavour to the B–⁰_s state. The phase difference between the two interfering amplitudes changes sign under CP transformations, denoted ϕ_s for B⁰_s or –ϕ_s for B⁰_s decays. A time-dependent CP asymmetry can arise if the phase difference ϕ_s is nonzero. The asymmetry between the decay rates of initial B⁰_s and B⁰_s mesons to the ϕϕ state as a function of the decay time follows a sine wave with amplitude sin(ϕ_s) and frequency Δm_s/2π. In the Standard Model (SM) the phase difference is predicted to be consistent with zero, ϕ^SM_s  = 0.00 ± 0.02 rad.

This is the most precise single measurement to date

The observed asymmetry as a function of the B⁰_s → ϕϕ decay time and the projection of the best fit are shown in figure 1 for the Run 2 data sample. The measured asymmetry is diluted by the finite decay-time resolution and the nonzero flavour mis-identification rate of the initial B⁰_s or B⁰_s state, and averaged over two types of linear polarisation states of the ϕϕ system that have CP asymmetries with opposite signs. Taking these effects into account, LHCb measured the CP-violating phase using the full Run 2 data sample. The result, when combined with the Run 1 measurement, is ϕ_s = –0.074 ± 0.069 rad, which agrees with the SM prediction and improves significantly upon the previous LHCb measurement. In addition to the increased data sample size, the new analysis benefits from improvements in the algorithms for vertex reconstruction and determination of the initial flavour of the B⁰_s or B⁰_s mesons.

This is the most precise single measurement to date of time-dependent CP asymmetry in any b → s transition. With no evidence for CP violation, the result can be used to derive stringent constraints on the parameter space of physics beyond the SM. Looking to the future, the upgraded LHCb experiment and a planned future phase II upgrade will offer unique opportunities to further explore new-physics effects in b → s decays, which could potentially provide insights into the fundamental origin of the puzzling matter–antimatter asymmetry.

The post New insights into CP violation via penguin decays appeared first on CERN Courier.

The LHCb collaboration is never idle. While building and commissioning its brand new Upgrade I detector, which entered operation last year with the start of LHC Run 3, planning for Upgrade II was already under way. This proposed new detector, envisioned to be installed during Long Shutdown 4 in time for High-Luminosity LHC (HL-LHC) operations continuing in Run 5, scheduled to begin in 2034/2035, would operate at a peak luminosity of 1.5 × 10³⁴ cm^–2 s^–1. This is 7.5 times higher than at Run 3 and would generate data samples of heavy-flavoured hadron decays six times larger than those obtainable at the LHC, allowing the collaboration to explore a wide range of flavour-physics observables with extreme precision. Unprecedented tests of the CP-violation paradigm (see “On point” figure) and searches for new physics at double the mass scales possible during Run 3 are among the physics goals on offer. 

Attaining the same excellent performance as the original detector has been a pivotal constraint in the design of LHCb Upgrade I. While achieving the same in the much harsher collision environments at the HL-LHC remains the guiding principle for Upgrade II, the LHCb collaboration is investigating the possibilities to go even further. And these challenges need to be met while keeping the existing footprint and arrangement of the detector (see “Looking forward” figure). Radiation-hard and fast 3D silicon pixels, a new generation of extremely fast and efficient photodetectors, and front-end electronics chips based on 28 nm semiconductor technology are just a few examples of the innovations foreseen for LHCb Upgrade II, and will also set the direction of R&D for future experiments.

Rethinking the data acquisition, trigger and data processing, along with intense use of hardware accelerators such as field-programmable gate arrays (FPGAs) and graphics processing units (GPUs), will be fundamental to manage the expected five-times higher average data rate than in Upgrade I. The Upgrade II “framework technical design report”, completed in 2022, is also the first to consider the experiment’s energy consumption and greenhouse-gas emissions, as part of a close collaboration with CERN to define an effective environmental protection strategy.

Extreme tracking 

At the maximum expected luminosity of the HL-LHC, around 2000 charged particles will be produced per bunch crossing within the LHCb apparatus. Efficiently reconstructing these particles and their associated decay vertices in real time represents a significant challenge. It requires the existing detector components to be modified to increase the granularity, reduce the amount of material and benefit from the use of precision timing.

The future VELO will be a true 4D-tracking detector

The new Vertex Locator (VELO) will be based, as it was for Upgrade I (CERN Courier May/June 2022 p38), on high-granularity pixels operated in vacuum in close proximity to the LHC beams. For Upgrade II, the trigger and online reconstruction will rely on the selection of events, or parts of events, with displaced tracks at the early stage of the event. The VELO must therefore be capable of independently reconstructing primary vertices and identifying displaced tracks, while coping with a dramatic increase in event rate and radiation dose. Excellent spatial resolution will not be sufficient, given the large density of primary interactions along the beam axis expected under HL-LHC conditions. A new coordinate – time – must be introduced. The future VELO will be a true 4D-tracking detector that includes timing information with a precision of better than 50 ps per hit, leading to a track time-stamp resolution of about 20 ps (see “Precision timing” figure). 

The new VELO sensors, which include 28 nm technology application-specific integrated circuits (ASICs), will need to achieve this time resolution while being radiation-hard. The important goal of a 10 ps time resolution has recently been achieved with irradiated prototype 3D-trench silicon sensors. Depending on the rate-capability of the new detectors, the pitch may have to be reduced and the mat­erial budget significantly decreased to reach comparable spatial resolution to the current Run 3 detector. The VELO mechanics have to be redesigned, in particular to reduce the material of the radio-frequency foil that separates the secondary vacuum – where the sensors are located – from the machine vacuum. The detector must be built with micron-level precision to control systematic uncertainties.

The tracking system will take advantage of a detector located upstream of the dipole magnet, the Upstream Tracker (UT), and of a detector made of three tracking stations, the Mighty Tracker (MT), located downstream of the magnet. In conjunction with the VELO, the tracking system ensures the ability to reconstruct the trajectory of charged particles bending through the detector due to the magnetic field, and provides a high-precision momentum measurement for each particle. The track direction is a necessary input to the photon-ring searches in Ring Imaging Cherenkov (RICH) detectors, which identify the particle species. Efficient real-time charged-particle reconstruction in a very high particle-density environment requires not only good detector efficiency and granularity, but also the ability to quickly reject combinations of hits not produced by the same particle. 

The UT and the inner region of the MT will be instrumented with high-granularity silicon pixels. The emerging radiation-hard monolithic active pixel sensor (MAPS) technology is a strong candidate for these detectors. LHCb Upgrade II would represent the first large-scale implementation of MAPS in a high-radiation environment, with the first prototypes currently being tested (see “Mighty pixels” figure). The outer region of the MT will be covered by scintillating fibres, as in Run 3, with significant developments foreseen to cope with the radiation damage. The availability of high-precision vertical-coordinate hit information in the tracking, provided for the first time in LHCb by pixels in the high-occupancy regions of the tracker, will be crucial to reject combinations of track segments or hits not produced by the same particle. To substantially extend the coverage of the tracking system to lower momenta, with consequent gains for physics measurements, the internal surfaces of the magnet side walls will be instrumented with scintillating bar detectors, the so-called magnet stations (MS). 

Extreme particle identification 

A key factor in the success of the LHCb experiment has been its excellent particle identification (PID) capabilities. PID is crucial to distinguish different decays with final-state topologies that are backgrounds to each other, and to tag the flavour of beauty mesons at production, which is a vital ingredient to many mixing and CP-violation measurements. For particle momenta from a few GeV/c up to 100 GeV/c, efficient hadron identification at LHCb is provided by two RICH detectors. Cherenkov light emitted by particles traversing the gaseous radiators of the RICHes is projected by mirrors onto a plane of photodetectors. To maintain Upgrade I performances, the maximum occupancy over the photodetector plane must be kept below 30%, the single-photon Cherenkov-angle resolution must be below 0.5 mrad, and the time resolution on single-photon hits should be well below 100 ps (see “RICH rewards” figure). 

Next-generation silicon photomultipliers (SiPMs) with improved timing and a pixel size of 1 × 1 mm², together with re-optimised optics, are deemed capable of delivering these specifications. The high “dark” rates of SiPMs, especially after elevated radiation doses, would be controlled with cryogenic cooling and neutron shielding. Vacuum tubes based on micro-channel plates (MCPs) are a potential alternative due to their excellent time resolution (30 ps) for single-photon hits and lower dark rate, but suffer in high-rate environments. New eco-friendly gaseous radiators with a lower refractive index can improve the PID performance at higher momenta (above 80 GeV/c), but meta-materials such as photonic crystals are also being studied. In the momentum region below 10 GeV/c, PID will profit from TORCH – an innovative 30 m² time-of-flight detector consisting of quartz plates where charged particles produce Cherenkov light. The light propagates by internal reflection to arrays of high-granularity MCP–PMTs optimised to operate at high rates, with a prototype already showing performances close to the target of 70 ps per photon.

Excellent photon and π⁰ reconstruction and e–π separation are provided by LHCb’s electromagnetic calorimeter (ECAL). But the harsh occupancy conditions of the HL-LHC impose the development of 5D calorimetry, which complements precise position and energy measurements of electromagnetic clusters with a time resolution of about 20 ps. The most crowded inner regions will be equipped with so-called spaghetti calorimeter (SPACAL) technology, which consists of arrays of scintillating fibres either made of plastic or garnet crystals arranged along the beam direction, embedded in a lead or tungsten matrix. The less-crowded outer regions of the calorimeter will continue to be instrumented with the current “Shashlik” technology with refurbished modules and increased granularity. A timing layer, either based on MCPs or on alternated tungsten and silicon-sensor layers placed within the front and back ECAL sections, is also a possibility to achieve the ultimate time resolution. Several SPACAL prototypes have already demonstrated that time resolutions down to an impressive 15 ps are feasible (see “Spaghetti calorimetry” image).

The final main LHCb subdetector is the muon system, based on four stations of multiwire proportional chambers (MWPCs) interleaved with iron absorbers. For Upgrade II, it is proposed that MWPCs in the inner regions, where the rate will be as high as a few MHz/cm², are replaced with new-generation micro-pattern gaseous detectors, the micro-RWELL, a prototype of which has proved able to reach a detection efficiency of approximately 97% and a rate-capability of around 10 MHz/cm². The outer regions, characterised by lower rates, will be instrumented either by reusing a large fraction (∼95%) of the current MWPCs or by implementing other solutions based on resistive plate chambers or scintillating-tile-based detectors. As with all Upgrade II subdetectors, dedicated ASICs in the front-end electronics, which integrate fast time-to-digital converters or high-frequency waveform samplers, will be necessary to measure time with the required precision.

Trigger and computing 

The detectors for LHCb Upgrade II will produce data at a rate of up to 200 Tbit/s (see “On the up” figure), which for practical reasons needs to be reduced by four orders of magnitude before being written to permanent storage. The data acquisition therefore needs to be reliable, scalable and cost-efficient. It will consist of a single type of custom-made readout board combined with readily available data-centre hardware. The readout boards collect the data from the various sub-detectors using the radiation-hard, low-power GBit transceiver links developed at CERN and transfer the data to a farm of readout servers via next- generation “PCI Express” connections or Ethernet. For every collision, the information from the subdetectors is merged by passing through a local area network to the builder server farm.

With up to 40 proton–proton interactions, every bunch crossing at the HL-LHC will contain multiple heavy-flavour hadrons within the LHCb acceptance. For efficient event selection, hits not associated with the proton–proton collision of interest need to be discarded as early as possible in the data-processing chain. The real-time analysis system performs reconstruction and data reduction in two high-level-trigger (HLT) stages. HLT1 performs track reconstruction and partial PID to apply inclusive selections, after which the data is stored in a large disk buffer while alignment and calibration tasks run in semi real-time. The final data reduction occurs at the HLT2 level, with exclusive selections based on full offline-quality event reconstruction. Starting from Upgrade I, all HLT1 algorithms are running on a farm of GPUs, which enabled, for the first time at the LHC, track reconstruction to be performed at a rate of 30 MHz. The HLT2 sequence, on the other hand, is run on a farm of CPU servers – a model that would be prohibitively costly for Upgrade II. Given the current evolution of processor performance, the baseline approach for Upgrade II is to perform the reconstruction algorithms of both HLT1 and HLT2 on GPUs. A strong R&D activity is also foreseen to explore alternative co-processors such as FPGAs and new emerging architectures.

The second computing challenge for LHCb Upgrade II derives from detector simulations. A naive extrapolation from the computing needs of the current detector implies that 2.5 million cores will be needed for simulation in Run 5, which is one order of magnitude above what is available with a flat budget assuming a 10% performance increase of processors per year. All experiments in high-energy physics face this challenge, motivating a vigorous R&D programme across the community to improve the processing time of simulation tools such as GEANT4, both by exploiting co-processors and by parametrising the detector response with machine-learning algorithms.

Intimately linked with digital technologies today are energy consumption and efficiency. Already in Run 3, the GPU-based HLT1 is up to 30% more energy-efficient than the originally planned CPU-based version. The data centre is designed for the highest energy-efficiency, resulting in a power usage that compares favourably with other large computing centres. Also for Upgrade II, special focus will be placed on designing efficient code and fully exploiting efficient technologies, as well as designing a compact data acquisition system and optimally using the data centre.

A flavour of the future 

The LHC is a remarkable machine that has already made a paradigm-shifting discovery with the observation of the Higgs boson. Exploration of the flavour-physics domain, which is a complementary but equally powerful way to search for new particles in high-energy collisions, is essential to pursue the next major milestone. The proposed LHCb Upgrade II detector will be able to accomplish this by exploring energy scales well beyond those reachable by direct searches. The proposal has received strong support from the 2020 update of the European strategy for particle physics, and the framework technical design report was positively reviewed by the LHC experiments committee. The challenges of performing precision flavour physics in the very harsh conditions of the HL-LHC are daunting, triggering a vast R&D programme at the forefront of technology. The goal of the LHCb teams is to begin construction of all detector components in the next few years, ready to install the new detector at the time of Long Shutdown 4.

The post LHCb looks forward to the 2030s appeared first on CERN Courier.

The Cabibbo–Kobayashi–Maskawa (CKM) matrix describes the couplings between the quarks and the weak charged current, and contains within it a phase γ that changes sign under consideration of antiquarks rather than quarks. In the Standard Model (SM), this phase is the only known difference in the interactions of matter and anti-matter, a consequence of the breaking of charge-parity (CP) symmetry. While the differences within the SM are known to be far too small to explain the matter-dominated universe, it is still of paramount importance to precisely determine this phase to provide a benchmark against which any contribution from new physics can be compared. 

A new measurement recently presented by the LHCb collaboration uses a novel method to determine γ using decays of the type B^± → D[K^∓π^±π^±π^∓]h^± (h = π, K). CP violation in such decays is a consequence of the interference between two tree-level processes with a weak phase that differs by γ, and thus provide a theor­etically clean probe of the SM. The new aspect of this measurement compared to those performed previously lies in the partitioning of the five-dimensional phase space of the D-decay into a series of independent regions, or bins. In these bins, the asymmetries between B⁺ and B^– meson decay rates can receive large enhancements from the hadronic interactions in the D-meson decay. The enhancement for one of such bins can be seen in figure 1, which shows the invariant mass spectrum of the B⁺ and B^– meson candidates, where the correctly reconstructed decays peak at around 5.3 GeV. The observed asymmetry in this region is around 85%, which is the largest difference in the behaviour of matter and antimatter ever measured. Observables from the different bins are combined with information on the hadronic interactions in the D-meson decay from charm-threshold experiments to obtain γ = 55 ± 9°, which is compatible with  previous determinations and is the second most precise single measurement.

The matter–antimatter asymmetry reaches 85% in a certain region, the largest ever observed

The LHCb average value of γ is then determined by combining this analysis with the measurements in many other B and D decays, where in all cases the SM contribution is expected to be dominant. Measurements of charm decays are also included to better constrain both the parameters of charm mixing, which also play an important role in the measurements of B-meson decays at the current level of precision and help to constrain the hadronic interactions in some of the D decays. In particular, included for the first time in this combination is a measurement of y_CP, which is proportional to the difference in lifetimes of the two neutral charm mesons, and was determined using two-body decays of the D meson using the entire LHCb data set collected so far. 

The overall impact of these additional analyses reduces the uncertainty on γ by more than 10%, corresponding to adding around a year of data taking across all decay modes. 

The improvements in the knowledge of y_CP is also dramatic, reducing the uncertainty by around 40%. While the value of γ is found to be compatible with determinations that would be more susceptible to new physics, the precision of the comparison is starting to approach the level of a few degrees, at which discrepancies may start to be observable. 

Given that the current uncertainties on many of the key input analyses to the combination are predominately statistical in nature, measurements of these fundamental flavour-physics parameters with the upgraded LHCb detector, and beyond, are an intriguing prospect for new-physics searches.

The post B to D decays reduce uncertainty on γ appeared first on CERN Courier.

At a seminar held at CERN today, the LHCb collaboration presented new measurements of rare B-meson decays that provide a high-precision test of lepton flavour universality, a key feature of the Standard Model (SM). Previous studies of these decays had hinted at intriguing tensions with predictions, but the results of an improved and wider-reaching analysis of the full LHCb dataset are in agreement with the SM.

A central mystery of particle physics is why the 12 elementary quarks and leptons are arranged in pairs across three generations, identical in all but mass. Lepton flavour universality (LFU) states that the SM gauge bosons are indifferent to which generation a charged lepton belongs, implying that certain decays of hadrons involving leptons from different generations should occur at the same rates. In recent years, however, an accumulation of results has suggested a possible violation of LFU in B-meson decays involving fundamental b- to s-quark transitions, such as the decay of a B into a K meson. Such processes are highly suppressed in the SM because they proceed through higher-order diagrams, making them promising channels in which to detect the possible influence of new particles.

A powerful test of LFU is to measure the relative rates of the processes B → Kμ⁺μ^– and B → Ke⁺e^–, a quantity called R(K), and the equivalent ratio for decays involving an excited kaon, R(K*). The SM predicts such ratios to be equal to unity once differences in the lepton masses are accounted for. In 2021, based on data collected during LHC Run 1 and Run 2, LHCb found R(K) to lie 3.1 σ below the SM prediction. For R(K*), measurements in 2017 based on Run 1 data were consistent with the SM at the level of 2–2.5 σ.

Earlier LHCb indications of anomalies with lepton flavour universality triggered immense excitement

The latest LHCb analysis simultaneously measures R(K) and R(K*) using the full Run 1 and Run 2 datasets. A sequence of multivariate selections and strict particle-identification requirements produced a higher signal purity and a better statistical sensitivity than the previous analysis. The two ratios were also computed in two bins of the squared di-lepton momentum-transfer q², thereby producing four independent measurements. The measured values of R(K) and R(K*) are now compatible with the SM within 1 σ and supersede previous LHCb publications on these topics. The new value of R(K*) is based on an integrated luminosity three times larger than that used in 2017, and the two results are in broad agreement. For R(K) in the central q² region, on the other hand, the new value is significantly higher than the 2021 result.

“Although a component of this shift can be attributed to statistical effects, it is understood that this change is primarily due to systematic effects,” explains LHCb spokesperson Chris Parkes of the University of Manchester. “The systematic shift in R(K) in the central q² region compared to the 2021 result stems from an improved understanding of misidentified hadronic backgrounds to electrons, due to an underestimation of such backgrounds and the description of the distribution of these components in the fit. New datasets will allow us to further research this interesting topic, along with other key measurements relevant to the flavour anomalies.”

The search goes on

The flavour anomalies are a set of discrepancies observed over the past several years in processes involving b → s and b → c quark transitions. Among the former is the parameter P₅′ based on angular distributions of the decay products of B-meson decays. Although these remain unaffected by the new LHCb result, tests of LFU via R(K)-type measurements are theoretically cleaner. On 18 October, complementing previous results by Belle, BaBar and LHCb, the LHCb collaboration made the first simultaneous measurement at a hadron collider of the parameter R(D), which compares the rates of B → Dτν and B → Dμν decays, and its counterpart R(D*). Involving b → c quark transitions, such decays proceed via the tree-level exchange of a virtual W boson. Based on Run 1 data, the new values of R(D) and R(D*) are compatible both with the current world average and with the SM prediction at 2.2 σ and 2.3 σ, respectively.

“Earlier LHCb indications of anomalies with lepton flavour universality triggered immense excitement, not least because possible new-physics explanations resonated with other hints of deviations from the SM,” says CERN theorist Michelangelo Mangano. “That such anomalies could have been real shows how little we know about the deep origin of flavour symmetries and their relation with the Higgs, and highlights the key role of experimental guidance. Theoretical efforts to interpret the anomalies explored novel avenues, exposing a myriad of unanticipated phenomena possibly emerging at distances shorter than those so far described by the SM. The latest LHCb findings take nothing away from our mission to push further the boundary of our knowledge, and the search for anomalies goes on!”

The post LHCb brings leptons into line appeared first on CERN Courier.

The conference opened with an overview of the LHCb experiment, where the first milestones of the Upgrade I commissioning were presented. The new fully software trigger scheme of LHCb, with the highest data processing scheme of any LHC experiment, has been successfully implemented for the full LHCb detector.

The hot-off-the-press result on the simultaneous determination of the ratios R(D*)= BR(B→D*τ^–ν_τ)/BR(B→D*μ^–ν_μ) and R(D)= BR(B^–→D⁰τ^–ν_τ)/BR(B^–→D⁰μ^–ν_μ) was shown. This result, which superseded the previous LHCb measurement, is 1.9 σ away from the Standard Model (SM) expectation. Another highlight was the first observation of the decay Λ⁰_b→Λ⁺_cτ^–ν_τ, and its use to test lepton flavour universality using the ratio of the tauonic to muonic decay, R(Λ⁺_c), which is yet in agreement with the SM. The newest precision extractions of the moduli of the Cabibbo-Kobayashi-Maskawa (CKM) matrix elements |V_cb| and |V_ub| were discussed, showing that the long-standing puzzle of inclusive versus exclusive measurements keeps being a hot topic with many upcoming developments in the near future.

Within the mixing and CP violation (CPV) stream, a major highlight was the measurement of the time-integrated CP asymmetry in D^o→K⁺K^– decays, leading to the first determination of the direct CP asymmetries in both D^o→K⁺K^– and D^o→π⁺π^– in the latter case constituting the first evidence for CPV in a single charm decay. These results led to exciting discussions about the size of U-spin breaking and possible underlying mechanisms. A new theoretical methodology for the derivation of amplitude U-spin sum rules was presented, making sum rules feasible for any system at any order in the expansion in the symmetry-breaking terms.

Further major results were the determination of the charm mixing parameters
y_CP-y^Kπ_CP, very large local CP asymmetries seen in B⁺→h⁺h’^– h’⁺ (with h,h’=π,K), as well as a new simultaneous determination of the weak phase γ together with charm mixing and decay parameters. On the theory side, it was also presented the completion of the next-to-next-to-leading order (NNLO) QCD calculation of the width difference of B^o_s mesons, allowing for an improved comparison with the corresponding experimental results.

The versatility of LHCb was showcased by covering rare beauty, kaon and charm decays, along with new tests on lepton-flavor universality violation

The rare decays session again showcased the versatility of LHCb by covering rare beauty, kaon and charm decays, including the most recent results on lepton-flavor universality violation. Recent progress on handling QCD corrections of b→sℓ⁺ℓ^–, which are important for the interpretation of the B anomalies, were presented, and it was shown a new method for the extraction of CKM matrix elements using time-dependent kaon decays. Lots of future opportunities lie in the measurements of rare charmed baryon decays which are very little probed so far.

New exciting results were shown in the spectroscopy stream, where one new pentaquark and three new tetraquark states were presented, showing the leading contribution of LHCb to the discovery of exotics and yet-not-understood states. Progress on QCD predictions along with new data-driven and machine-learning based methods were discussed.

The BSM session gave a great overview of a diverse range of beyond-the-SM models including leptoquarks, Z’ models, axion-like particles (ALPs) as well as models with extra dimensions. Importantly, these models induce correlations between the B anomalies and other anomalies like g-2 or the Cabibbo angle anomaly. Complementary and partially competitive constraints on the viable model space come from direct searches and high-p_T observables.

Interesting discussions took place in the electroweak precision-measurements session, where the LHCb W-boson mass measurement was presented, which is in line with the world average and in tension with the recent precise CDF measurement at the 4σ level. This measurement will soon be complemented with the full Run 2 dataset.

The workshop closed with a grand overview given in the keynote talk by Alexander Lenz. The next instance of the Implications Workshop will take place at CERN in October 2023.

The post LHCb experiment meets theory appeared first on CERN Courier.

Studies of rare B-meson decays at the LHC provide a sensitive probe of physics beyond the Standard Model (SM) and allow us to explore energy scales much higher than those directly accessible. A key factor in the success of these studies is the availability of precise theoretical predictions that can be compared with experimentally accessible processes. The dimuon decays B⁰_S → μ⁺μ^– and B⁰ → μ⁺μ^– are a case in point. In particular, studies of these decays could help researchers to understand the nature of several anomalies seen in other rare B-meson decays.

The CMS collaboration recently reported a new measurement of the B⁰_S → μ⁺μ^– branching fraction and effective lifetime, as well as the result of a search for the B⁰ → μ⁺μ^– decay, using data recorded during LHC Run 2. This new study benefits not only from a large event sample but also from advanced machine-learning algorithms, which are used to uncover the rare signal events out of the overwhelming background. The B⁰_S → μ⁺μ^– signal is very clearly seen (see figure 1), leading to more precise measurements than previously achieved. The B⁰_S → μ⁺μ^– branching fraction is measured to be (3.8 ± 0.4) × 10^–9, the relative uncertainty of 11% being a remarkable improvement with respect to that of the previous CMS result, 23%.

This measured value is consistent with the SM prediction of (3.7 ± 0.1) × 10^–9, and reduces a previous tension between theory and experiment, which was based on the combination of the previous CMS result with the ATLAS and LHCb values. The variation in the central value of the CMS measurements is mostly driven by the use of a larger data sample and by the change of the B-hadron fragmentation fraction ratio (by about 8%). The measured effective lifetime of the B⁰_S → μ⁺μ^–  decay, 1.8 ± 0.2 ps, is also consistent with the SM prediction. The precision of this measurement is approaching the level necessary to probe the CP properties of B⁰_S → μ⁺μ^–, which could differ from the SM prediction. Finally, the B⁰ → μ⁺μ^– decay remains unseen.

CMS physicists are looking forward to continuing these rare-decay studies with the large data samples to be collected during LHC Run 3. Besides the improved precision expected for B⁰_S → μ⁺μ^– measurements, seeing the first evidence of B⁰ → μ⁺μ^– is high on their wish list.

The post Rare B-meson decays to two muons appeared first on CERN Courier.

The LHCb experiment is designed to study heavy-flavour particles containing beauty and charm quarks. Nevertheless, thanks to the large strangeness production cross-sections at the LHC as well as the excellent reconstruction performance of LHCb at low momenta, the experiment is also able to produce precise results in strange decays, complementary to those from dedicated experiments such as NA62 and KOTO. The collaboration has recently released a “trillio-scale” upper limit on the branching fraction of the decay K⁰_S → μ⁺μ^–μ⁺μ^–, being the first at this scale at the LHC. The same dataset was used to search for K⁰_L → μ⁺μ^–μ⁺μ^–, yielding the world best upper limit f and the first LHC result on a K⁰_L decay.

According to the Standard Model (SM), K⁰_S (K⁰_L) mesons decay into four muons at a very small rate of a few 10^–14 (10^–13). The decay rates of these processes are very sensitive to possible contributions from new, yet-to-be discovered particles such as dark photons, which could significantly enhance or suppress the decay rate via quantum interference with the SM amplitude. Despite the unprecedented K⁰-meson production rate at the LHC, performing this search is challenging due to the low transverse momentum (typically a few hundred MeV) of the muons. LHCb exploits its unique capability to select, in real time, low transverse-momentum muons – a capability that has improved in recent years thanks to the versatility of its online trigger system. The analysis used machine learning to discriminate long-lived particles from combinatorial background, as well as a data-driven and detailed map of the detector material around the interaction point. The invariant mass of the four-muon system is used as a control variable to statistically separate the potential signal from the remaining combinatorial background.

No selected event consistent with the decay of K⁰_S into four muons, which should appear in the region around the K⁰_S mass of 498 MeV, was observed (see figure 1). In the absence of a signal, upper limits on the respective branching fractions are set to 5.1 × 10^–12 for the K⁰_S decay mode and 2.3 × 10^–9 for the K⁰_L mode at 90% CL. These results represent the world’s most precise searches for these decays, and the branching fraction for K⁰_S → μ⁺μ^–μ⁺μ^– is the most stringent upper limit on a K⁰_S decay mode. 

The upgraded LHCb detector, which started data-taking this year, offers excellent opportunities to further improve the search precision and eventually find evidence of this decay. In addition to the increased luminosity, the LHCb upgrade has a fully software trigger, which is expected to significantly improve the efficiency for K⁰ decays into four muons and other decays with very soft final-state particles.

The post Spotting kaon decays into four muons appeared first on CERN Courier.

Lepton-flavour universality holds that aside from mass differences, all interactions must couple identically to different leptons. As such, the rate of B-meson decays to different leptons is expected to be the same, apart from known differences due to their different masses. Global fits of R(D^(*)) measurements, which probe b → c quark transitions, show that the ratio of B-meson to D-meson decays tends to be larger (by about 3.2 σ) than the SM prediction. The ratios of electronic to muonic B-meson decays, R(K), which probe b → s quark transitions, are also under scrutiny to test this basic principle of the SM.

To reconstruct b → cτ^– ν_τ decays, LHCb used the leptonic τ^–→μ^–νν decay to identify the visible decay products D^(*) and µ^–. “We use the measurement of the B flight direction to constrain the kinematics of the unreconstructed particles, and with an approximation reconstruct the rest frame kinematic quantities,” says LHCb’s Greg Ciezarek, who presented the results. “The challenge is then to understand the modelling of the various background processes which also produce the same visible decay products but have additional missing particles different distributions in the rest frame quantities. We use control samples selected based on these missing particles to constrain the modelling of background processes and justify our level of understanding.”

The respective SM predictions for the ratios R(D) and R(D*) are very clean because they are independent of uncertainties induced by the CKM-matrix element V_cb and hadronic matrix elements. The new values of R(D) and R(D*) are compatible both with the current world average compiled by the HFLAV collaboration, and with the SM prediction (at 2.2σ and 2.3σ). The combined LHCb result provides improved sensitivity to a possible lepton-universality breaking process.

“Rare B-meson decays and ratios such as R(K) and R(D^(*)) are powerful probes to search for beyond the Standard Model particles, which are not directly detectable at the LHC,” says Ben Allanach, theorist at the University of Cambridge.

The post LHCb tests lepton-flavour universality in b → c transitions appeared first on CERN Courier.

With so many new hadronic states being discovered at the LHC (67 and counting, with the vast majority seen by LHCb), it can be difficult to keep track of what’s what. While most are variations of known mesons and baryons, LHCb is uncovering an increasing number of exotic hadrons, namely tetraquarks and pentaquarks. A case in point is its recent discovery, announced at CERN on 5 July, of a new strange pentaquark (with quark content ccuds) and a new tetraquark pair: one constituting the first doubly charged open-charm tetraquark (csud) and the other a neutral isospin partner (csud). The situation has prompted the LHCb collaboration to introduce a new naming scheme. “We’re creating ‘particle zoo 2.0’,” says Niels Tuning, LHCb physics coordinator. “We’re witnessing a period of discovery similar to the 1950s, when a ‘zoo’ of hadrons ultimately led to the quark model of conventional hadrons in the 1960s.” 

While the quark model allows the existence of multiquark states beyond two- and three-quark mesons and baryons, the traditional naming scheme for hadrons doesn’t make much allowance for what these particles should be called. When the first tetraquark candidate was discovered at the Belle experiment in 2003, it was denoted by “X” because it didn’t seem to be a conventional charmonium state. Shortly afterwards, a similarly mysterious but different state turned up at BaBar and was denoted “Y”. Subsequent exotic states seen at Belle and BESIII were dubbed “Z”, and more recently tetraquarks discovered at LHCb were labelled “T”. 

Complicating matters further, the subscripts added to differentiate between the various states lack consistency. For example, the first known tetraquark states contained both charm and anticharm quarks, so a subscript “c” was added. But the recent discoveries of tetraquarks and pentaquarks containing a single strange quark require an extra subscript “s”. On top of all of that, explains LHCb’s Tim Gershon, who initiated the new naming scheme, tetraquarks discovered by LHCb in 2020 contain a single charm quark. “We couldn’t assign the subscript ‘c’ because we’ve always used that to denote states containing charm and anticharm, so we didn’t know what symbols to use,” he explains. “Things were starting to become a bit confusing, so we thought it was time to bring some kind of logic to the naming scheme. We have done this over an extended period, not only within LHCb but also involving other experiments and theorists in this field.”  

Helpfully, the new proposal labels all tetraquarks “T” and all pentaquarks “P”, with a set of rules regarding the necessary subscripts and superscripts. In this scheme, the two different spin states of the open-charm tetraquarks discovered by LHCb in 2020 become T_cs0(2900)⁰ and T_cs1(2900)⁰ instead of X₀(2900)⁰ and X₁(2900)⁰, for example, while the latest pentaquark is denoted P^Λ_ψs(4338)⁰. The collaboration hopes that the new scheme, which can be extended to six- or seven-quark hadrons, will make it easier for experts to communicate while also helping newcomers to the field. 

The new scheme could make it easier to spot patterns that might have been missed before

Importantly, it could make it easier to spot patterns that might have been missed before, perhaps shedding light on the central question of whether exotic hadrons are compact tightly bound multi-quark states or more loosely bound molecular-like states. The new LHCb scheme might even help researchers predict new exotic hadrons, just as the multiplets arising from the quark model made it possible to predict new mesons and baryons such as the Ω^–. 

“Before this new scheme it was almost like a Tower of Babel situation where it was difficult to communicate,” says Gershon. “We have created a document that people can use as a kind of dictionary, in the hope that it will help the field to progress more rapidly.” 

The post Exotic hadrons brought into order by LHCb appeared first on CERN Courier.

As the LHCb experiment prepares for data taking with an upgraded detector for LHC Run 3, the rich harvest of results using data collected in Run 1 and Run 2 of the LHC continues.

A fascinating area of study is the quantum-mechanical oscillation of neutral mesons between their particle and antiparticle states, implying a coupled system of two mesons with different lifetimes. The phenomenology of the B_s system is particularly interesting as it provides a sensitive probe to physics beyond the Standard Model. A B_s meson oscillates with a frequency of about 3 × 10¹² Hz, or on average about nine times during its lifetime, τ. In addition, a sizeable difference between the decay widths of the heavy (Γ_H) and light (Γ_L) mass eigenstates is expected. Measuring the lifetime of a CP-even B_s-decay mode determines τ_L = 1/Γ_L. 

LHCb has recently released a new and precise measurement of this parameter, making use of B_s → J/ψη decays selected from 5.7 fb^–1 of Run 2 data. The study improves the previous Run 1 precision by a factor of two. Due to the combinatorial background, the reconstruction of the η meson via its two-photon decay mode is a particular challenge for this analysis. Despite this, and even with the modest energy resolution of the calorimeter leading to a relatively broad mass peak overlapping partially with the signal from the B⁰ → J/ψη decay, a competitive accuracy has been achieved. By exploiting the latest machine-learning techniques to reduce the background and the well understood LHCb detector, the B_s → J/ψη decay is observed (figure 1), and τ_L is extracted from a two-dimensional fit to the mass and decay time.

The analysis finds τ_L = 1.445 ± 0.016 (stat) ± 0.008 (syst) ps, which is the most precise measurement of this quantity. Combined with the LHCb Run 1 study of this and the B_s → D_s⁺ D_s^– decay mode, τ_L = 1.437 ± 0.014 ps, which agrees well both with the Standard Model expectation (τ_L = 1.422 ± 0.013 ps) and the value inferred from measurements of Γ_s and ΔΓ_s in B_s → J/ψφ decays. Further improvement in the knowledge of τ_L is expected both by considering other CP-even B_s decays to final states containing η or η′ mesons, the B_s → D_s⁺ D_s^– dataset collected during Run 2 and from the upcoming Run 3. 

The post Determining the lifetime of the B_s appeared first on CERN Courier.

To explain the large matter–antimatter asymmetry in the universe, the laws of nature need to be asymmetric under a combination of charge-conjugation (C) and parity (P) transformations. The Standard Model (SM) provides a mechanism for CP violation, but it is insufficient to explain the observed baryon asymmetry in the universe. Thus, searching for new sources of CP violation is important.

The non-invariance of the fundamental forces under CP can lead to different rates between a particle and an antiparticle decay. The CP violation in the decay of a particle is quantified through the parameter A_CP, equal to the relative difference between the decay rate of the process and the decay rate of the CP-conjugated process. Three years ago, the LHCb collaboration reported the first observation of CP violation in the decay of charmed hadrons by measuring the difference between the time-integrated A_CP in D⁰ → K^–K⁺ and D⁰ → π^– π⁺ decays, ΔA_CP. This difference was found to lie at the upper end of the SM expectation, prompting renewed interest in the charm-physics community. There is now an ongoing effort to understand whether this signal is consistent with the SM or a sign of new physics.

At the 41^st ICHEP conference in Bologna on 7 July, the LHCb collaboration announced a new measurement of the individual time-integrated CP asymmetry in the D⁰ → K^–K⁺ decay using the data sample collected during LHC Run 2. The measured value, A_CP(K^–K⁺) = [6.8 ± 5.4 (stat) ± 1.6 (syst)] × 10^–4, is almost three times more precise than the previous LHCb determination obtained with Run 1 data. This was thanks not only to a larger data sample but also the inclusion of additional control channels D_s⁺ → K^– K⁺ π⁺ and D_s⁺ → K_s⁰ K⁺. Together with the previous control channels, D⁺ → K^– π⁺ π⁺ and D⁺ → K_s⁰ π⁺, these decays allow the separation between tiny signals of CP asymmetries from the much larger bias due to the asymmetric meson production and instrumental effects.

The combination of the measured values with the previously obtained ones of A_CP(K^–K⁺) and ΔA_CP by LHCb allowed the determination of the direct CP asymmetries in the D⁰ → π^– π⁺ and D⁰ → K^– K⁺ decays: [23.2 ± 6.1] × 10^–4 and [7.7 ± 5.7] × 10^–4, respectively, with correlated uncertainties (ρ = 0.88). This is the first evidence of direct CP violation in an individual charm–hadron decay (D⁰ → π^– π⁺), with a significance of 3.8σ.

The sum of the two direct asymmetries, which is expected to be equal to 0 in the limit of s–d quark symmetry (called U-spin symmetry), is equal to [30.8 ± 11.4] × 10^–4. This corresponds to a departure from U-spin symmetry of 2.7σ. In addition, this result is essential to the theory community in the quest to clarify the theoretical picture of CP-violation in the charm system. Since the measurement is statistically limited, its precision will improve with the larger dataset collected during Run 3.

The post LHCb digs deeper in CP-violating charm decays appeared first on CERN Courier.

The famous “November revolution” in particle physics in winter 1974 was sparked by the discovery of the charm quark by two independent groups at Brookhaven and SLAC. It signalled the existence of a second generation of fermions, and was therefore a milestone in establishing the Standard Model (SM). Less widely known is that, four years earlier, the Glashow–Iliopoulos–Maiani (GIM) mechanism had postulated the existence of the charm quark to explain the smallness of the K⁰ → μ⁺μ^– branching fraction. In addition, in the summer of 1974, the puzzling smallness of the mass difference between neutral kaons, which was apparent from kaon mixing, led Gaillard and Lee to conclude, correctly, that the charm mass should be below 1.5 GeV. 

Many historical discoveries in particle physics have followed this pattern: a measurement in flavour physics generated a theoretical breakthrough, which in turn led to a direct discovery. The 1977 discovery of the beauty quark at Fermilab was a confirmation of the Cabibbo–Kobayashi–Maskawa (CKM) mechanism postulating the existence of three generations of fermions, which was put forward following the experimental discovery of CP violation in the kaon system in 1964. In 1987, hints of a surprisingly large value for the top-quark mass were inferred from the first measurement of B⁰-meson oscillations at the Argus experiment, and confirmed in 1995 by the discovery of the top quark at the Tevatron. 

This critical role of the flavour sector in particle physics is by no means accidental. Since new particles can contribute virtually via loops or box diagrams, precision measurements in flavour physics in tandem with precise theoretical predictions can provide sensitive probes to indirectly search for new particles or interactions at high energy scales. Could the historical role of flavour measurements in elucidating new-particle discoveries be about to repeat itself at the LHC? 

The flavour promise

Following the Higgs-boson discovery in 2012, the next target at the LHC was clear: to search for an indisputable sign of an eagerly awaited mass peak as a signature for a new particle beyond the SM. So far, however, it seems nature might have something else in store. To unearth the new physics that is strongly motivated to exist – to explain phenomena such as the arbitrary mass hierarchy of elementary particles, the matter–antimatter imbalance in the universe and the origin of the CKM matrix – we should also consider the historically successful route though flavour physics. 

Flavour processes are governed by loop diagrams such as “box” and “penguin” diagrams (see “Virtual production” figure), in which new heavy particles can contribute virtually and alter our expectations. The key word here is “virtually”. This peculiarity of quantum physics allows us to probe new physics at very high energy scales, even if the collision energy is not sufficient to produce new particles directly. Any significant discrepancy between flavour measurements and theoretical calculations would provide us with a valuable lead towards hidden new physics. 

Cooking up a storm

On the experimental side, the main ingredient required is a large sample of beauty and charm hadrons. This makes the LHC, and the LHCb experiment in particular, the ideal place to carefully test the flavour structure of the SM. Not only does the LHC have a record energy reach, it also combines a large production cross-section for beauty and charm hadrons with a very high instantaneous luminosity. There is one catch, however. Due to the nature of quantum-chromodynamics, a large number of hadrons are produced in proton–proton collisions, saturating the different sub-detectors (see “Asymmetric complexity” image). Flavour measurements require a full understanding of this complex event environment, which is a much more challenging task compared to that at e⁺e^– colliders where only a low number of particles is produced in each collision.

Since the inauguration of the LHC, its four main experiments have discovered more than 50 new hadronic states. Most follow the expected pattern of the original quark model, whereas some are new forms of matter such as the doubly-heavy “tetraquark” T_cc⁺ or bound states of five quarks, the so-called pentaquarks, discovered by LHCb. Since the early planning of the LHC, the mission of the flavour community was to better understand the behaviour of beauty and charm quarks. Indeed, in 2019 LHCb was the first single experiment to observe the mixing and CP violation of neutral charm mesons. Similarly for beauty decays, the first observation of time-integrated and time-dependent CP-violating B_s decays was made at the LHC. The unique properties and structure of the CKM matrix connect seemingly unrelated flavour observables, most of which are accessible through B decays. Accurate flavour measurements thus simultaneously allow the CKM matrix to be probed, and precise theory predictions to be scrutinised.

Unturned stones 

Today, the LHC dominates the flavour sector, with an important parallel programme ongoing at Belle II in Japan. Between them, the LHC experiments have made the most precise measurement of matter–antimatter oscillations in the neutral B system, measured CP violation in B mesons, discovered rare B decays and determined CKM elements such as V_tb. So far no measurement has yielded a significant disagreement with SM expectations. However, some interesting hints have emerged, and a couple of stones have not yet been turned.

Since the inauguration of the LHC, its four main experiments have discovered more than 50 new hadronic states

A promising opportunity to probe physics beyond the SM arises through b → sℓℓ and b → cℓν transitions in various hadron decays. The latter proceed through tree-level transitions in processes that are abundant and well-understood: the decay is mediated by a charged W boson that changes the b quark into a c quark, emitting a lepton and an antineutrino. In b → sℓℓ processes, the quark flavour changes through the emission of a Z boson or a photon. This flavour-changing neutral-current process occurs through a higher order penguin diagram, and underlies a breed of suppressed and thus rare hadron decays. The SM makes a slew of precise predictions for flavour observables for both types of transitions. However, new-physics models include yet-unobserved particles that can potentially contribute virtually.

A number of flavour observables are particularly well predicted within the SM. Well-known examples are the lepton-flavour-universality observables R(K), which compare the decay rates of b → sℓℓ decays containing muons to those containing electrons, and R(D), which compares b → cℓν decay rates with muons and tau leptons in the final state. The theoretical precision for these ratios reach an impressive relative uncertainty of about 1%. But other measurable flavour quantities in these two transitions, such as absolute decay rates or angular observables, are more challenging due to the limited knowledge of gluon exchange between hadrons in the initial and final states. 

Intriguingly, all b → sℓℓ flavour observables measured by the b-factories LHCb, Belle and BaBar, and also ATLAS and CMS, collectively, point in a similar direction away from SM predictions. This has led to speculation that new heavy particles are changing the rate of B-meson decays to different lepton flavours, violating the SM principle of lepton-flavour universality. The contributions of such particles are quantified “effectively” – similar to the way Fermi described weak decays in terms of a single coupling constant instead of the underlying W-boson propagator. New particles that contribute to B decays can affect many different types of couplings, depending on their spin or handedness. Remarkably, the current flavour anomalies seem to affect only one or two effective couplings (left-handed vector and axial couplings, known as C₉ and C₁₀), and these can be visualised in a single two-dimensional plane of new-physics contributions (see “New couplings” figure). Data from b → cℓν transitions also exhibit hints for anomalous lepton-flavour non-universality.

The picture that seems to be emerging could be explained by models that involve leptoquarks or Z′ bosons (CERN Courier May/June 2019 p33). The flavour anomalies measured at the LHC disagree with the SM at the level of 2–3.5σ, which is insufficient to confirm the presence of new physics. To address these and other unanswered questions in the flavour sector, the available data sample need to be expanded.

Luminous future  

The LHCb experiment will operate at Run 3 at an increased instantaneous luminosity, and with an improved data acquisition system. Together, this will enable a 10-fold increase of the sample size. The price to pay for this increased luminosity is the daunting number of overlapping collisions in a single proton-bunch crossing, which makes the task of sifting through billions of collisions to identify interesting topologies a challenge. Novel technologies such as graphics processing units have been incorporated in LHCb’s trigger system to speed up the processing of busy hadronic events, while new detectors have been built to reconstruct charged particle tracks, find the vertex position and to identify the particle species using state-of-the-art readout electronics (CERN Courier May/June 2022 p38). The LHCb upgrades completed during LS2 will also serve the experiment for Run 4 beginning in 2029, which is the start of the ambitious High-Luminosity LHC (HL-LHC) project. 

During the next few years of Run 3, the LHCb experiment is expected to collect an integrated luminosity of 20–25 fb^–1 (compared to 6 fb^–1 in Run 2). This will enable significant improvements on the precision of CP-violation observables and rare B-decay measurements. The expectation is to improve the precision on possible CP violation in B_s⁰ – B_s⁰ mixing to 10^–3, on CP violation in the interference between mixing and decay in B_s → J/ψϕ decays to about 14 mrad, and on the CKM angle γ to 1.5°. Further probes of possible lepton-flavour non-universality are another key target. The ratios of electroweak penguin processes involving b → sℓℓ transitions, R(K) and R(K^*), are expected to be determined with a precision between three to two per cent, and ratios of semileptonic b → cℓν processes R(D^*) to a precision below one per cent (see “Anomaly squeeze” figure). 

The flavour sector delivered a great harvest in the first 10 years of LHC operations

The flavour programme in the era of the HL-LHC is even more rich and diverse. Many directions are being pursued, including precision measurements targeting CP violation and mixing in charm and beauty, and measurements of CP-conserving quantities such as the magnitudes of the CKM elements V_ub, V_cb and V_tb. The end goal is to study every possible constraint to scrutinise the overall CKM picture within the SM (see “Triangulating” figure). Regarding the anomalous b → sℓℓ and b → cℓν transitions, the long-term projections for the LHC experiments are clear: if new phenomena are found, then their detailed characteristics will be established. The large data sample at the end of the HL-LHC will also allow tests of lepton-flavour violation in b → sℓℓ transitions involving tau leptons. The power of such indirect searches is their ability to elucidate the energy scale at which new particles might be present, and could point the way for the next generation of colliders.

The flavour sector delivered a great harvest in the first 10 years of LHC operations: new particles and new forms of matter were discovered, new behaviour of matter was established, stringent constraints on the CKM matrix were set and intriguing flavour anomalies have appeared. That success is only the beginning. The higher luminosity phase of the LHC beginning with Run 3 will undoubtedly generate further knowledge of particle physics, and might unveil deeper layers of nature beyond the SM.

The post A flavour of Run 3 physics appeared first on CERN Courier.

Lepton-flavour universality (LFU) is the principle that the weak interaction couples to electrons, muons and tau leptons equally. Decays of hadrons to electrons, muons and tau leptons are therefore predicted to occur at the same rate, once differences in the lepton masses are taken into account.

During the past few years, physicists have seen hints that some processes might not respect LFU. One of the strongest comes from b→cℓ^–ν_ℓ (ℓ=μ,τ) transitions in B-meson decays, as quantified by the parameter R(D^∗), which measures the ratio of the branching fractions of B →D^∗τ⁻ν_τ and B→D^∗ℓ⁻ν_ℓ. The combined deviation from precise Standard Model predictions of R(D*) and R(D) as measured by the BaBar, Belle and LHCb collaborations amounts to around 3.4σ. R(J/ψ), which concerns the branching ratios of B_c⁺→J/ψτ⁺ν_τ and B_c⁺→J/ψμ⁺ν_μ, was also found by LHCb to be larger than expected, but only at the level of around 2σ. Another key test of LFU involves the flavour-changing neutral current (FCNC) quark transition b→sℓ⁺ℓ^–, for which several channels suggest that electrons are produced at a greater rate than muons. The largest effect comes from the decay B⁺→K⁺e⁺e^–, for which LHCb finds R(K) to lie 3.1σ from the Standard Model expectation.

Taken individually, none of the measurements are significant. But together they present an intriguing pattern. New-physics models based on leptoquarks have been proposed as possible explanations for the anomalies observed in semileptonic B-meson decays and in FCNC reactions.

Baryons entered the fray in late 2019, when LHCb compared the rates of Λ_b⁰→pK^–e⁺e^– and Λ_b⁰→pK^–μ⁺μ^– decays. Although R(pK) also erred on the side of fewer muons than electrons, it was found to be in agreement with the Standard Model within the limited statistics. The latest LHCb analysis, which compared the branching ratio of Λ_b⁰→ Λ_c⁺τ^–ν_τ from a sample of around 350 events selected from LHC Run 1 to that of Λ_b⁰→ Λ_c⁺μ^– ν_μ measured by the former DELPHI experiment at LEP, found R(Λ_c⁺) = 0.242±0.026(stat)±0.040(syst)±0.059(ext), in good agreement (approximately 1σ ) with the Standard Model prediction of 0.324±0.004.

R(D*) can be large and R(Λ_c⁺) small in one new-physics scenario, or R(D*) large and R(Λ_c⁺) even larger in another

Guy Wormser

Baryon decays provide complementary constraints on potential violations of LFU to those from meson decays due to the different spin of the initial state. This allows constraints to be placed on possible new-physics scenarios, explains Guy Wormser of IJCLab, who led the LHCb analysis: “R(D*) can be large and R(Λ_c⁺) small in one new-physics scenario or R(D*) large and R(Λ_c⁺) even larger in another. The spin of the accompanying hadron changes the way new-physics couples into the reaction, and it depends also of the spin of particle present in the new-physics model, usually a leptoquark which can be a scalar, pseudoscalar, vector, axial vector or tensor. Our result excludes phase-space regions in some of these scenarios. In the future, a combined measurement of LFU violation — if it is confirmed — in mesons and baryons can therefore help to pin down the characteristics of the new-physics mediator.”

The latest LHCb result concerning R(Λ_c⁺) is likely to trigger intensive discussions among theorists, says the collaboration, with future measurements of this and other “R” measurements using Run-2 and Run-3 data keenly anticipated.

The post LHCb probes lepton universality with taus appeared first on CERN Courier.

The CP-violating angle γ of the Cabibbo–Kobayashi–Maskawa (CKM) quark-mixing matrix is a benchmark of the Standard Model, since it can be determined from tree-level beauty decays in an entirely data-driven way with negligible theoretical uncertainty. Comparisons between direct and indirect measurements of γ therefore provide a potent test for new physics. Before LHCb began taking data, γ was one of the poorest known constraints of the CKM unitarity triangle, but that is no longer the case. 

A new result from LHCb marks an important change in strategy, by including not only results from beauty decays sensitive to γ but additionally exploiting the sensitivity to CP violation and mixing of charm meson (D⁰) decays. Mixing in the D⁰–D⁰ system proceeds via flavour-changing neutral currents, which may also be affected by contributions from new heavy particles. The process is described by two parameters: the mass difference, x, and width difference, y, between the two charm flavour states (see figure 1).

The latest combination takes the results of more than 20 LHCb beauty and charm measurements to determine γ = (65.4 _–4.2^+3.8)°, which is the most precise measurement from a single experiment (see figure 2). Furthermore, various charm-mixing parameters were determined by combining, for the first time, both the beauty and charm datasets, which results in x = (0.400)% and y = (0.630)%. The latter is a factor-of-two more precise than the current world average, which is entirely due to the new methodology that harnesses additional sensitivity to the charm sector from beauty decays.

This demonstrates that LHCb has already achieved better precision than its original design goals. When the redesigned LHCb detector restarts operations in 2022, the target of sub-degree precision on γ, and the chance to observe CP violation in charm mixing, comes ever closer.

The post Beauty enhances precision of CKM angle γ appeared first on CERN Courier.

The inaugural CERN Flavour Anomalies Workshop took place on 20 October as part of this year’s Implications of LHCb Measurements and Future Prospects meeting. More than 500 experimentalists and theorists met in a hybrid format via Zoom and in person. Discussion centered on the longstanding tensions in B-physics measurements, and new project ideas. The workshop was dedicated to the memory of long-time LHCb collaborator Sheldon Stone (Syracuse), who made a plentiful contribution to CERN’s flavour programme.

The central topic of the workshop was the b anomalies: a persistent set of tensions between predictions and measurements in a number of semileptonic b-decays which are not as clear as unexpected peaks in invariant mass distributions. Instead, they manifest themselves as modifications to the branching fractions and angular distributions of certain flavour-changing neutral-current (FCNC) b-decays which have become more significant over the past decade. The latest LHCb measurement of the ratio (R_K) of B⁺ decays to a kaon and a muon or electron pair differs from the Standard Model (SM) by more than 3σ, and the ratio (R_K*) of B⁰ decays to an excited kaon and a muon or electron pair differs by more than 2σ. LHCb has also seen several departures from theory in measurements of angular distributions at the level of roughly 3σ significance. Finally, and coherent with these FCNC effects, BaBar, Belle and LHCb analyses of charged-current b→cτ^–ν̄  decays support lepton-flavour-universality (LFU) violation at a combined significance of roughly 3σ. Though no single measurement is statistically significant, the collective pattern is intriguing. 

Four of the major fitting groups showed a stunning agreement in fits to effective-field-theory parameters

But how robust are the SM predictions for these observables? Efforts include both theory-only and data-driven approaches for distinguishing genuine signs of beyond-the-SM (BSM) effects from hard-to-understand hadronic effects. A further aim is to understand what type of BSM models could produce the observed effects. Of particular interest was the question of how to incorporate information from high-pT searches at the LHC experiments. ATLAS and CMS are ramping up their efforts, and their ongoing B-physics programmes will hopefully soon confirm and complement LHCb’s results. Both experiments reported on work to address the main bottlenecks: the reconstruction of low-momentum leptons, and trigger challenges foreseen as a result of increased luminosities in Run 3. The complementarity of B-physics and direct searches was clear from results such as ATLAS and CMS searches for leptoquarks compatible with the flavour anomalies.

Theory consensus

The workshop saw, for the first time, a joint theory presentation by four of the major b→sℓ⁺ℓ^– fitting groups. They showed a stunning agreement in fits to effective-field-theory parameters which register as nonzero in the presence of BSM physics (see figure). The fits use observables that either probe LFU or help to constrain troublesome hadronic uncertainties. The observables include the now famous R_K, R_K* and R_pK (which studies Λ_b⁰ baryon decays to a proton, a charged kaon and a pair of muons or electrons), whose measurements are dominated by LHCb results; and results on the branching fraction for B_s→μ⁺μ^– from ATLAS, CMS and LHCb. Though the level of agreement diminishes when other observables and measurements are included, dominantly due to the different theoretical assumptions made by the four groups, all agree that substantial tensions with the SM are unavoidable.

New results from LHCb included first measurements of the LFU-sensitive ratios R_K*+ (which concerns B⁺→K*⁺ℓ⁺ℓ^– decays) and R_Ks (which concerns B⁰→K_S⁰ℓ⁺ℓ^– decays), and new measurements of branching fractions and angular observables for the decay B_s→ϕμ⁺μ^–, which is at present hampered by significant theory uncertainties. By contrast, many theoretical predictions for b→cτ^–ν̄ processes are now more precise than measurements, with the promise of further improvements thanks to dedicated lattice-QCD studies. Larger and more diverse datasets will be needed to reduce the experimental uncertainties.

As the end of the year approaches, it may not be too early to collect wishes for 2022. The most prevalent wishes involve new analysis results from ATLAS, CMS and LHCb on these burning topics, and a 2022 workshop to happen in person!

The post Beyond bumps appeared first on CERN Courier.

The possibility that the proton wave function may contain a |uudcc> component in addition to the g → cc splitting arising from perturbative gluon radiation has been debated for decades. In favour of such “intrinsic charm” (IC), light-front QCD (LFQCD) calculations predict that non-perturbative IC manifests as percent-level valence-like charm content in the parton distribution functions (PDFs) of the proton. On the other hand, if the charm–quark content is entirely perturbative in nature, the charm PDF should resemble that of the gluon and decrease sharply at large momentum fractions, x. The proton could also contain intrinsic beauty, but suppressed by a factor of order m²_c/m²_b. The picture for intrinsic strangeness is somewhat murkier due to the lighter mass of the strange quark.

Measurements of charm-hadron production in deep-inelastic scattering and in fixed-target experiments, with typical momentum transfers below Q = 10 GeV, have been interpreted as evidence both for and against the IC predicted by LFQCD. Even though such experiments are in principle sensitive to valence-like c-quark content, interpreting low-Q data is challenging since it requires a careful theoretical treatment of hadronic and nuclear effects. Recent global PDF analy­ses, which also include measurements by ATLAS, CMS and LHCb, are inconclusive and can only exclude a relatively large IC component carrying more than a few percent of the momentum of the proton.

Using its Run-2 data, LHCb recently studied IC by making the first measurement of the fraction of Z+jet events that contain a charm jet in the forward region of proton–proton collisions. Since Zc production is inherently at large Q, above the electroweak scale, hadronic effects are small. A leading-order Zc production mechanism is gc → Zc scattering (figure 1), where in the forward region one of the initial partons must have large x, hence Zc production probes the valence-like region.

The spectrum observed by LHCb exhibits a sizable enhancement at forward Z rapidities (figure 2), consistent with the effect expected if the proton wave function contains the |uudcc> component predicted by LFQCD. Incorporating these results into global PDF analyses should strongly constrain the large-x charm PDF, both in size and shape – and could reveal that the proton contains valence-like intrinsic charm.

These results demonstrate the unique sensitivity of the LHCb experiment to the valence-like content of the proton. Looking forward to Run 3, increased luminosity will lead to a substantial improvement in the precision of this measurement, which should provide an even clearer picture of just how charming the proton is. 

The post LHCb studies the intrinsic charm of the proton appeared first on CERN Courier.

At a seminar at CERN today, the LHCb collaboration presented new tests of lepton universality in rare B-meson decays. While limited in statistical sensitivity, the results fit an intriguing pattern of recent results in the flavour sector, says the collaboration.

Since 2013, several measurements have hinted at deviations from lepton-flavour universality (LFU), a tenet of the Standard Model (SM) which treats charged leptons, ℓ, as identical apart from their masses. The measurements concern decay processes involving the transition between a bottom and a strange quark b→sℓ⁺ℓ^–, which are strongly suppressed by the SM because they involve quantum corrections at the one-loop level (leading to branching fractions of one part in 10⁶ or less). A powerful way to probe LFU is therefore to measure the ratio of B-meson decays to muons and electrons, for which the SM prediction, close-to-unity, is theoretically very clean.

In March this year, an LHCb measurement of R_K = BR(B⁺→K⁺μ⁺μ^–)/BR(B⁺→K⁺e⁺e^–) based on the full LHC Run 1 and 2 dataset showed a 3.1σ difference from the SM prediction. This followed departures at the level of 2.2—2.5σ in the ratio R_K*⁰ (which probes B⁰→K*⁰ℓ⁺ℓ^– decays) reported by LHCb in 2017. The collaboration has also seen slight deficits in the ratio R_pK, and departures from theory in measurements of the angular distribution of final-state particles and of branching fractions in neutral B-meson decays. None of the results is individually significant enough to constitute evidence of new physics. But taken together, say theorists, they point to a coherent pattern.

We are seeing a similar deficit of rare muon decays to rare electron decays that we have seen in other LFU tests

Harry Cliff

The latest LHCb analysis clocked the ratio of muons to electrons in the isospin-partner B-decays: B⁰→ K_S⁰ℓ⁺ℓ^– and B⁺→K*⁺ℓ⁺ℓ^–. As well as being a first at the LHC, it’s the first single-experiment observation of these decays, and the most precise measurement yet of their branching ratios. Being difficult to reconstruct due to the presence of a long-lived K_S⁰ in the final state, however, the sensitivity of the results is lower than for previous “R_K” analyses. LHCb found R(K_S⁰) = 0.66+0.2/-0.15 (stat) +0.02/-0.04 (syst) and R(K*⁺) = 0.70+0.18/-0.13 (stat) +0.03/-0.04 (syst), which are consistent with the SM at the level of 1.5 and 1.4σ, respectively.

“What is interesting is that we are seeing a similar deficit of rare muon decays to rare electron decays that we have seen in other LFU tests,” said Harry Cliff of the University of Cambridge, who presented the result on behalf of LHCb (in parallel with a presentation at Rencontres de Blois by Cambridge PhD student John Smeaton). “With many other LFU tests in progress using Run 1 and 2 data, there will be more to come on this puzzle soon. Then we have Run 3, where we expect to really zoom in on the measurements and obtain a detailed understanding.”

The experimental and theoretical status of the flavour anomalies in b→sℓ⁺ℓ and semi-leptonic B-decays will be the focus of the Flavour Anomaly Workshop at CERN on Wednesday 20 October, at which ATLAS and CMS activities will also be discussed, along with perspectives from theorists.

The post LHCb tests lepton universality in new channels appeared first on CERN Courier.

The LHCb experiment recently presented new results on the b → sμμ decay of a B_s meson to a φ meson and a dimuon pair, reinforcing an anomaly last reported in 2015 with improved statistics and theory calculations. Such decays of b hadrons via b → s quark transitions are strongly suppressed in the Standard Model (SM) and therefore constitute sensitive probes for hypothetical new particles. In recent years, several measurements of rare semileptonic b → sℓℓ decays have shown tensions with SM predictions. Anomalies have been spotted in measurements of branching fractions, angular analyses and tests of lepton flavour universality (LFU), leading to cautious excitement that new physics might be at play.

Calculating the Standard Model prediction is more challenging than for lepton-flavour universality

At the SM@LHC conference in April, LHCb presented the most precise determination to date of the branching fraction for the decay using data collected during LHC Run 1 and Run 2 (figure 1). The branching fraction is measured as a function of the dimuon invariant mass (q²) and found to lie below the SM prediction at the level of 3.6 standard deviations in the low-q² region. This deficit of muons is consistent with the pattern seen in LFU tests of b → sℓℓ transitions, however calculating the SM prediction for the B_s → φμμ branching fraction is more challenging than for LFU tests as it involves the calculation of non-perturbative hadronic effects. 

Calculations based on light-cone sum rules are most precise at low q², while lattice-QCD calculations do better at high q². A combination is expected to give the best precision over the full q² range. If lattice-QCD calculations are not used in the comparison, increased theory errors reduce the tension to 1.8 standard deviations in the low-q² region. The previous 2015 measurement by LHCb, which was based exclusively on Run-1 data (grey data points), was reported at the time to be approximately three standard deviations below the best theoretical predictions that were available at the time. Since then, theo­retical calculations have generally become more precise with regard to form factors, but more conservatively evaluated with regard to non-local hadronic effects.

Angular information

The angular distribution of the B_s → φμμ decay products offers complementary information. At the international FPCP conference in June, LHCb presented a measurement of the angular distribution of these decays in different q² regions using data collected during LHC Run 1 and Run 2. Figure 2 shows the longitudinal polarisation fraction F_L – one of several variables sensitive to anomalous b → sμμ couplings. The results are consistent with SM predictions at the level of two standard deviations, but may also hint at the same pattern of unexpected behaviour seen in angular analyses of other b → sμμ decays and in branching-fraction measurements.

For both analyses, LHC Run 3 will be crucial to better understanding the anomalous behaviour seen so far in B_s → φμμ decays.

The post B_s decays remain anomalous appeared first on CERN Courier.

It was once questioned whether it would be possible to successfully operate an asymmetric “forward” detector at a hadron collider. In such a high-occupancy environment, it is much harder to reconstruct decay vertices and tracks than it is at a lepton collider. Following its successes during LHC Run 1 and Run 2, however, LHCb has rewritten the forward-physics rulebook, and is now preparing to take on bigger challenges.

During Long Shutdown 2, which comes to an end early next year, the LHCb detector is being almost entirely rebuilt to allow data to be collected at a rate up to 10 times higher during Run 3 and Run 4. This will improve the precision of numerous world-best results, such as constraints on the angles of the CKM triangle, while further scrutinising intriguing results in B-meson decays, which hint at departures from the Standard Model. 

At the core of the LHCb upgrade project are new detectors capable of sustaining an instantaneous luminosity up to five times that seen at Run 2, and which enable a pioneering software-only trigger that will enable LHCb to process signal data in an upgraded computing farm at the frenetic rate of 40 MHz. The vertex locator (VELO) will be replaced with a pixel version, the upstream silicon-strip tracker will be replaced with a lighter version (the UT) located closer to the beamline, and the electronics for LHCb’s muon stations and calorimeters are being upgraded for 40 MHz readout. 

Recently, three further detector systems key to dealing with the higher occupancies ahead were lowered into the LHCb cavern for installation: the upgraded ring-imaging Cherenkov detectors RICH1 and RICH2 for sharper particle identification, and the brand new “SciFi” (scintillating fibre) tracker. 

SciFi tracking

The components of LHCb’s SciFi tracker may not seem futuristic at first glance. Its core elements are constructed from what is essentially paper, plastic, some carbon fibre and glue. However, its materials components conceal advanced technologies which, when coupled together, produce a very light and uniform, high-performance detector that is needed to cope with the higher number of particle tracks expected during Run 3.

Located behind the LHCb magnet (see “Asymmetric anatomy” image), the SciFi represents a challenge, not only due to its complexity, but also because the technology – plastic scintillating fibres and silicon photomultiplier arrays – has never been used for such a large area in such a harsh radiation environment. Many of the underlying technologies have been pushed to the extreme during the past decade to allow the SciFi to successfully operate under LHC conditions in an affordable and effective way. 

More than 11,000 km of 0.25 mm-diameter polystyrene fibre was delivered to CERN before undergoing meticulous quality checks. Excessive diameter variations were removed to prevent disruptions of the closely packed fibre matrix produced during the winding procedure, and clear improvements from the early batches to the production phase were made by working closely with the industrial manufacturer. From the raw fibres, nearly 1400 multi-layered fibre mats were wound in four of the LHCb collaboration’s institutes (see “SciFi spools” image), before being cut and bonded in modules, tested, and shipped to CERN where they were assembled with the cold boxes. The SciFi tracker contains 128 stiff and robust 5 × 0.5 m² modules made of eight mats bonded with two fire-resistant honeycomb and carbon-fibre panels, along with some mechanics and a light-injection system. In total, the design produces nearly 320 m² of detector surface over the 12 layers of the tracking stations. 

The scintillating fibres emit photons at blue-green wavelengths when a particle interacts with them. Secondary scintillator dyes added to the polystyrene amplify the light and shift it to longer wavelengths so it can be read out by custom-made silicon photomultipliers (SiPMs). SiPMs have become a strong alternative to conventional photomultiplier tubes in recent years, due to their smaller channel sizes, easier operation and insensitivity to magnetic fields. This makes them ideal to read out the higher number of channels necessary to identify separate but nearby tracks in LHCb during Run 3. 

The width of the SiPM channels, 0.25 mm, is designed to match that of the fibres. Though they need not align perfectly, this provides a better separation power for tracking than the previously used 5 mm gas straw tubes in the outer regions of the detector, while providing a similar performance to the silicon-strip tracker. The tiny channel size results in over 524,288 SiPM channels to collect light from 130 m of fibre-mat edges. A custom ASIC, called the PACIFIC, outputs two bits per channel based on three signal-amplitude thresholds. A field-programmable gate array (FPGA) assigned to each SiPM then groups these signals into clusters, where the location of each cluster is sent to the computing farm. Despite clustering and noise suppression, this still results in an enormous data rate of 20 Tb/s – nearly half of the total data bandwidth of the upgraded LHCb detector.

One of the key factors in the success of LHCb’s flavour-physics programme is its ability to identify charged particles

LHCb’s SciFi tracker is the first large-scale use of SiPMs for tracking, and takes advantage of improvements in the technology in the 10 years since the SciFi was proposed. The photon-detection efficiency of SiPMs has nearly doubled thanks to improvements in the design and production of the underlying pixel structures, while the probability of crosstalk between the pixels (which creates multiple fake signals by causing a single pixel to randomly fire without incident light following radiation damage) has been reduced from more than 20% to a few percent by the introduction of microscopic trenches between the pixels. The dark-single-pixel firing rate can also be reduced by cooling the SiPM. Together, these two methods greatly reduce the number of fake-signal clusters such that the tracker can effectively function after several years of operation in the LHCb cavern. 

The LHCb collaboration assembled commercial SiPMs on flex cables and bonded them in groups of 16 to a 0.5 m-long 3D-printed titanium cooling bar to form precisely assembled photodetection units for the SciFi modules. By circulating a coolant at a temperature of –50 °C through the cold bar, the dark-noise rate was reduced by a factor of 60. Furthermore, in a first for a CERN experiment, it was decided to use a new single-phase liquid coolant called Novec-649 from 3M for its non-toxic properties and low greenhouse warming potential (GWP = 1). Historically, C₆F₁₄ – which has a GWP = 7400 – was the thermo-transfer fluid of choice. Although several challenges had to be faced in learning how to work with the new fluid, wider use of Novec-649 and similar products could contribute significantly to the reduction of CERN’s carbon footprint. Additionally, since the narrow envelope of the tracking stations precludes the use of standard foam insulation of the coolant lines, a significant engineering effort has been required to vacuum insulate the 48 transfer lines from the 24 rows of SiPMs and 256 cold-bars where leaks are possible at every connection. 

To date, LHCb collaborators have tirelessly assembled and tested nearly half of the SciFi tracker above ground, where only two defective channels out of the 262,144 tested in the full signal chain were unrecoverable. Four out of 12 “C-frames” containing the fibre modules (see “Tracking tall” image) are now installed and waiting to be connected and commissioned, with a further two installed in mid-July. The remaining six will be completed and installed before the start of operations early next year.

New riches

One of the key factors in the success of LHCb’s flavour-physics programme is its ability to identify charged particles, which reduces the background in selected final states and assists in the flavour tagging of b quarks. Two ring-imaging Cherenkov (RICH) detectors, RICH1 and RICH2, located upstream and downstream of the LHCb magnet 1 and 10 m away from the collision point, provide excellent particle identification over a very wide momentum range. They comprise a large volume of fluorocarbon gas (the radiator), in which photons are emitted by charged particles travelling at speeds higher than the speed of light in the gas; spherical and flat mirrors to focus and reflect this Cherenkov light; and two photon-detector planes where the Cherenkov rings are detected and read out by the front-end electronics.

The original RICH detectors are currently being refurbished to cope with the more challenging data-taking conditions of Run 3, requiring a variety of technological challenges to be overcome. The photon detection system, for example, has been redesigned to adapt to the highly non-uniform occupancy expected in the RICH system, running from an unprecedented peak occupancy of ~35% in the central region of RICH1 down to 5% in the peripheral region of RICH2. Two types of 64-channel multi-anode photomultiplier tubes (MaPMTs) have been selected for the task which, thanks to their exceptional quantum efficiency in the relevant wavelength range, are capable of detecting single photons while providing excellent spatial resolution and very low background noise. These are key requirements to allow pattern-recognition algorithms to reconstruct Cherenkov rings even in the high-occupancy region. 

More than 3000 MaPMT units, for a total of 196,608 channels, are needed to fully instrument both upgraded RICH detectors. The already large active area (83%) of the devices has been maximised by arranging the units in a compact and modular “elementary cell” containing a custom-developed, radiation-hard eight-channel ASIC called the Claro chip, which is able to digitise the MaPMT signal at a rate of 40 MHz. The readout is controlled by FPGAs connected to around 170 channels each. The prompt nature of Cherenkov radiation combined with the performance of the new opto-electronics chain will allow the RICH systems to operate within the LHC’s 25 ns time window, dictated by the bunch-crossing period, while applying a time-gate of less than 6 ns to provide background rejection.

To keep the new RICHes as compact as possible, the hosting mechanics has been designed to provide both structural support and active cooling. Recent manufacturing techniques have enabled us to drill two 6 mm-diameter ducts over a length of 1.5 m into the spine of the support, through which a coolant (the more environmentally friendly Novec649, as in the SciFi tracker) is circulated. Each element of the opto-electronics chain has been produced and fully validated within a dedicated quality-assurance programme, allowing the position of the photon detectors and their operating conditions to be fine-tuned across the RICH detectors. In February, the first photon-detector plane of RICH2 (see “RICH2 to go” image) became the first active element of the LHCb upgrade to be installed in the cavern. The two planes of RICH2, located at the sides of the beampipe, were commissioned in early summer and will see first Cherenkov light during an LHC beam test in October. 

RICH1 presents an even bigger challenge. To reduce the number of photons in the hottest region, its optics have been redesigned to spread the Cherenkov rings over a larger surface. The spatial envelope of RICH1 is also constrained by its magnetic shield, demanding even more compact mechanics for the photon-detector planes. To accommodate the new design of RICH1, a new gas enclosure for the radiator is needed. A volume of 3.8 m³ of C₄F₁₀ is enclosed in an aluminium structure directly fastened to the VELO tank on one side and sealed with a low-mass window on the other, with particular effort placed on building a leak-less system to limit potential environmental impact. Installing these fragile components in a very limited space has been a delicate process, and the last element to complete the gas-enclosure sealing was installed at the beginning of June.

The optical system is the final element of the RICH1 mechanics. The ~2 m² spherical mirrors placed inside the gas enclosure are made of carbon fibre composite to limit the material budget (see “Cherenkov curves” image), while the two 1.3 m² planes of flat mirrors are made of borosilicate glass for high optical quality. All the mirror segments are individually coated, glued on supports and finally aligned before installation in the detector. The full RICH1 installation is expected to be completed in the autumn, followed by the challenging commissioning phase to tune the operating parameters to be ready for Run 3.

Surpassing expectations

In its first 10 years of operations, the LHCb experiment has already surpassed expectations. It has enabled physicists to make numerous important measurements in the heavy-flavour sector, including the first observation of the rare decay B⁰_s → µ⁺µ^–, precise measurements of quark-mixing parameters, the discovery of CP violation in the charm sector, and the observation of more than 50 new hadrons including tetraquark and pentaquark states. However, many crucial measurements are currently statistically limited, including those underpinning the so-called flavour anomalies (see B_s decays remain anomalous). Together with the tracker, trigger and other upgrades taking place during LS2, the new SciFi and revamped RICH detectors will put LHCb in prime position to explore these and other searches for new physics for the next 10 years and beyond.

The post Building the future of LHCb appeared first on CERN Courier.

The COMPASS experiment at CERN has reported the first direct evidence for a long-hypothesised interplay between hadron decays which can masquerade as a resonance. The analysis, which was published last week in Physical Review Letters, suggests that the “a₁(1420)” signal observed by the collaboration in 2015 is not a new exotic hadron after all, but the first sighting of a so-called triangle singularity.

“Triangle singularities are a mechanism for generating a bump in the decay spectrum that does not correspond to a resonance,” explains analyst Mikhail Mikhasenko of the ORIGINS Cluster in Munich. “One gets a peak that has all features of a new hadron, but whose true nature is a virtual loop with known particles.” 

“This is a prime example of an aphorism which is commonly attributed to Dick Dalitz,” agrees fellow analyst Bernhard Ketzer, of the University of Bonn: “Not every bump is a resonance, and not every resonance is a bump!”

Triangle singularities take their name from the triangle in a Feynman diagram when a secondary decay product fuses with a primary decay product. If the particle masses line up such that the process can proceed as a cascade of on-mass-shell hadron decays, the matrix element is enhanced by a so-called logarithmic singularity which can easily be mistaken for a resonance. But the effect is usually rather small, requiring a record 50 million π^–p→π^–π⁺π^–p events, and painstaking work by the COMPASS collaboration to make certain that the a₁(1420) signal, which makes up less than 1% of the three-pion sample, wasn’t an artefact of the analysis procedure.

Hadron experiments are reaching the precision needed to see one of the most peculiar multi-body features of QCD

Mikhail Mikhasenko

“The correspondence of this small signal with a triangle singularity is noteworthy because it shows that hadron experiments are finally reaching the precision and statistics needed to see one of the most peculiar features of the multi-body non-perturbative regime of quantum chromodynamics,” says Mikhasenko.

Triangle singularities were dreamt up independently by Lev Landau and Richard Cutkosky in 1959. After five decades of calculations and speculations, physicists at the Institute for High-Energy Physics in Beijing in 2012 used a triangle singularity to explain why intermediate f₀(980) mesons in J/ψ meson decays at the BESIII experiment at the Beijing Electron–Positron Collider II were unusually long-lived. In 2019, the LHCb collaboration ruled out triangle singularities as the origin of the pentaquark states they discovered that year. The new COMPASS analysis is the first time that a “bump” in a decay spectrum has been convincingly explained as more likely due to a triangle singularity than a resonance.

COMPASS collides a secondary beam of charged pions from CERN’s Super Proton Synchrotron with a hydrogen target in the laboratory’s North Area. In this analysis, gluons emitted by protons in the target excite the incident pions, producing the final state of three charged pions which is observed by the COMPASS spectrometer. Intermediate resonances display a variety of angular momentum, spin and parity configurations. In 2015, the collaboration observed a small but unmistakable “P-wave” (L=1) component of the f₀(980)π system with a peak at 1420 MeV and J^PC=1⁺⁺. Dubbed a₁(1420), the apparent resonance was suspected to be exotic, as it was narrower, and hence more stable, than the ground-state meson with the same quantum numbers, a₁(1260). It was also surprisingly light, with a mass just above the K*K threshold of 1.39 GeV. A tempting interpretation was that a₁(1420) might be a dsūs̄ tetraquark, and thus the first exotic hadronic state with no charm quarks, and a charged cousin of the famous exotic X(3872) at the D*D threshold to boot, explains Mikhasenko.

According to the new COMPASS analysis, however, the bump at 1420 MeV can more simply be explained by a triangle singularity, whereby an a₁(1260) decays to a K*K pair, and the kaon from the resulting K*→Kπ decay annihilates with the initial anti-kaon to create a light unflavoured f₀(980) meson which decays to a pair of charged pions. Crucially, the mass of f₀(980) is just above the KK threshold, and the roughly 300 MeV width of the conventional a₁(1260) meson is wide enough for the particle to be said to decay to K*K on-mass-shell.

A new resonance is not required. That is phenomenologically significant.

Ian Aitchison

“The COMPASS collaboration have obviously done a very thorough job, being in possession of a complete partial-wave analysis,” says Ian Aitchison, emeritus professor at the University of Oxford, who in 1964 was among the first to propose that triangle graphs with an unstable internal line (in this case the K*) could lead to observable effects. This enables the whole process to occur nearly on-shell for all particles, which in turn means that the singularities of the amplitude will be near the physical region, and hence observable, explains Aitchison. “This is not unambiguous evidence for the observation of a triangle singularity, but the paper shows pretty convincingly that it is sufficient to explain the data, and that a new resonance is not required. That is phenomenologically significant.”

The collaboration now plans further studies of this new phenomenon, including its interference with the direct decay of the a₁(1260). Meanwhile, observation by Belle II of the a₁(1420) phenomenon in decays of the tau meson to three pions should confirm our understanding and provide an even cleaner signal, says Mikhasenko.

The post COMPASS points to triangle singularity appeared first on CERN Courier.

All the exotic hadrons that have been observed so far decay rapidly via the strong interaction. The ccūd̄ tetraquark (T_cc⁺ ) just discovered by the LHCb collaboration is no exception. However, it is the longest-lived state yet, and reinforces expectations that its beautiful cousin, bbūd , will be stable with respect to the strong interaction when its peak emerges in future data.

“We have discovered a ccūd tetraquark with a mass just below the D^*+D⁰ threshold which, according to most models, indicates that it is a bound state,” says LHCb analyst Ivan Polyakov (Syracuse University). “It still decays to D mesons via the strong interaction, but much less intensively than other exotic hadrons.”

Most of the exotic hadronic states discovered in the past 20 years or so are cc̄qq̄ tetraquarks or cc̄qqq pentaquarks, where q represents an up, down or strange quark. A year ago LHCb also discovered a hidden-double-charm cc̄cc̄ tetraquark, X(6900), and two open-charm csūd tetraquarks, X₀(2900) and X₁(2900). The new ccūd state, presented today at the European Physical Society conference on high-energy physics to have been observed with a significance substantially in excess of five standard deviations, is the first exotic hadronic state with so-called double open heavy flavour — in this case, two charm quarks unaccompanied by antiparticles of the same flavour.

Astoundingly, its observation by LHCb reveals that it is a mere 270 keV below the threshold

Prime Candidate

Tetraquark states with two heavy quarks and two light antiquarks have been the prime candidates for stable exotic hadronic states since the 1980s. LHCb’s discovery, four years ago, of the Ξ_cc⁺⁺ (ccu) baryon allowed QCD phenomenologists to firmly predict the existence of a stable bbūd tetraquark, however the stability of a potential ccūd state remained unclear. Predictions of the mass of the ccūd state varied substantially, from 250 MeV below to 200 MeV above the D^*+D⁰ mass threshold, say the team. Astoundingly, its observation by LHCb reveals that it is a mere 273 ± 61 keV below the threshold — a bound state, then, but with the threshold for strong decays to D^*+D⁰ lying within the observed resonance’s narrow width of 410 ± 165 keV, prescribed by the uncertainty principle. The T_cc⁺ tetraquark can therefore decay via the strong interaction, but strikingly slowly. By contrast, most exotic hadronic states have widths from tens to several hundreds of MeV.

“Such closeness to the threshold is not very common in heavy-hadron spectroscopy,” says analyst Vanya Belyaev (Kurchatov Institute/ITEP). “Until now, the only similar closeness was observed for the enigmatic χ_c1(3872) state, whose mass coincides with the D^*0D⁰ threshold with a precision of about 120 keV.” As it is wider, however, it is not yet known whether the χ_c1(3872) is below or above threshold.

I am fascinated by the idea that a strong coupling to a decay channel might attract the bare mass of the hadron

Mikhail Mikhasenko

“The surprising proximity of T_cc⁺ and χ_c1(3872) to the D^*D thresholds must have deep reasoning,” adds analyst Mikhail Mikhasenko (ORIGINS, Munich). “I am fascinated by the idea that, roughly speaking, a strong coupling to a decay channel might attract the bare mass of the hadron. Tremendous progress in lattice QCD over the past 10 years gives us hope that we will discover the answer soon.”

The cause of this attraction, says Mikhasenko, could be linked to a “quantum admixture” of two models that vie to explain the structure of the new tetraquark: it could be a D^*+ and a D⁰ meson, bound by the exchange of colourneutral objects such as light mesons, or a colour-charged cc “diquark” tightly bound via gluon exchange to up and down antiquarks (see “Jumbled together” figure). Diquarks are a frequently employed mathematical construct in low-energy quantum chromodynamics (QCD): if two heavy quarks are sufficiently close together, QCD becomes perturbative, and they may be shown to attract each other and exhibit effective anticolour charge. For example, a red-green cc diquark would have a wavefunction similar to an anti-blue anti-quark, and could pair up with a blue quark to form a baryon — or, hypothetically, a blue anti-diquark, to form a colour-neutral tetraquark.

“The question is if the D and D^* are more or less separated, jumbled together to such a degree that all quarks are intertwined in a compact object, or something in between,” says Polyakov. “The first scenario resembles a relatively large ~4 fm deuteron, whereas the second can be compared to a relatively compact ~2 fm alpha particle.”

The new T_cc⁺ tetraquark is an enticing target for further study. Its narrow decay into a D⁰D⁰π⁺ final state — the virtual D^*+ decays promptly into D⁰π⁺ — includes no particles that are difficult to detect, leading to a better precision on its mass than for existing measurements of charmed baryons. This, in turn, can provide a stringent test for existing theoretical models and could potentially probe previously unreachable QCD effects, says the team. And, if detected, its beautiful cousin would be an even bigger boon. “Observing a tightly bound exotic hadron that would be stable with respect to the strong interaction would be a cornerstone in understanding QCD at the scale of hadrons,” says Polyakov. “The bbūd , which is believed to satisfy this requirement, is produced rarely and is out of reach of the current luminosity of the LHC. However, it may become accessible in LHC Run 3 or at the High-Luminosity LHC.” In the meantime, there is no shortage of work in hadron spectroscopy, jokes Belyaev. “We definitely have more peaks than researchers!”

The post New tetraquark a whisker away from stability appeared first on CERN Courier.

The LHCP format traditionally has plenary sessions in the mornings and late afternoons, with parallel sessions in the middle of the day. This “shape” was kept for the online meeting, with a shorter day to improve the practicality of joining from distant time zones. This resulted in a dense format with seven-fold parallel sessions, allowing all parts of the LHC programme, both experimental and theoretical, to be explored in detail. The overall vitality of the programme is illustrated by the raw statistics: a grand total of 238 talks and 122 posters were presented.

Last year saw a strong focus on the couplings to the second generation

Nine years on from the discovery of the 125 GeV Higgs boson, measurements have progressed to a new level of precision with the full Run-2 data. Both ATLAS and CMS presented new results on Higgs production, helping constrain the dynamics of the production mechanisms via differential and “simplified template” cross-section measurements. While the couplings of the Higgs to third-generation fermions are now established, last year saw a strong focus on the couplings to the second generation. After first evidence for Higgs decays to muons was reported from CMS and ATLAS results earlier in the year, ATLAS presented a new search with the full Run-2 data for Higgs decays to charm quarks using powerful new charm-tagging techniques. Both CMS and ATLAS showed updated searches for Higgs-pair production, with ATLAS being able to exclude a production rate more than 4.1 times the Standard Model (SM) prediction at 95% confidence. This is a process that should be observable with High-Luminosity LHC statistics, if it is as predicted in the SM. A host of searches were also reported, some using the Higgs as a tool to probe for new physics.

Puzzling hints

The most puzzling hints from the LHC Run 1 seem to strengthen in Run 2. LHCb presented analyses relating to the “flavour anomalies” found most notably in b→sµ⁺µ⁻ decays, updated to the full data statistics, in multiple channels. While no result yet passes a 5σ difference from SM expectations, the significances continue to creep upwards. Searches by ATLAS and CMS for potential new particles or effects at high masses that could indicate an associated new-physics mechanism continue to draw a blank, however. This remains a dilemma to be studied with more precision and data in Run 3. Other results in the flavour sector from LHCb included a new measurement of the lifetime of the Ω_c, four times longer than previous measurements (CERN Courier July/August 2021 p17) and the first observation of a mass difference between the mixed D⁰–D⁰ meson mass eigenstates (CERN Courier July/August 2021 p8).

A wealth of results was presented from heavy-ion collisions. Measurements with heavy quarks were prominent here as well. ALICE reported various studies of the differences in heavy-flavour hadron production in proton–proton and heavy-ion collisions, for example using D mesons. CMS reported the first observation of B_c meson production in heavy-ion collisions, and also first evidence for top-quark pair production in lead–lead collisions. ATLAS used heavy-flavour decays to muons to compare suppression of b- and c-hadron production in lead–lead and proton–proton collisions. Beyond the ions, ALICE also showed intriguing new results demonstrating that the relative rates of different types of c-hadron production differ in proton–proton collisions compared to earlier experiments using e⁺e⁻ and ep collisions at LEP and HERA.

Looking forward, the experiments reported on their preparations for the coming LHC Run 3, including substantial upgrades. While some work has been slowed by the pandemic, recommissioning of the detectors has begun in preparation for physics data taking in spring 2022, with the brighter beams expected from the upgraded CERN accelerator chain. One constant to rely on, however, is that LHCP will continue to showcase the fantastic panoply of physics at the LHC.

The post LHCP sees a host of new results appeared first on CERN Courier.

Which charmed baryon decays first? The LHCb collaboration recently challenged the received wisdom of fixed-target experiments by almost quadrupling the measured lifetime of the doubly strange Ω_c⁰. Now, a follow-up measurement by the collaboration confirms the revised hierarchy, offering valuable input to theoretical models of the decays.

The situation changed dramatically in 2018

Ground-state baryons containing a charm quark (c), such as Λ_c⁺ (udc), Ξ_c⁺ (usc), Ξ_c⁰ (dsc) and Ω_c⁰ (ssc), decay via the weak interaction. The ordering of their lifetimes has long been thought to be τ(Ξ_c⁺) > τ(Λ_c⁺) > τ(Ξ_c⁰) > τ(Ω_c⁰), based on measurements from fixed-target experiments nearly 20 years ago. However, the situation changed dramatically in 2018 when LHCb joined the game using a sample of Ω_c⁰ baryons obtained from bottom- baryon semileptonic decays. That LHCb study measured the Ω_c⁰ lifetime to be nearly four times larger than previously measured, transforming the hierarchy into τ(Ξ_c⁺) > τ(Ω_c⁰) > τ(Λ_c⁺) > τ(Ξ_c⁰). One year later, LHCb significantly improved the precisions of the lifetimes of the other three charmed baryons using the same method, also finding the lifetime of the Ξ_c⁰ baryon to be larger than the world-average value by about 3σ (figure 1).

Theoretically challenging

The corresponding theoretical calculations are challenging. In the charm sector, an effective theory of heavy-quark expansion is taken to calculate lifetimes of charmed baryons through an expansion in powers of 1/m_c, where m_c is the constituent charm–quark mass. Calculations up to order 1/m_c³ imply a lifetime hierarchy consistent with the original fixed-target measurements, though only qualitatively. Attempts at higher-order calculations up to order 1/m_c⁴, however, cannot accommodate the old hierarchy, but can explain the new one if a suppression factor to the constructive Pauli-interference and semileptonic terms is written in. The origin of the suppression factor is still unknown, but probably due to even higher order effects. An independent measurement was therefore needed to confirm the experimental situation.

The charmed-baryon lifetime puzzle has now been resolved by a new measurement from LHCb using a much larger sample of Ω_c⁰ and Ξ_c⁰ baryons produced directly in proton–proton collisions. Both particles are detected in the final state pK^–K^–π⁺. The measurement is made relative to the lifetime distribution of the charmed meson D⁰ via D⁰ → K⁺K^–π⁺π^– decays, in order to control systematic uncertainties. Taking advantage of the performance and detailed understanding of the LHCb detector, the lifetimes of the Ω_c⁰ and Ξ_c⁰ baryons are found to be τ(Ω_c⁰) = 276.5 ± 13.4 (stat) ± 4.5 (syst) fs and τ(Ξ_c⁰) = 148.0 ± 2.3 (stat) ± 2.2 (syst) fs, respectively, where the precision of the Ω_c⁰ lifetime is improved by a factor of two compared to the semileptonic measurement. The new results are consistent with the previous LHCb measurements, and hence establish the new lifetime hierarchy. Combining this measurement with the previous LHCb results gives τ(Ω_c⁰) = 274.5 ± 12.4 fs and τ(Ξ_c⁰) = 152.0 ± 2.0 fs, the most precise charm-baryon lifetimes to date. The newly confirmed lifetime hierarchy will help improve our knowledge of QCD dynamics in charm hadrons, and provides a crucial input to calibrate theoretical calculations.

The post New charmed-baryon lifetime hierarchy cast in stone appeared first on CERN Courier.

The ability of certain neutral mesons to oscillate between their matter and antimatter states at distinctly unworldly rates is a spectacular feature of quantum mechanics. The phenomenon arises when the states are orthogonal combinations of narrowly split mass eigenstates that gain a relative phase as the wavefunction evolves, allowing quarks and antiquarks to be interchanged at a rate that depends on the mass difference. Forbidden at tree level, proceeding instead via loops, such fl avour-changing neutral-current processes offer a powerful test of the Standard Model and a sensitive probe of physics beyond it.

Only four known meson systems can oscillate

Predicted by Gell-Mann and Pais in the 1950s, only four known meson systems (those containing quarks from different generations) can oscillate. K⁰–K⁰ oscillations were observed in 1955, B⁰–B⁰ oscillations in 1986 at the ARGUS experiment at DESY, and B_s⁰–B_s⁰ oscillations in 2006 by the CDF experiment at Fermilab. Following the first evidence of charmed-meson oscillations (D⁰–D⁰) at Belle and BaBar in 2007, LHCb made the first single-experiment observation confirming the process in 2012. Being relatively slow (more than 100 times the average lifetime of a D⁰ meson), the full oscillation period cannot be observed. Instead, the collaboration looked for small changes in the flavour mixture of the D⁰ mesons as a function of the time at which they decay via the Kπ final state.

On 4 June, during the 10th International Workshop on CHARM Physics, the LHCb collaboration reported the first observation of the mass difference between the D⁰–D⁰states, precisely determining the frequency of the oscillations. The value represents one of the smallest ever mass differences between two particles: 6.4 × 10^–6 eV, corresponding to an oscillation rate of around 1.5 × 10⁹ per second. Until now, the measured value of the mass-difference between the underlying D⁰ and D⁰ eigenstates was marginally compatible with zero. By establishing a non-zero value with high significance, the LHCb team was able to show that the data are consistent with the Standard Model, while significantly improving limits on mixing-induced CP violation in the charm sector.

“In the future we hope to discover time-dependent CP violation in the charm system, and the precision and luminosity expected from LHCb upgrades I and II may make this possible,” explains Nathan Jurik, a CERN fellow who worked on the analysis.

The latest measurements of neutral charm–meson oscillations follow hot on the heels of an updated LHCb measurement of the B_s⁰–B_s⁰ oscillation frequency announced in April, based on the heavy and light strange-beauty-meson mass difference. The very high precision of the B_s⁰–B_s⁰ measurement provides one of the strongest constraints on physics beyond the Standard Model. Using a large sample of B_s⁰ → D_s^– π⁺ decays, the new measurement improves upon the previous precision of the oscillation frequency by a factor of two: Δm_s = 17.7683 ± 0.0051 (stat) ± 0.0032 (sys) ps^–1 which, when combined with previous LHCb measurements, gives a value of 17.7656 ± 0.0057 ps^–1. This corresponds to an oscillation rate of around 3 × 10¹² per second, the highest of all four meson systems.

The post Charmed matter–antimatter flips clocked by LHCb appeared first on CERN Courier.

Beauty baryons are a subject of great interest at the LHC, offering unique insights into the nature of the strong interaction and the mechanisms by which hadrons are formed. While the ground states Λ_b⁰, Σ_b^±, Ξ_b^–, Ξ_b⁰, Ω_b^– were observed at the Tevatron at Fermilab and the SPS at CERN, the LHC’s higher energy and orders-of-magnitude larger integrated luminosity have allowed the discovery of more than a dozen excited beauty baryon states among the 59 new hadrons observed at the LHC so far (see LHCb observes four new tetraquarks).

Many hadrons with one c or b quark are quite similar. Interchanging heavy-quark flavours does not significantly change the physics predicted by effective models assuming “heavy quark symmetry”. The well-established charm baryons and their excitations therefore provide excellent input for theories modelling the less well understood spectrum of beauty-baryons. A number of the lightest excited b baryons, such as Λ_b(5912)⁰, Λ_b(5920)⁰, and several excited Ξ_b and Ω_b^– states, have been observed, and are consistent with their charm partners. By contrast, however, heavier excitations, such as the Λ_b(6072)⁰ and Ξ_b(6227) isodoublet (particles that differ only by an up or down quark), cannot yet be readily associated with charmed partners.

New particles

The first particle observed by the CMS experiment, in 2012, was the beauty- strange baryon Ξ_b(5945)⁰ (CERN Courier June 2012 p6). It is consistent with being the beauty partner of the Ξ_c(2645)⁺ with spin-parity 3/2⁺, while the Ξ_b(5955)^– and Ξ_b(5935)^– states observed by LHCb are its isospin partner and the beauty partner of the Ξ_c′⁰, respectively. The charm sector also suggests the existence of prominent heavier isodoublets, called Ξ_b^**: the lightest orbital Ξ_b excitations with orbital momentum between a light diquark (a pairing of a s quark with either a d or a u quark) and a heavy b quark. The isodoublet with spin-parity 1/2^– decays into Ξ_b′ π^± and the one with 3/2^– into Ξ_b^* π^±.

The CMS collaboration has now observed such a baryon, Ξ_b(6100)^–, via the decay sequence Ξ_b(6100)^– → Ξ_b(5945)⁰π^– → Ξ_b^– π⁺ π^–. The new state’s measured mass is 6100.3 ± 0.6 MeV, and the upper limit on its natural width is 1.9 MeV at 95% confidence level. The Ξ_b^– ground state was reconstructed in two channels: J/ψ Ξ^– and J/ψ Λ K^–. The latter channel also includes partially reconstructed J/ψ Σ⁰ K^– (where the photon from the Σ⁰ → Λ γ decay is too soft to be reconstructed).

If the Ξ_b(6100)^– baryon were only 13 MeV heavier, it would be above the Λ_b⁰ K^– mass threshold

The observation of this baryon and the measurement of its properties are useful for distinguishing between different theoretical models predicting the excited beauty baryon states. It is curious to note that if the Ξ_b(6100)^– baryon were only 13 MeV heavier, a tiny 0.2% change, it would be above the Λ_b⁰ K^– mass threshold and could decay to this final state. The Ξ_b(6100)^– might also shed light on the nature of previous discoveries: if it is the 3/2^– member of the lightest orbital excitation isodoublet, then the Ξ_b(6227) isodoublet recently found by the LHCb collaboration could be the 3/2^– orbital excitation of Ξ_b′ or Ξ_b^* baryons. 

The post New excited beauty-strange baryon observed appeared first on CERN Courier.

The electroweak session of the Rencontres de Moriond convened more than 200 participants virtually from 22 to 27 March in a new format, with pre-recorded plenary talks and group-chat channels that went online in advance of live discussion sessions. The following week, the QCD and high-energy interactions session took place with a more conventional virtual organisation.

The highlight of both conferences was the new LHCb result on R_K based on the full Run 1 and Run 2 data, and corresponding to an integrated luminosity of 9 fb^–1, which led to the claim of the first evidence for lepton-flavour-universality (LFU) violation from a single measurement. R_K is the ratio of the branching fractions for the decays B⁺ → K⁺ μ⁺ μ^– and B⁺ → K⁺ e⁺ e^–. LHCb measured this ratio to be 3.1σ below unity, despite the fact that the two branching fractions are expected to be equal by virtue of the well-established property of lepton universality (see New data strengthens R_K flavour anomaly). Coupled with previously reported anomalies of angular variables and the R_K*, R_D and R_D* branching-fraction ratios by several experiments, it further reinforces the indications that LFU may be violated in the B sector. Global fits and possible theoretical interpretations with new particles were also discussed. 

Important contributions

Results from Belle II and BES III were reported. Some of the highlights were a first measurement of the B⁺ → K⁺ νν decay and the most stringent limits to date for masses of axions between 0.2 and 1 GeV from Belle II, based on the first data they collected, and searches for LFU violation in the charm sector from BES III that for the moment give negative results. Belle II is expected to give important contributions to the LFU studies soon and to accumulate an integrated luminosity of 50 ab^–1 10 years from now.

ATLAS and CMS presented tens of new results each on Standard Model (SM) measurements and searches for new phenomena in the two conferences. Highlights included the CMS measurement of the W leptonic and hadronic branching fraction with an accuracy larger than that measured at LEP for the branching fractions to the electron and muon, and the updated ATLAS evidence of the four-top-production process at 4.7σ (with 2.6σ expected). ATLAS and CMS have not yet found any indications of new physics but continue to perform many searches, expanding the scope to as-yet unexplored areas, and many improved limits on new-physics scenarios were reported for the first time at both conference sessions.

Several results and prospects of electroweak precision measurements were presented and discussed, including a new measurement of the fine structure constant with a precision of 80 parts per trillion, and a measurement at PSI of the null electric dipole moment of the neutron with an uncertainty of 1.1 × 10^–26 e∙cm. Theoretical predictions of (g–2)_μ were discussed, including the recent lattice calculation from the Budapest–Marseille–Wuppertal group of the hadronic–vacuum–polarisation contribution, which, if used in comparison with the experimental measurement, would bring the tension with the (g–2)_μ prediction to within 2σ.

In the neutrino session, the most relevant recent new results of last year were discussed. KATRIN reported updated upper limits on the neutrino mass, obtained from the direct measurement of the endpoint of the electron spectrum of the tritium β decay, while T2K showed the most recent results concerning CP violation in the neutrino sector, obtained from the simultaneous measurement of the ν_μ and ν_μ disappearance, and ν_e and ν_e  appearance. The measurement disfavours at 90% CL the CP-conserving values 0 and π of the CP-violating parameter of the neutrino mixing matrix, δ_CP, and all values between 0 and π.

The quest for dark matter is in full swing and is expanding on all fronts. XENON1T updated delegates on an intriguing small excess in the low-energy part of the electron-recoil spectrum, from 1 to 7 keV, which could be interpreted as originating from new particles but that is also consistent with an increased background from tritium contamination. Upcoming new data from the upgraded XENONnT detector are expected to be able to disentangle the different possibilities, should the excess be confirmed. The Axion Dark Matter eXperiment (ADMX) is by far the most sensitive experiment to detect axions in the explored range around 2 μeV. ADMX showed near-future prospects and the plans for upgrading the detector to scan a much wider mass range, up to 20 μeV, in the next few years. The search for dark matter also continues at accelerators, where it could be directly produced or be detected in the decays of SM particles such as the Higgs boson.

The quest for dark matter is in full swing and is expanding on all fronts

ATLAS and CMS also presented new results at the Moriond QCD and high-energy-interactions conference. Highlights of the new results are: the ATLAS full Run-2 search for double-Higgs-boson production in the bbγγ channel, which yielded the tightest constraints to date on the Higgs-boson self-coupling, and the measurement of the top-quark mass by CMS in the single-top-production channel that for the first time reached an accuracy of less than 1 GeV, now becoming relevant to future top-mass combinations. Several recent heavy-ion results were also presented by the LHC experiments, and by STAR and PHENIX at RHIC, in the dedicated heavy-ion session. One highlight was a result from ALICE on the measurement of the Λ_c⁺ transverse-momentum spectrum and the Λ_c⁺ /D⁰ ratio in pp and p–Pb collisions, showing discrepancies with perturbative QCD predictions.

The above is only a snapshot of the many interesting results presented at this year’s Rencontres de Moriond, representing the hard work and dedication of countless physicists, many at the early-career stage. As ever, the SM stands strong, though intriguing results provoked lively debate during many virtual discussions.

The post Anomalies intrigue at Moriond appeared first on CERN Courier.

The principle that the charged leptons have identical electroweak interaction strengths is a distinctive feature of the Standard Model (SM). However, this lepton-flavour universality (LFU) is an accidental symmetry in the SM, which may not hold in theories beyond the SM. The LHCb collaboration has used a number of rare decays mediated by flavour-changing neutral currents, where the SM contribution is suppressed, to test for deviations from LFU. During the past few years, these and other measurements, together with results from B-factories, hint at possible departures from the SM.

In a new measurement of a LFU-sensitive parameter “R_K” with increased precision and statistical power, reported today at the Rencontres de Moriond, LHCb has strengthened the significance of the flavour anomalies. The value R_K  probes the ratio of B-meson decays to muons and electrons: R_K = BR(B⁺→K⁺μ⁺μ^–)/BR(B⁺→K⁺e⁺e^–). Testing LFU in such b→sℓ⁺ℓ^– transitions has the advantage that not only are SM contributions suppressed, but the theoretical predictions are very precise. Therefore, any significant deviation of R_K from unity would imply physics beyond the SM.

The experimental challenge lies in the fact that, while electrons and muons interact via the electroweak force in the same way, the small electron mass means it interacts with detector material much more than muons. For example, electrons radiate a significant number of bremsstrahlung photons when traversing the LHCb detector, which degrades reconstruction efficiency and signal resolution compared to muons. The key to control this effect is to use the decays J/ψ→e⁺e^– and J/ψ→μ⁺μ^–, which are known to have the same decay probability and can be used to calibrate and test electron reconstruction efficiencies. High precision tests with the J/ψ are compatible with LFU, which provides a powerful cross-check on the experimental analysis.

Previous LHCb measurements of R_K and R_K* (which probes B⁰→K*ℓ⁺ℓ^– decays) in 2019 and 2017 respectively, provide hints of deviations from unity. The latest analysis of R_K, which uses the full dataset collected by the experiment in Run 1 and Run 2 of the LHC, represents a substantial improvement in precision on the previous measurement (see figure) thanks to doubling the dataset. The R_K ratio is measured to be three standard deviations from the SM prediction (see figure). This is the first time that a departure from LFU above this level has been seen in any individual B-meson decay, with a value of R_K=0.846^+0.042_-0.039 (stat.) ^+0.013_-0.012 (syst.).

Although it is too early to conclude anything definitive at this stage, this deviation is consistent with a pattern of anomalies which have manifested themselves in b→s ℓ⁺ℓ^– and similar processes over the course of the past decade. In particular, the strengthening R_K anomaly may be considered alongside hints from other measurements of these transitions, including angular asymmetries and decay rates.

The LHCb experiment is well placed to clarify the potential existence of new-physics effects in these decays. Updates on a suite of  b→s ℓ⁺ℓ^– related measurements with the full Run 1 and Run 2 dataset are underway. A major upgrade to the detector during the ongoing second long shutdown of the LHC will offer a step change in precision in Run 3 and beyond.

The post New data strengthens R_K flavour anomaly appeared first on CERN Courier.

The knowledge of f_s/f_d – the ratio of the fragmentation fraction of a b quark to a B_s⁰ or a B⁰ meson – is thus a key parameter at the LHC. So far it has been measured with limited precision and has been the dominant systematic uncertainty for most B⁰_s branching fractions. Now, however, the LHCb collaboration has, in a recent publication, combined the efforts of five different analyses with information on this parameter. The f_s/f_d ratio was measured in previous publications through semi­leptonic decays, hadronic decays with D mesons and hadronic decays with J/ψ mesons in the final state. Some of these measurements are only sensitive to the product of the fragmentation fraction and the branching fractions. This new work analyses these results simultaneously, obtaining a precise measurement of f_s/f_d as well as branching fraction measurements of two important decays, B⁰_s → D^–_s π⁺ and B⁰_s → J/ψ φ. These are golden channels for mixing and CP violation measurements in the B⁰_s sector.

Precision leap

The results reduce the uncertainty on f_s/f_d by roughly a factor of two for collisions at 7 TeV, and a factor of 1.5 for collisions at 13 TeV, yielding a precision of about 3%. They also confirm the dependence of f_s/f_d on the transverse momentum of the B⁰_s meson, and indicate a slight dependence on the centre-of-mass energy of proton–proton collisions (figure 1). The results are used in this work to update the previous branching-fraction measurements of about 50 different B⁰_s decay channels, significantly improving their precision, and boosting several searches for new physics.

The post Precision leap for B_s⁰ fragmentation and decay appeared first on CERN Courier.

Most travellers know that it is essential to have a good travel guide when setting out into unexplored territory. A book where one can learn what previous travellers discovered about these surroundings, with both global information on the language, history and traditions of the land to be explored, and practical details on how to overcome day-to-day difficulties. Andrzej Buras’s recent book, Gauge Theory of Weak Decays, is the ideal guide for both new physicists and seasoned travellers, and experimentalists and theoreticians alike, who wish to start a new expedition into the fascinating world of weak meson decays, in pursuit of new physics.

The physics of weak decays is one of the most active and interesting frontiers in particle physics, from both the theoretical and the experimental points of view. Major steps in the construction of the Standard Model (SM) have been made possible only thanks to key observations in weak decays. The most famous example is probably the suppression of flavour-changing neutral currents in kaon decays, which led Glashow, Iliopoulos and Maiani to postulate, in 1970, the existence of the charm quark, well before its direct discovery. But there are many other examples, such as the heaviness of the top quark, inferred from the large matter-to-antimatter oscillation frequency of neutral B mesons, again well before its discovery. In the post-Higgs-discovery era, weak decays are a privileged observatory in which to search for signals of physics beyond the SM. The recent “B-physics anomalies”, reported by LHCb and other experiments, could indeed be the first hint of new physics. The strategic role that weak decays play in the search for new physics is further reinforced by the absence on the horizon, at least in the near future, of a collider with a centre-of-mass energy exceeding that of the LHC, while the LHC and other facilities still have a large margin of improvement in precision measurements.

As Buras describes with clarity, signals of new physics in the weak decays of K, D, and B mesons, and other rare low-energy processes, could manifest themselves as deviations from the precise predictions of the corresponding decay rates that we are able to derive within the SM. In the absence of a reference beyond-the-SM theory, it is not clear where, and at which level of precision, these deviations could show up. But general quantum field theory arguments suggest that weak decays are particularly sensitive probes of new physics, as they can often be predicted with high accuracy within the SM.

The two necessary ingredients for a journey in the realm of weak decays are therefore precise SM predictions on the one hand, and a broad-spectrum investigation of beyond-the-SM sensitivity on the other. These are precisely the two ingredients of Buras’s book. In the first part, he guides the reader though all the steps necessary to arrive to the most up-to-date predictions for rare decays. This part of the book offers different paths to different readers: students are guided, in a very pedagogical way, from tree-level calculations to high-precision multi-loop calculations. Experienced readers can directly find up-to-date phenomenological expressions that summarise the present knowledge on virtually any process of current experimental interest. This part of the book can also be viewed as a well-thought-out summary of the history of precise SM calculations for weak decays, written by one of its most relevant protagonists.

Beyond the Standard Model

The second part of the book is devoted to physics beyond the SM. Here the style is quite different: less pedagogical and more encyclopaedic. Employing a pragmatic approach, which is well motivated to discuss low-energy processes, extensions of the SM are classified according to properties of hypothetical new heavy particles, from Z′ bosons to leptoquarks, and from charged Higgs bosons to “vector-like” fermions. This allows Buras to analyse the impact of such models on rare processes in a systematic way, with great attention paid to correlations between observables.

To my knowledge, this book is the first comprehensive monograph of this type, covering far more than just the general aspects of SM physics, as may be found in many other texts on quantum field theory. The uniqueness of this book lies in its precious details on a wide variety of interesting rare processes. It is a key reference that was previously missing, and promises to be extremely useful in the coming decades.

The post The hitchhiker’s guide to weak decays appeared first on CERN Courier.

The new exotic states were observed in an almost pure sample of 24 thousand B⁺→J/ψφK⁺ decays, which, as a three-body decay, may be visualised using a Dalitz plot (see “Mountain ridges” figure). Horizontal and vertical bands indicate the temporary production of tetraquark resonances which subsequently decay to a J/ψ meson and a K⁺ meson or a J/ψ meson and a φ meson, respectively. The most prominent vertical bands correspond to the cc̄ss̄ tetraquarks X(4140), X(4274), X(4500) and X(4700) which were first observed in June 2016. The collaboration has now resolved two new horizontal bands corresponding to the cc̄us̄ states Z_cs(4000)⁺ and Z_cs(4220)⁺, and two additional vertical bands corresponding to the cc̄ss̄ states X(4685) and X(4630).

These states may have very different inner structures

Liming Zhang

The results have already triggered theoretical head scratching. In November, the BESIII collaboration at the Beijing Electron–Positron Collider II reported the discovery of the first candidate for a charged hidden-charm tetraquark with strangeness, tentatively dubbed Z_cs(3985)^– (CERN Courier January/February 2021 p12). It is unclear whether the new Z_cs(4000)⁺ tetraquark can be identified with this state, say physicists. Though their masses are consistent, the width of the BESIII particle is ten times smaller. “These states may have very different inner structures,” says lead analyst Liming Zhang of the LHCb collaboration. “The one seen by BESIII is a narrow and longer-lived particle, and is easier to understand with a nuclear-like hadronic molecular picture, where two hadrons interact via a residual strong force. The one we observed is much broader, which would make it more natural to interpret as a compact multiquark candidate.”

59 hadrons

The new observations take the tally of new hadronic states discovered at the LHC – which includes several pentaquarks as well as rare and excited mesons and baryons – to 59 (see “Diagram of discovery” figure). Though quantum chromodynamics naturally allows the existence of states beyond conventional two- and three-quark mesons and baryons, the detailed mechanisms responsible for binding multi-quark states are still largely mysterious. Tetraquarks, for example, could be tightly bound pairs of diquarks or loosely bound meson-meson molecules – or even both, depending on the production process.

Who would have guessed we’d find so many exotic hadrons?

Patrick Koppenburg

“Who would have guessed we’d find so many exotic hadrons?” says former LHCb physics coordinator Patrick Koppenburg, who put the plot together. “I hope that they bring us to a better modelling of the strong interaction, which is very much needed to understand, for instance, the anomalies we see in B-meson decays.”

The post LHCb observes four new tetraquarks appeared first on CERN Courier.

|V_ub| and |V_cb| are the focus of a longstanding puzzle. When comparing the world-average values derived from inclusive and exclusive B-meson decays, respectively, the inclusive and exclusive measurements disagree by more than three standard deviations, for measurements of both |V_ub| and |V_cb|. Traditionally, the exclusive |V_ub| determination requires the reconstruction of the semileptonic b → u decay B⁰ → π^–μ⁺ν_μ. LHCb also has access to B_s⁰ meson and b-baryon decays, but the missing neutrino makes it difficult to isolate the signal from the copious background. Defying expectations, however, in 2015 LHCb managed to observe the Λ_b⁰ → pμ^–ν_μ decay, and used the normalisation channel Λ_b⁰ → Λ⁺_cμ^–ν_μ to determine |V_ub|/ |V_cb|. The main difficulty in this type of analysis resides in the fact that only two charged particles are reconstructed in decays such as B_s⁰ → K^–μ⁺ν_μ and Λ_b⁰ → pμ^–ν_μ. A huge background arising from other sources dominates the selected data sample. Machine-learning algorithms are therefore used to isolate the signal from the various background categories consisting of decays with additional charged and/or neutral particles in the final state. The remaining irreducible background is modelled by using both simulation and control samples extracted from data.

This is the first experimental test of the form-factor calculations

First observation

In a recent paper, the LHCb collaboration presented the first observation of the decay B_s⁰ → K^–μ⁺ν_μ. The decay B_s⁰ → D_s^– μ⁺ν_μ is used as a normalisation channel to minimise experimental systematic uncertainties. The study was performed in two regions of the squared invariant mass (or momentum transfer) q² of the muon and the neutrino below and above 7 GeV². The observed total yield was about 13,000 events, corresponding to a branching fraction of (1.06 ± 0.10) × 10^–4, of which about one third stemmed from the low q² range (figure 1).

The extraction of the ratio |V_ub|/|V_cb| requires external knowledge of the form factors describing the strong B_s⁰ → K^– and B_s⁰ → D_s^– transitions, to account for the interactions of the quarks bound in mesons. These vary with the momentum transfer and are calculated using non-perturbative techniques, such as lattice QCD (LQCD) and light-cone sum rules (LCSR). As LQCD and LCSR calculations are more accurate at high and low q², respectively, they are used in the corresponding q² regions. The obtained value of |V_ub|/|V_cb| = 0.095 ± 0.008 in the high q² interval shows agreement with the world average of exclusive measurements, and with the LHCb result using Λ_b⁰ → pμ^–ν_μ decays, while in the low q² region, |V_ub|/|V_cb| = 0.061 ± 0.004 is significantly lower (figure 2). This is the first experimental test of the form-factor calculations, and new results are expected in the near future. These will help settle the exclusive versus inclusive debate surrounding the values of |V_ub| and |V_cb|, and provide further constraints on the unitarity triangle.

The post LHCb sheds light on V_ub puzzle appeared first on CERN Courier.

The international conference devoted to b-hadron physics at frontier machines, Beauty 2020, took place from 21 to 24 September, hosted virtually by Kavli IPMU, University of Tokyo. This year’s edition, the 19th in the series, attracted around 350 registrants, significantly more than have attended physical Beauty conferences in the past. Two days were devoted to parallel sessions, a change in approach necessitated by the online format, stimulating lively discussion. There were 64 invited talks, of which 13 were overviews given by theorists.

Studies of beauty hadrons have great sensitivity to possible physics beyond the Standard Model (SM), as was stressed by Gino Isidori (University of Zurich) in the opening talk of the conference. Possible lepton-universality anomalies that have emerged from analyses of decays into pairs of leptons and accompanying hadrons are particularly tantalising, as they show significant deviations from the SM in a manner that could be explained by the existence of new particles such as leptoquarks or Z′ bosons. We will know much more when LHCb releases measurements from the updated analysis of the full Run-2 data set. In the meantime, the combined results from ATLAS, CMS and LHCb for the branching ratio of the ultra-rare decay B_s → μ⁺μ^– generated much discussion. This final state is produced only a few times every billion B_s decays, but is now measured to a remarkable precision of 13%. Intriguingly, the observed value of the branching ratio lies two standard deviations below the SM prediction (see “Ultra-rare” figure) – an effect that some commentators have noted could be driven by the same new particles invoked to explain the other flavour anomalies.

Recent impressive results were shown in the field of CP violation. LHCb presented the first ever observation of time-dependent CP violation in the B_s system – a phenomenon that has eluded previous experiments on account of the very fast (a rate of about 3 × 10¹² Hz) B_s oscillations and inadequate sample sizes. In addition, new LHCb results were shown for the CP-violating phase γ. The most precise of these comes from an analysis that isolates B → DK decays which are followed by D → K_Sπ⁺π^– decays, and the distributions of the final-state particles compared depending on whether they originate from B^– or B⁺ mesons. This analysis is based on the full Run 1 and Run 2 data sets and constrains γ to a precision of five degrees, which from this single analysis alone represents around a four-fold improvement compared to when the LHC began operation. Further improvements are expected over the coming years.

Participants were eager to learn about the progress of the SuperKEKB accelerator and Belle II experiment. SuperKEKB is now operating at higher luminosity than any previous electron–positron machine, and the data set collected by Belle II (of the order 100 fb^–1) is already sufficient to demonstrate the capabilities of the detector and to allow for important early physics studies, which were shown during the week. Belle II has superior performance to the first-generation B-factory experiments, BaBar and Belle, in areas such as flavour tagging and proper-time resolution, and will collect around 50 times the integrated luminosity. By the end of the decade Belle II will have accumulated 50 ab^–1 of data, from which many precise and exciting physics measurements are expected.

Recent impressive results were shown in the field of CP violation

Studies of kaon decays provide important insights into flavour physics that are complementary to those obtained from b-hadrons. The NA62 collaboration presented its updated branching ratio for the ultra-rare decay K⁺ → π⁺νv, which is predicted to be around 10^–10 in the SM. The data set is now sufficiently large to see a signal with a significance of more than three standard deviations. Future running is planned to allow a measurement to be made with a 10–20% precision, which will provide a powerful test of the SM prediction (CERN Courier September/October 2020 p9).

The concluding plenary session focused on the future of flavour physics. The LHCb experiment is currently being upgraded, and a further upgrade is foreseen at the end of the decade. In parallel, the upgrades of ATLAS and CMS will increase their capabilities for beauty studies. In the electron–positron domain, Belle II will continue to accumulate data, and there is the exciting possibility of a super-tau-charm factory, situated in either China or Russia, which will collect very large data sets at lower energies. These prospects were surveyed by Phillip Urquijo (University of Melbourne) in the summary talk of the conference, who stressed the importance of exploiting these ongoing and future facilities to the maximum. Flavour studies have a bright future, and they are sure to retain a central role in our search for physics beyond the SM.

The post Heavy-flavour highlights from Beauty 2020 appeared first on CERN Courier.

Quarkonia, the bound states of charm and anti-charm or bottom and anti-bottom quarks, are an important tool to test our knowledge of quantum chromodynamics (QCD). At the LHC, the study of quarkonia polarisations offers a valuable new window onto how heavy quarks bind together in such states. Understanding quarkonium polarisation has already proven to be difficult at lower energies, however, and measurements at the LHC pose significant further challenges.

ALICE measures quarkonia spin orientations with respect to a chosen axis via a measurement of the anisotropy in the angular distribution of the decay products. The angular distribution is parametrised in terms of the polarisation parameters, λ_θ, λ_φ and λ_θφ, where θ and φ are the polar and azimuthal emission angles. If all of them are null, no polarisation is present, whereas (λ_θ = 1, λ_φ = 0, λ_θφ = 0) and (λ_θ = –1, λ_φ = 0, λ_θφ = 0) indicate a polarisation of the spin in the transverse and longitudinal directions, respectively.

Polarisation studies represent a valuable tool for the study of the properties of quark–gluon plasma

In pp collisions, polarisation has been mainly used to investigate the J/ψ production mechanism. Reproducing the small values of polarisation parameter λ_θ observed at the LHC is a challenge for many theoretical models. Until recently, no corresponding results were available for nucleus–nucleus collisions, and in this domain polarisation studies represent a valuable tool for the study of the properties of quark–gluon plasma (QGP). The formation of this deconfined, strongly interacting medium impacts differently on the various quarkonium resonances, inducing a larger suppression on the less bound excited states ψ(2S) and χ_c, and modifying their feed-down fractions into the ground state, J/ψ. This effect may lead to a variation of the overall polarisation values since different charmonium states are expected to be produced with different polarisations. In addition, the recombination of uncorrelated heavy-quark pairs inside the QGP gives rise to an extra source of J/ψ, which can further modify the overall polarisation with respect to pp collisions.

The ALICE experiment has recently made the first measurements of the J/ψ and ϒ(1S) polarisation in Pb–Pb collisions. The data correspond to a centre-of-mass energy √(s_NN) = 5.02 TeV, and the rapidity range 2.5 < y < 4. The measurements were carried out in the dimuon decay channel, and results were obtained in two different reference frames, helicity and Collins–Soper, each of them with its own definition of the quantisation axis. In the helicity frame, the quarkonium momentum direction in the laboratory is chosen, while the bisector of the angle formed by the two colliding beams boosted in the quarkonium rest frame is used in the Collins–Soper frame. The J/ψ polarisation parameters, evaluated in three p_T bins covering the range between 2 and 10 GeV, are close to zero, but with a maximum positive deviation for λ_θ (corresponding to a transverse polarisation) of 2σ for 2 < p_T < 4 GeV in the helicity reference frame. Interestingly, the corresponding LHCb pp result for prompt J/ψ at √(s_NN) = 7 TeV instead shows a small but significant longitudinal polarisation.

The observation of a significant difference between J/ψ polarisation results in pp and Pb–Pb collisions motivates further experimental and theoretical studies, with the main goal of connecting this observable with the known suppression and regeneration mechanisms in heavy-ion collisions. For the rarer ϒ(1S), a bound state of a bottom and an antibottom quark, the inclusive polarisation parameters were found to be compatible with zero within sizeable uncertainties. A higher precision and momentum-differential measurement will be enabled by the ten-fold larger Pb–Pb luminosity expected in Run 3 of the LHC.

The post J/ψ polarisation differs in lead collisions appeared first on CERN Courier.

Exotic charmonium-like states are a very active field of study at the LHC. These states have atypical properties such as non-zero electric charges and strong decays that violate isospin symmetry. The exotic X(3872) charmonium state discovered by the Belle collaboration in 2003 displays such isospin-violating strong decays and has a natural width of about 1 MeV, which is unexpectedly narrow for a state with mass very close to the D^*0 D⁰ threshold.

Luciano Maiani and collaborators pointed out that the new CMS measurement can naturally be explained by a tetraquark model of X(3872)

Several theoretical interpretations of the internal structure of these charmonium-like states have been proposed to explain their peculiar properties. To choose the most adequate model for each state, we must continue studying their properties and improving the determination of their parameters. As for the X(3872), although it is inconsistent with the predicted conventional charmonium states and does not have a definite isospin, its production partially resembles that of ordinary charmonium states such as ψ(2S) or χ_c1(1P). One of the ways to evaluate the degree of similarity between X(3872) and ψ(2S) is to compare their production rates in exclusive b-hadron decays. In the case of ψ(2S), which is a conventional charmonium state, the branching fractions of the decays B⁰_s → ψ(2S)φ, B⁺ → ψ(2S)K⁺, and B⁰ → ψ(2S)K⁰, are approximately equal to each other. Recent CMS measurements of the corresponding rates for decays to X(3872) show differences, however, which may provide a clue to the nature of this exotic charmonium-like state.

Recently the CMS collaboration observed the decay B⁰_s → X(3872)φ for the first time, with a significance exceeding five standard deviations. The X(3872) is reconstructed via its decay to J/ψπ⁺π^–, followed by a decay of the J/ψ meson into a pair of muons, and of the φ meson to a pair of charged kaons (figure 1).

Diquark hypothesis

At a simple Feynman-diagram level, this decay is a close analogue to the B⁺ → X(3872)K⁺ and B⁰ → X(3872)K⁰ decays that have previously been observed. The ratio of the branching fractions of this new B⁰_s decay to that of the B⁺ decay is significantly below unity at 0.48 ± 0.10, while a similar ratio for the decays involving ψ(2S) is consistent with unity. This is not expected from naive “spectator-quark” considerations, based on a simple tree-level diagram, and assuming X(3872) is a pure charmonium state. The measured ratio also happens to be consistent with the analogous ratio for the B⁰ → X(3872)K⁰ to B⁺ → X(3872)K⁺ decays, though the latter ratio has not yet been measured with high accuracy. The results suggest that spectator quarks behave differently in the B⁺ and B⁰_(s) two-body decays into X(3872) and a light meson. In a recent theoretical paper, former CERN Director-General Luciano Maiani and collaborators pointed out that the new CMS measurement can naturally be explained by a tetraquark model of X(3872), which describes this exotic particle as a bound state of a diquark (charm and up quarks) and its anti-diquark.

Further studies of X(3872) are now important in order to gain a deeper understanding of its exotic properties and uncover its mysterious nature. The results may have interesting consequences for our understanding of quantum chromodynamics.

The post CMS reaffirms exotic nature of the X(3872) appeared first on CERN Courier.

This was the first ICHEP meeting since the publication of the update of the European strategy for particle physics, which presented an ambitious vision for the future of CERN. Though VIP-guest Peter Gabriel – rock star and human rights advocate – may not have been aware of this when delivering his opening remarks, his urging that delegates speak up for science and engage with politicians resonated with the physicists virtually present.

Many scientific highlights were covered at ICHEP and it is only possible to scratch the surface here. The results from all four major LHC experiments were particularly impressive and the collective progress in understanding the properties of neutrinos shows no sign of slowing down.

Higgs physics
ATLAS and CMS presented the first evidence for the decay of the Higgs boson into a pair of muons. Combined, the results provide strong evidence for the coupling of the Higgs boson to the muon, with the strength of the coupling consistent with that predicted in the Standard Model. Prior to these new results, the Higgs had only been observed to couple to the much heavier third-generation fermions and the W and Z gauge bosons. The measurements also provide further evidence for the linearity of the Higgs coupling, now over four orders of magnitude (from the muon to top quark), indicating the universality of the Standard-Model Higgs boson as the mechanism through which all Standard Model particles acquire mass. These are highly non-trivial statements.

ATLAS also presented a combined measurement of the Higgs signal strength, which describes a common scaling of the expected Higgs-boson yields in all processes, of 1.06 ± 0.07. In this measurement, the experimental and theoretical uncertainties are now roughly equal, emphasising the ever-increasing importance of theoretical developments in keeping up with the experimental progress; a feature that will ultimately determine the precision that will be reached by the LHC and high-luminosity LHC (HL-LHC) Higgs physics programmes.

The range of Standard Model measurements presented at ICHEP 2020 by ATLAS and CMS was truly impressive

More generally, the precision we are seeing from the ATLAS and CMS Run 2 proton–proton data is truly impressive, and an exciting indication of what is to come as the integrated luminosity accumulated by the experiments ramps up, and then ramps up again in the HL-LHC era. One interesting new example was the first observation of WW production from photon–photon collisions, where the photons are radiated from the incoming proton beams. This is a neat measurement that demonstrates the breadth of physics accessible at the LHC.

Overall, the range of Standard Model measurements presented at ICHEP 2020 by ATLAS and CMS was truly impressive and we should not forget that it is still relatively early in the LHC programme. In parallel, direct searches for new phenomena, such as supersymmetry and the “unexpected”, continues apace. Results from direct searches at the energy frontier were covered in numerous parallel session presentations. The current status was summarised succinctly by Paris Sphicas (Athens) in his conference summary talk: “Looked for a lot of possible new things. Nothing has turned up yet. Still looking intensively.”

Flavour physics
Over the last few years, a number of deviations from theoretical predictions have been observed in B-meson decays to final states with leptons. Discrepancies have been observed in ratios of decays to different lepton species, and in the angular distribution of decay products. Taken alone, each of these discrepancies are not particularly significant, but collectively they may be telling us something new about nature. At ICHEP 2020, the LHCb experiment presented their recently published results on the angular analysis in B⁰ → K^*0 μ⁺μ^–. The overall picture remains unchanged. The full analysis of the LHCb Run-2 data set, including updated measurements of the relative rates of the muon and electron decay modes (R_K and R_K*), is eagerly awaited.

The search for rare kaon decays continues to attract interest

The search for rare kaon decays continues to attract interest. One of the most impressive results presented at ICHEP was the recent observation by NA62 of the extremely rare kaon decay, K⁺ → π⁺νν̄. Occurring only once in every 10 billion decays, this is an incredibly challenging measurement and the new NA62 result is the first statistically significant observation of this decay, based on just 17 events. Whilst the observed rate is consistent with the Standard Model expectation, its observation opens up a new future avenue for exploring the possible effects of new physics.

Neutrino physics
Neutrino physics continues to be one of the most active areas of research in particle physics, so it was not surprising that the neutrino parallel sessions were the best attended of the conference. This is a particularly interesting time, with long-baseline neutrino oscillation experiments becoming sensitive to the neutrino mass ordering, and beginning to provide constraints on CP violation. Updates were presented by the NOvA experiment in the USA and the T2K experiment in Japan. Both experiments favour the normal-ordering hypothesis, although not definitively, and there is currently a slight tension between the CP violation results from the two experiments. It is worth noting that the combined interpretation of the two experiments is quite complex. The NOvA and T2K collaborations are working on a combined analysis to clarify the situation.

There were also a number of presentations on the next generation of long-baseline neutrino oscillation experiments, DUNE in the US and Hyper-Kamiokande in Japan, which aim to make the definitive discovery of CP violation in the neutrino sector. In the context of DUNE, the progress with liquid-argon time-projection- chamber (LArTPC) detector technology is impressive. It was particularly pleasing to see a number of physics results from MicroBooNE at Fermilab, and the single-phase DUNE detector prototype at CERN (ProtoDUNE-SP), that are based on the automatic reconstruction of LArTPC images – a longstanding challenge.

Virtual success
A vast range of high-qualify scientific research was covered in the 800 parallel session presentations and summarised in the 44 plenary talks at ICHEP 2020. The quality of the presentations was high, and speakers coped well with the challenge of pre-recording talks. The “replay” sessions worked extremely well too – an innovation that is likely to persist in the post-COVID world. About 3000 people registered for the meeting, which is more than double the previous two events. It was particularly pleasing to learn that almost 2500 connected to the parallel sessions.

Despite the many successes, we all missed the opportunity to meet colleagues in person; it is often the informal discussions over coffee or in restaurants and bars that generate new ideas and, importantly, lead to new collaborations. Whilst virtual conferences are likely to remain a feature in the post- COVID world, they will not replace in-person events.

The post ICHEP’s online success appeared first on CERN Courier.

An important virtue of K⁺→π⁺νν̄ is its clean theoretical character

Andrzej Buras

A flavour-changing, neutral-current process, K⁺→π⁺νν̄ is highly suppressed in the Standard Model, with contributions from Z-penguin and box diagrams with W, top quark and charm exchanges. The measured branching fraction of 110⁺⁴⁰_-35 per trillion K⁺→π⁺νν̄ decays is in agreement with the Standard Model prediction of 84 ± 10 per trillion (JHEP 11 033). “A particular and very important virtue of K⁺→π⁺νν̄ is its clean theoretical character, which can only be matched among meson decays by K_L→π⁰νν̄, and possibly B_s,d→μ⁺μ^–,” says Andrzej Buras of the Institute for Advanced Study in Garching, Germany. “This is related to the fact that the low-energy hadronic matrix elements are just those of the quark currents between the hadronic states, which can be extracted from the leading semileptonic decay K⁺→π⁰e⁺ν,” he explains, noting that higher-order QCD and electroweak corrections are already well known, and lattice QCD calculations should soon tackle the small, “long-distance” contributions to the amplitude.

NA62 observes the 6% of positively charged kaons that are produced when 450 GeV protons from the Super Proton Synchrotron strike a beryllium target. The analysis is challenging because of the tiny branching fraction and the presence of a neutrino pair in the final state. Pioneering the technique of observing kaon decays in flight, the collaboration measures the kinematics of both the initial kaon and the final-state pion to isolate the kinematic signature of K⁺→π⁺νν̄, before then suppressing other decay modes by a further eight orders of magnitude using particle-identification techniques.

The collaboration’s new result adds a further 17 events to its previous analysis (arXiv:2007.08218, submitted to JHEP), wherein three events observed in 2016 and 2017 yielded an estimated branching fraction of 47 ⁺⁷²_-47 decays per trillion. The previous best measurement was by Brookhaven National Laboratory’s E787 and E949 experiments in the 2000s, which together inferred a branching fraction of 173 ⁺¹¹⁵_-105 per trillion (Phys. Rev. Lett. 101 191802).

Meanwhile in Japan

The NA62 result is expected soon to be complemented by a measurement of the related CP-violating K_L→π⁰νν̄ decay by the KOTO collaboration at the J-PARC research facility in Tokai, Japan. This even rarer process has a predicted Standard Model branching fraction of just 34 ± 6 per trillion. KOTO’s 2015 data yielded no event candidates and a 90% confidence upper limit on the branching fraction of 3.0 per billion (Phys. Rev. Lett. 122 021802). The collaboration is now finalising its results from the 2016–2018 run, and plans to improve its sensitivity to less than 0.1 per billion by increasing the beam intensity and upgrading the KOTO detectors.

As experimental uncertainties are expected to approach the theoretical precision in coming years, explains Buras, K⁺→π⁺νν̄ and K_L→π⁰νν̄ decays can probe scales as high as a few hundred TeV – beyond the reach of most B-meson decays. “K⁺→π⁺νν̄ is most sensitive to hypothetical Z′ gauge bosons, vector-like quark models, supersymmetry and some leptoquark models,” he says. “LHCb studies of K_S→ μ⁺μ^– and Belle II studies of B→ K(K*)νν̄ will also have a part to play, allowing a global analysis to test not only the concept of minimal flavour violation, but also probe new CP-violating phases and right-handed currents.”

Theorists expect to reach an accuracy of 5% on the predicted K⁺→π⁺νν̄ branching ratio towards the end of the decade. In the same period, the NA62 team is seeking to hone its resolution from the current 30% down to 10%. The collaboration will resume data taking in 2021, following upgrades to both beam and detector taking place during the ongoing second long shutdown of CERN’s accelerator complex.

Sensitivity to decay rates below the 10^–11 level is now in sight

Cristina Lazzeroni

“The horizon of a new-physics programme with a sensitivity to decay rates well below the 10^–11 level is now in sight,” says NA62 spokesperson Cristina Lazzeroni of the University of Birmingham, UK. “The instruments and techniques developed for the NA62 experiment will lead to the next generation of rare-kaon-decay experiments. For the longer term future, a high-intensity kaon-beam programme is starting to take shape at CERN, with prospects to measure the K⁺→π⁺νν̄ decay to a few per cent, address the analogous decay of the neutral kaon, and reach extreme sensitivities to a large variety of rare kaon decays that are complementary to investigations in the beauty-quark sector.”

The post Double digits for ultra-rare kaon decay appeared first on CERN Courier.

LHCP20 presentations covered a wide assortment of topics and several new results with significantly enhanced sensitivity than was previously possible. These included both precision measurements with excellent potential to uncover discrepancies that can be explained only by beyond the Standard Model (SM) physics and direct searches using innovative techniques and advanced analysis methods to look for new particles.

The first observation of the combined production of three massive vector bosons was reported by CMS

The first observation of the combined production of three massive vector bosons (VVV with V = W or Z) was reported by the CMS experiment. In the nearly 40 years that have followed the discovery of the W and Z boson, their properties have been measured very precisely, including via “diboson” measurements of the simultaneous production of two vector bosons. However, “triboson” simultaneous production of three massive vector bosons had eluded us so far, as the cross sections are small and the background contributions are rather large. Such measurements are crucial to undertake, both to test the underlying theory and to probe non-standard interactions. For example, if new physics beyond the SM is present at high mass scales not far above 1 TeV, then cross section measurements for triboson final states might deviate from SM predictions. The CMS experiment took advantage of the large Run 2 dataset and machine learning techniques to search for these rare processes. Leveraging the relatively background-free leptonic final states, CMS collaborators were able to combine searches for different decay modes and different types of triboson production (WWW, WWZ, WZZ and ZZZ) to achieve the first observation of combined heavy triboson production (with an observed significance of 5.7 standard deviations) and at the same time evidence for WWW and WWZ production with observed significances of 3.3 and 3.4 standard deviations, respectively. While the results obtained so far are in agreement with SM predictions, more data is needed for the individual measurements of the WZZ and ZZZ processes.

Four-top-quark production

The first evidence for four-top-quark production was announced by ATLAS. The top-quark discovery in 1995 launched a rich programme of top-quark studies that includes precision measurements of its properties as well as the observation of single-top-quark production. In particular, since the large mass of the top quark is a result of its interaction with the Higgs field, studies of rare processes such as the simultaneous production of four top quarks can provide insights into properties of the Higgs boson. Within the SM, this process is extremely rare, occurring just once for every 70 thousand pairs of top quarks created at the LHC; on the other hand, numerous extensions of the SM predict exotic particles that couple to top quarks and lead to significantly higher production rates. The ATLAS experiment performed this challenging measurement using the full Run-2 dataset using sophisticated techniques and machine-learning methods applied to the multilepton final state to obtain strong evidence for this process. The observed signal significance was found to be 4.3 standard deviations, in excess of the expected sensitivity of 2.4, assuming SM four-top-quark-production properties. While the measured value of the cross section was found to consistent with the SM prediction within 1.7 standard deviations, the data collected during Run 3 will shed further light on this rare process.

The LHCb collaboration presented, with unprecedented precision, measurements of two properties of the mysterious X(3872) particle. Originally discovered by the Belle experiment in 2003 as a narrow state in the J/ψπ⁺π⁻ mass spectrum of B⁺→J/ψπ⁺π⁻K⁺ decays, this particle has puzzled particle physicists ever since. The nature of the state is still unclear and several hypotheses have been proposed, such as its being an exotic tetraquark (a system of four quarks bound together), a two-quark hadron, or a molecular state consisting of two D mesons. LHCb collaborators reported the most precise mass measurement yet, and measured, for the first time, and with 5 standard-deviations significance, the width of the resonance (see LHCb interrogates X(3872) line-shape). Though the results favour its interpretation as a quasi-bound D⁰D^*0 molecule, more data and additional analyses are needed to rule out other hypotheses.

Antideuterons could be produced during the annihilation or decay of neutralinos or sneutrinos

The ALICE collaboration presented a first measurement of the inelastic low-energy antideuteron cross section using p-Pb collisions at a centre-of-mass energy per nucleon–nucleon pair of 5.02 TeV. Low-energy antideuterons (composed of an antiproton and an antineutron) are predicted by some models to be a promising probe for indirect dark-matter searches. In particular, antideuterons could be produced during the annihilation or decay of neutralinos or sneutrinos, which are hypothetical dark-matter particles. Contributions from cosmic-ray interactions in the low-energy range below 1-2 GeV per nucleon are expected to be small. ALICE collaborators used a novel technique that utilised the detector material as an absorber for antideuterons to measure the production and annihilation rates of low energy antideuterons. The results from this measurement can be used in propagation models of antideuterons within the interstellar medium for interpreting dark-matter searches, including intriguing results from the AMS experiment. Future analyses with higher statistics data will improve the modelling as well as extend these studies to heavier antinuclei.

The above are just a few of the many excellent results that were presented at LHCP2020. The extraordinary performance of the LHC coupled with progress reported by the theory community, and the excellent data collected by the experiments, has inspired LHC physicists to continue with their rich harvest of physics results despite the current world crisis. Results presented at the conference showed that huge progress has been made on several fronts, and that Run 3 and the High-Luminosity LHC upgrade programme will enable further exploration of particle physics at the energy frontier.

The post LHC physics shines amid COVID-19 crisis appeared first on CERN Courier.

In 2003, the Belle collaboration reported the discovery of a mysterious new hadron, the X(3872), in the decay B⁺→X(3872)K⁺. Their analysis suggested an extremely small width, consistent with zero, and a mass remarkably close to the sum of the masses of the D⁰ and D^*0 mesons. The particle’s existence was later confirmed by the CDF, D0, and BaBar experiments. LHCb first reported studies of the X(3872) in the data sample taken in 2010, and later unambiguously determined its quantum numbers to be 1⁺⁺, leading the Particle Data Group to change the name of the particle to χ_c1(3872).

The nature of this state is still unclear. Until now, only an upper limit on the width of the χ_c1(3872) of 1.2 MeV has been available. No conventional hadron is expected to have such a narrow width in this part of the otherwise very well understood charmonium spectrum. Among the possible explanations are that it is a tetraquark, a molecular state, a hybrid state where the gluon field contributes to its quantum numbers, or a glueball without any valence quarks at all. A mixture of these explanations is also possible.

Two new measurements

As reported at the LHCP conference this week, the LHCb collaboration has now published two new measurements of the width of the χ_c1(3872), based on minimally overlapping data sets. The first uses Run 1 data corresponding to an integrated luminosity of 3 fb^-1, in which (15.5±0.4)×10³ χ_c1(3872) particles were selected inclusively from the decays of hadrons containing b quarks. The second analysis selected (4.23±0.07)×10³ fully reconstructed B⁺→χ_c1(3872)K⁺ decays from the full Run 1–2 data set, which corresponds to an integrated luminosity of 9 fb^-1. In both cases, the χ_c1(3872) particles were reconstructed through decays to the final state J/ψπ⁺π⁻. For the first time the measured Breit-Wigner width was found to be non-zero, with a value close to the previous upper limit from Belle (see figure).

Combining the two analyses, the mass of the χ_c1(3872) was found to be 3871.64±0.06 MeV – just 70±120 keV below the D⁰D^*0 threshold. The proximity of the χ_c1(3872) to this threshold puts a question mark on measuring the width using a simple fit to the well-known Breit-Wigner function, as this approach neglects potential distortions. Conversely, a precise measurement of the line-shape could help elucidate the nature of the χ_c1(3872). This has led LHCb to explore a more sophisticated Flatté parametrisation and report a measurement of the χ_c1(3872) line-shape with this model, including the pole positions of the complex amplitude. The results favour the interpretation of the state as a quasi-bound D⁰D^*0 molecule, but other possibilities cannot yet be ruled out. Further studies are ongoing. Physicists from other collaborations are also keenly interested in the nature of the χ_c1(3872), and the very recent observation by CMS of the decay process B_s⁰→χ_c1(3872)? suggests another laboratory for studying its properties.

The post LHCb interrogates X(3872) line-shape appeared first on CERN Courier.

The family of charged leptons is composed of the electron, muon (μ) and tau lepton (τ). According to the Standard Model (SM), these particles only differ in their mass: the muon is heavier than the electron and the tau is heavier than the muon. A remarkable feature of the SM is that each flavour is equally likely to interact with a W boson. This is known as lepton flavour universality.

In a new ATLAS measurement reported this week at the LHCP conference, a novel technique using events with top-quark pairs has been exploited to test the ratio of the probabilities for tau leptons and muons to be produced in W boson decays, R(τ/μ). In the SM, R(τ/μ) is expected to be unity, but a longstanding tension with this prediction has existed since the LEP era in the 1990s, where, from a combination of the four experiments, R(τ/μ) was measured to be 1.070 ± 0.026, deviating from the SM expectation by 2.7σ. This strongly motivated the need for new measurements with higher precision. If the LEP result were confirmed it would correspond to an unambiguous discovery of beyond the SM physics.

Tag and probe

To conclusively prove either that the LEP discrepancy is real or that it was just a statistical fluctuation, a precision of at least 1–2% is required — something previously not thought possible at a hadron collider like the LHC, where inclusive W bosons, albeit produced abundantly, suffer from large backgrounds and kinematic biases due to the online selection in the trigger. The key to achieving this is to obtain a sample of muons and tau leptons from W boson decays that is as insensitive as possible to the details of the trigger and object reconstruction used to select them. ATLAS has achieved this by exploiting both the LHC’s large sample of over 100 million top-quark pairs produced in the latest run, and the fact that top quarks decay almost exclusively to a W boson and a b quark. In a tag-and-probe approach, one W boson is used to select the events and the other is used, independently of the first, to measure the fractions of decays to tau-leptons and muons.

The analysis focuses on tau-lepton decays to a muon, rather than hadronic tau decays which are more complicated to reconstruct, thus reducing the systematic uncertainties associated with the object reconstruction. The lifetime of the tau lepton and its lower momentum decay products are exploited by the precise muon reconstruction available from the ATLAS detector to separate muons from tau-lepton decays and muons produced directly by a W decay (so-called prompt muons). Specifically, the absolute distance of closest approach of muon tracks in the plane perpendicular to the beam line, |d₀^μ| (figure 1), and the transverse momentum, p_T^μ, of the muons, are used to isolate these contributions. These variables, in particular |d₀^μ|, are calibrated using a pure sample of prompt muons from Z→μμ data.

The extraction of R(τ/μ) is performed using a fit to |d₀^μ| and p_T^μ where the cancellation of several systematic uncertainties is observed as they are correlated between the prompt μ and τ→μ contributions. This includes, for example, uncertainties related to jet reconstruction, flavour tagging and trigger efficiencies. As a result, the measurement obtains very high precision, surpassing that of the previous LEP measurement.

The measured value is R(τ/μ) = 0.992 ± 0.013 [ ± 0.007 (stat) ± 0.011 (syst) ], forming the most precise measurement of this ratio, with an uncertainty half the size of that from the combination of LEP results (figure 2). It is in agreement with the Standard Model expectation and suggests that the previous LEP discrepancy may be due to a fluctuation.

Though surviving this latest test, the principle of lepton flavour universality will not quite be out of the woods until the anomalies in B-meson decays recorded by the LHCb experiment (CERN Courier May/June 2020 p10) have also been definitively probed.

The post LEP-era universality discrepancy unravelled appeared first on CERN Courier.

LHCb will soon become the first LHC experiment able to run simultaneously with two separate interaction regions. As part of the ongoing major upgrade of the LHCb detector, the new SMOG2 fixed‑target system will be installed in long shutdown 2. SMOG2 will replace the previous System for Measuring the Overlap with Gas (SMOG), which injected noble gases into the vacuum vessel of LHCb’s vertex detector (VELO) at a low rate with the initial goal of calibrating luminosity measurements. The new system has several advantages, including the ability to reach effective area densities (and thus luminosities) up to two orders of magnitude higher for the same injected gas flux.

SMOG2 is a gas target confined within a 20 cm‑long aluminium storage cell that is mounted at the upstream edge of the VELO, 30 cm from the main interaction point, and coaxial with the LHC beam (figure 1). The storage‑cell technology allows a very limited amount of gas to be injected in a well defined volume within the LHC beam pipe, keeping the gas pressure and density profile under precise control, and ensuring that the beam‑pipe vacuum level stays at least two orders of magnitude below the upper threshold set by the LHC. With beam‑gas interactions occurring at roughly 4% of the proton–proton collision rate at LHCb, the lifetime of the beam will be essentially unaffected. The cell is made of two halves, attached to the VELO with an alignment precision of 200 μm. Like the VELO halves, they can be opened for safety during LHC beam injection and tuning, and closed for data‑taking. The cell is sufficiently narrow that as small a flow as 10^–15 particles per second will yield tens of pb^–1 of data per year. The new injection system will be able to switch between gases within a few minutes, and in principle is capable of injecting not just noble gases, from helium up to krypton and xenon, but also several other species, including H₂, D₂, N₂, and O₂.

SMOG2 will open a new window on QCD studies and astroparticle physics at the LHC

SMOG2 will open a new window on QCD studies and astroparticle physics at the LHC, performing precision measurements in poorly known kinematic regions. Collisions with the gas target will occur at a nucleon–nucleon centre‑of‑mass energy of 115 GeV for a proton beam of 7 TeV, and 72 GeV for a Pb beam of 2.76 TeV per nucleon. Due to the boost of the interacting system in the laboratory frame and the forward geometrical acceptance of LHCb, it will be possible to access the largely unexplored high‑x and intermediate Q² regions.

Combined with LHCb’s excellent particle identification capabilities and momentum resolution, the new gas target system will allow us to advance our understanding of the gluon, antiquark, and heavy‑quark components of nucleons and nuclei at large‑x. This will benefit searches for physics beyond the Standard Model at the LHC, by improving our knowledge of the parton distribution functions of both protons and nuclei, particularly at high‑x, where new particles are most often expected, and will inform the physics programmes of proposed next‑generation accelerators such as the Future Circular Collider. The gas target will also allow the dynamics and spin distributions of quarks and gluons inside unpolarised nucleons to be studied for the first time at the LHC, a decade before corresponding measurements at much higher accuracy are performed at the Electron‑Ion Collider in the US. Studying particles produced in collisions with light nuclei, such as He, and possibly N and O, will also allow LHCb to give important inputs to cosmic‑ray physics and dark‑matter searches. Last but not least, SMOG2 will allow LHCb to perform studies of heavy‑ion collisions at large rapidities, in an unexplored energy range between the SPS and RHIC, offering new insights into the QCD phase diagram.

The post New SMOG on the horizon appeared first on CERN Courier.

The Belle II collaboration at the SuperKEKB collider in Japan has published its first physics analysis: a search for Z′ bosons, which are hypothesised to couple the Standard Model (SM) with the dark sector. The team scoured four months of data from a pilot run in 2018 for evidence of invisibly decaying Z′ bosons in the process e⁺e⁻→μ⁺μ⁻Z′, and for  lepton-flavour violating Z′ bosons in e⁺e⁻→e^±μ^∓Z′, by looking for missing energy recoiling against two clean lepton tracks. “This is the first ever search for the process e⁺e⁻→μ⁺μ⁻Z′ where the Z′ decays invisibly,” says Belle II spokesperson Toru Iijima of Nagoya University.

The team did not find any excess of events, yielding preliminary sensitivity to the coupling g′ in the so-called L_μ−L_τ extension of the SM, wherein the Z′ couples only to muon and tau-lepton flavoured SM particles and the dark sector. This model also has the potential to explain anomalies in b → sμ⁺μ⁻ decays reported by LHCb and the longstanding muon g-2 anomaly, claims the team.

The results come a little over a year since the first collisions were recorded in the fully instrumented Belle II detector on 25 March 2019. Following in the footsteps of Belle at the KEKB facility, the new SuperKEKB b-factory plans to achieve a 40-fold increase on the luminosity of its predecessor, which ran from 1999 to 2010. First turns were achieved in February 2016, and first collisions between its asymmetric-energy electron and positron beams were achieved in April 2018. The machine has now reached a luminosity of 1.4 × 10³⁴ cm^-2 s^-1 and is currently integrating around 0.7 fb^-1 each day, exceeding the peak luminosity of the former PEP-II/BaBar facility at SLAC, notes Iijima.

By summer the team aims to exceed the Belle/KEKB record of 2.1 × 10³⁴ cm^-2 s^-1 by implementing a nonlinear “crab waist” focusing scheme. First used at the electron-positron collider DAΦNE at INFN Frascati, and not to be confused with the crab-crossing technology used to boost the luminosity at KEKB and planned for the high-luminosity LHC, the scheme stabilises e⁺e^– beam-beam blowup using carefully tuned sextupole magnets located symmetrically on either side of the interaction point. “The 100 fb^-1 sample which we plan to integrate by summer will allow us to provide our first interesting results in B physics,” says Tom Browder of the University of Hawaii, who was Belle II spokesperson until last year.

Flavour debut

Belle II will make its debut in flavour physics at a vibrant moment, complementing  efforts to resolve hints of anomalies seen at the LHC, such as the recent test of lepton-flavour universality in beauty-baryon decays by the LHCb collaboration.

We will then look for the star attraction of the dark sector, the dark photon

Tom Browder

As well as updating searches for  invisible decays of the Z′ with one to two orders of magnitude more data, Belle II will now conduct further dark-sector studies including a search for axion-like particles decaying to two photons, the Z′ decaying to visible final states and dark-Higgstrahlung with a μ⁺μ^– pair and missing energy, explains Browder. “We will then look for the star attraction of the dark sector, the dark photon, with the difficult signature of e⁺e^– to a photon and nothing else.”

The post First physics for Belle II appeared first on CERN Courier.

The LHCb collaboration has confirmed previous hints of odd behaviour in the way B mesons decay into a K^*and a pair of muons, bringing fresh intrigue to the pattern of flavour anomalies that has emerged during the past few years. At a seminar at CERN on 10 March, Eluned Smith of RWTH Aachen University presented an updated analysis of the angular distributions of B⁰→K^*0μ⁺μ^– decays based on around twice as many events than were used for the collaboration’s previous measurement reported in 2015. The result reveals a mild increase in the overall tension with the Standard Model (SM) prediction, though, at 3.3σ, more data are needed to determine the source of the effect.

The B⁰→K^*0μ⁺μ^– decay is a promising system with which to explore physics beyond the SM. A flavour-changing neutral-current process, it involves a quark transition (b→s) which is forbidden at the lowest perturbative order in the SM, and therefore occurs only around once for every million B decays. The decay proceeds instead via higher-order penguin and box processes, which are sensitive to the presence of new, heavy particles. Such particles would enter in competing processes and could significantly change the B⁰→K^*0μ⁺μ^– decay rate and the angular distribution of its final-state particles. Measuring angular distributions as a function of the invariant mass squared (q²) of the muon pair is of particular interest because it is possible to construct variables that depend less on hadronic modelling uncertainties.

Potentially anomalous behaviour in an angular variable called P₅′ came to light in 2013, when LHCb reported a 3.7σ local deviation with respect to the SM in one q² bin, based on 1fb^-1 of data. In 2015, a global fit of different angular distributions of the B⁰→K^*0μ⁺μ^– decays using the total Run 1 data sample of 3 fb^-1 reaffirmed the puzzle, showing discrepancies of 3.4σ (later reduced to 3.0σ when using new theory calculations with an updated description of potentially large hadronic effects). In 2016, the Belle experiment at KEK in Japan performed its own angular analysis of B⁰→K^*0μ⁺μ^– using data from electron—positron collisions and found a 2.1σ deviation in the same direction and in the same q² region as the LHCb anomaly.

We as a community have been eagerly waiting for this measurement and LHCb has not disappointed

Jure Zupan

The latest LHCb result includes additional Run 2 data collected during 2016, corresponding to a total integrated luminosity of 4.7fb^-1. It shows that the local tension of P₅′ in two q² bins between 4 and 8 GeV²/c⁴ reduces from 2.8 and 3.0σ, as observed in the previous analysis, to 2.5 and 2.9σ. However, a global fit to several angular observables shows that the overall tension with the SM increases from 3.0 to 3.3σ. The results of the fit also find a better overall agreement with predictions of new-physics models that contain additional vector or axial-vector contributions. However, the collaboration also makes it clear that the discrepancy could be explained by an unexpectedly large hadronic effect that is not accounted for in the SM predictions.

“We as a community have been eagerly waiting for this measurement and LHCb has not disappointed,” says theorist Jure Zupan of the University of Cincinnati. “The new measurements have moved closer to the SM predictions in the angular observables so that the combined significance of the excess remained essentially the same. It is thus becoming even more important to understand well and scrutinise the SM predictions and the claimed theory errors.”

Flavour puzzle
The latest result makes LHCb’s continued measurements of lepton-flavour universality even more important, he says. In recent years, LHCb has also found that the ratio of the rates of muonic and electronic B decays departs from the SM prediction, suggesting a violation of the key SM principle of lepton-flavour universality. Though not individually statistically significant, the measurements are theoretically very clean, and the most striking departure – in the variable known as R_K — concerns B decays that proceed via the same b→s transition as B⁰→K^*0μ⁺μ^–. This has led physicists to speculate that the two effects could be caused by the same new physics, with models involving leptoquarks or new gauge bosons in principle able to accommodate both sets of anomalies.

An update on R_K based on additional Run 2 data is hotly anticipated, and the collaboration is also planning to add data from 2017-18 to the B⁰→K^*0μ⁺μ^– angular analysis, as well as working on further analyses with b-quark transitions in mesons. LHCb also recently brought the decays of beauty baryons, which also depend on b→s transitions, to bear on the subject. Departures from the norm have also been spotted in B decays to D mesons, which involve tree-level b→c quark transitions. Such decays probe lepton-flavour universality via comparisons between tau leptons and muons and electrons but, as with R_K, the individual measurements are not highly significant.

“We have not seen evidence of new physics, but neither were the B physics anomalies ruled out,” says Zupan of the LHCb result. “The wait for the clear evidence of new physics continues.”

The post Anomalies persist in flavour-changing B decays appeared first on CERN Courier.

Three major conclusions can be drawn frofm these first 10 years. First and foremost, Run 1 has shown that the Higgs boson – the previously missing, last ingredient of the Standard Model (SM) – exists. Secondly, the exploration of energy scales as high as several TeV has further consolidated the robustness of the SM, providing no compelling evidence for phenomena beyond the SM (BSM). Nevertheless, several discoveries of new phenomena within the SM have emerged, underscoring the power of the LHC to extend and deepen our understanding of the SM dynamics, and showing the unparalleled diversity of phenomena that the LHC can probe with unprecedented precision.

Exceeding expectations

Last but not least, we note that 10 years of LHC operations, data taking and data interpretation, have overwhelmingly surpassed all of our most optimistic expectations. The accelerator has delivered a larger than expected luminosity, and the experiments have been able to operate at the top of their ideal performance and efficiency. Computing, in particular via the Worldwide LHC Computing Grid, has been another crucial driver of the LHC’s success. Key ingredients of precision measurements, such as the determination of the LHC luminosity, or of detection efficiencies and of backgrounds using data-driven techniques beyond anyone’s expectations, have been obtained thanks to novel and powerful techniques. The LHC has also successfully provided a variety of beam and optics configurations, matching the needs of different experiments and supporting a broad research programme. In addition to the core high-energy goals of the ATLAS and CMS experiments, this has enabled new studies of flavour physics and of hadron spectroscopy, of forward-particle production and total hadronic cross sections. The operations with beams of heavy nuclei have reached a degree of virtuosity that made it possible to collide not only the anticipated lead beams, but also beams of xenon, as well as combined proton–lead, photon–lead and photon-photon collisions, opening the way to a new generation of studies of matter at high density.

Theoretical calculations have evolved in parallel to the experimental progress. Calculations that were deemed of impossible complexity before the start of the LHC have matured and become reality. Next-to-leading-order (NLO) theoretical predictions are routinely used by the experiments, thanks to a new generation of automatic tools. The next frontier, next-to-next-to-leading order (NNLO), has been attained for many important processes, reaching, in a few cases, the next-to-next-to-next-to-leading order (N³LO), and more is coming.

Aside from having made these first 10 years an unconditional success, all these ingredients are the premise for confident extrapolations of the physics reach of the LHC programme to come.

To date, more than 2700 peer-reviewed physics papers have been published by the seven running LHC experiments (ALICE, ATLAS, CMS, LHCb, LHCf, MoEDAL and TOTEM). Approximately 10% of these are related to the Higgs boson, and 30% to searches for BSM phenomena. The remaining 1600 or so report measurements of SM particles and interactions, enriching our knowledge of the proton structure and of the dynamics of strong interactions, of electroweak (EW) interactions, of flavour properties, and more. In most cases, the variety, depth and precision of these measurements surpass those obtained by previous experiments using dedicated facilities. The multi-purpose nature of the LHC complex is unique, and encompasses scores of independent research directions. Here it is only possible to highlight a fraction of the milestone results from the LHC’s expedition so far.

Entering the Higgs world

The discovery by ATLAS and CMS of a new scalar boson in July 2012, just two years into LHC physics operations, was a crowning early success. Not only did it mark the end of a decades-long search, but it opened a new vista of exploration. At the time of the discovery, very little was known about the properties and interactions of the new boson. Eight years on, the picture has come into much sharper focus.

The structure of the Higgs-boson interactions revealed by the LHC experiments is still incomplete. Its couplings to the gauge bosons (W, Z, photon and gluons) and to the heavy third-generation fermions (bottom and top quarks, and tau leptons) have been detected, and the precision of these measurements is at best in the range of 5–10%. But the LHC findings so far have been key to establish that this new particle correctly embodies the main observational properties of the Higgs boson, as specified by the Brout–Englert–Guralnik–Hagen–Higgs–Kibble EW-symmetry breaking mechanism, referred hereafter as “BEH”, a cornerstone of the SM. To start with, the measured couplings to the W and Z bosons reflect the Higgs’ EW charges and are proportional to the W and Z masses, consistently with the properties of a scalar field breaking the SM EW symmetry. The mass dependence of the Higgs interactions with the SM fermions is confirmed by the recent ATLAS and CMS observations of the H → bb and H → ττ decays, and of the associated production of a Higgs boson together with a tt quark pair (see figure 1).

These measurements, which during Run 2 of the LHC have surpassed the five-sigma confidence level, provide the second critical confirmation that the Higgs fulfills the role envisaged by the BEH mechanism. The Higgs couplings to the photon and the gluon (g), which the LHC experiments have probed via the H → γγ decay and the gg → H production, provide a third, subtler test. These couplings arise from a combination of loop-level interactions with several SM particles, whose interplay could be modified by the presence of BSM particles, or interactions. The current agreement with data provides a strong validation of the SM scenario, while leaving open the possibility that small deviations could emerge from future higher statistics.

The process of firmly establishing the identification of the particle discovered in 2012 with the Higgs boson goes hand-in-hand with two research directions pioneered by the LHC: seeking the deep origin of the Higgs field and using the Higgs boson as a probe of BSM phenomena.

The breaking of the EW symmetry is a fact of nature, requiring the existence of a mechanism like BEH. But, if we aim beyond a merely anthropic justification for this mechanism (i.e. that, without it, physicists wouldn’t be here to ask why), there is no reason to assume that nature chose its minimal implementation, namely the SM Higgs field. In other words: where does the Higgs boson detected at the LHC come from? This summarises many questions raised by the possibility that the Higgs boson is not just “put in by hand” in the SM, but emerges from a larger sector of new particles, whose dynamics induces the breaking of the EW symmetry. Is the Higgs elementary, or a composite state resulting from new confining forces? What generates its mass and self-interaction? More generally, is the existence of the Higgs boson related to other mysteries, such as the origin of dark matter (DM), of neutrino masses or of flavour phenomena?

The Higgs boson is becoming an increasingly powerful exploratory tool to probe the origin of the Higgs itself

Ever since the Higgs-boson discovery, the LHC experiments have been searching for clues to address these questions, exploring a large number of observables. All of the dominant production channels (gg fusion, associated production with vector bosons and with top quarks, and vector-boson fusion) have been discovered, and decay rates to WW, ZZ, γγ, bb and ττ were measured. A theoretical framework (effective field theory, EFT) has been developed to interpret in a global fashion all these measurements, setting strong constraints on possible deviations from the SM. With the larger data set accumulated during Run 2, the production properties of the Higgs have been studied with greater detail, simultaneously testing the accuracy of theoretical calculations, and the resilience of SM predictions.

To explore the nature of the Higgs boson, what has not been seen as yet can be as important as what was seen. For example, lack of evidence for Higgs decays to the fermions of the first and second generation is consistent with the SM prediction that these should be very rare. The H → μμ decay rate is expected to be about 3 × 10^–3 times smaller than that of H → ττ; the current sensitivity is two times below, and ATLAS and CMS hope to first observe this decay during the forthcoming Run 3, testing for the first time the couplings of the Higgs boson to second-generation fermions. The SM Higgs boson is expected to conserve flavour, making decays such as H → μτ, H → eτ or t → Hc too small to be seen. Their observation would be a major revolution in physics, but no evidence has shown up in the data so far. Decays of the Higgs to invisible particles could be a signal of DM candidates, and constraints set by the LHC experiments are complementary to those from standard DM searches. Several BSM theories predict the existence of heavy particles decaying to a Higgs boson. For example, heavy top partners, T, could decay as T → Ht, and heavy bosons X decay as X → HV (V = W, Z). Heavy scalar partners of the Higgs, such as charged Higgs states, are expected in theories such as supersymmetry. Extensive and thorough searches of all these phenomena have been carried out, setting strong constraints on SM extensions.

As the programme of characterising the Higgs properties continues, with new challenging goals such as the measurement of the Higgs self-coupling through the observation of Higgs pair production, the Higgs boson is becoming an increasingly powerful exploratory tool to probe the origin of the Higgs itself, as well as a variety of solutions to other mysteries of particle physics.

Interactions weak and strong

The vast majority of LHC processes are controlled by strong interactions, described by the quantum-chromodynamics (QCD) sector of the SM. The predictions of production rates for particles like the Higgs or gauge bosons, top quarks or BSM states, rely on our understanding of the proton structure, in particular of the energy distribution of its quark and gluon components (the parton distribution functions, PDFs). The evolution of the final states, the internal structure of the jets emerging from quark and gluons, the kinematical correlations between different objects, are all governed by QCD. LHC measurements have been critical, not only to consolidate our understanding of QCD in all its dynamical domains, but also to improve the precision of the theoretical interpretation of data, and to increase the sensitivity to new phenomena and to the production of BSM particles.

Collisions galore

Approximately 10⁹ proton–proton (pp) collisions take place each second inside the LHC detectors. Most of them bear no obvious direct interest for the search of BSM phenomena, but even simple elastic collisions, pp → pp, which account for about 30% of this rate, have so far failed to be fully understood with first-principle QCD calculations. The ATLAS ALFA spectrometer and the TOTEM detector have studied these high-rate processes, measuring the total and elastic pp cross sections, at the various beam energies provided by the LHC. The energy dependence of the relation between the real and imaginary part of the pp forward scattering amplitude has revealed new features, possibly described by the exchange of the so-called odderon, a coherent state of three gluons predicted in the 1970s.

The structure of the final states in generic pp collisions, aside from defining the large background of particles that are superimposed on the rarer LHC processes, is of potential interest to understand cosmic-ray (CR) interactions in the atmosphere. The LHCf detector measured the forward production of the most energetic particles from the collision, those driving the development of the CR air showers. These data are a unique benchmark to tune the CR event generators, reducing the systematics in the determination of the nature of the highest-energy CR constituents (protons or heavy nuclei?), a step towards solving the puzzle of their origin.

On the opposite end of the spectrum, rare events with dijet pairs of mass up to 9 TeV have been observed by ATLAS and CMS. The study of their angular distribution, a Rutherford-like scattering experiment, has confirmed the point-like nature of quarks, down to 10^–18 cm. The overall set of production studies, including gauge bosons, jets and top quarks, underpins countless analyses. Huge samples of top quark pairs, produced at 15 Hz, enable the surgical scrutiny of this mysteriously heavy quark, through its production and decays. New reactions, unobservable before the LHC, were first detected. Gauge-boson scattering (e.g. W⁺ W⁺ → W⁺ W⁺), a key probe of electroweak symmetry breaking proposed in the 1970s, is just one example. By and large, all data show an extraordinary agreement with theoretical predictions resulting from decades of innovative work (figure 2). Global fits to these data refine the proton PDFs, improving the predictions for the production of Higgs bosons or BSM particles.

The cross sections σ of W and Z bosons provide the most precise QCD measurements, reaching a 2% systematic uncertainty, dominated by the luminosity uncertainty. Ratios such as σ(W⁺)/σ(W^–) or σ(W)/σ(Z), and the shapes of differential distributions, are known to a few parts in 1000. These data challenge the theoretical calculations’ accuracy, and require caution to assess whether small discrepancies are due to PDF effects, new physics or yet imprecise QCD calculations.

Precision is the keystone to consolidate our description of nature

As already mentioned, the success of the LHC owes a lot to its variety of beam and experimental conditions. In this context, the data at the different centre-of-mass energies provided in the two runs are a huge bonus, since the theoretical prediction for the energy-dependence of rates can be used to improve the PDF extraction, or to assess possible BSM interpretations. The LHCb data, furthermore, cover a forward kinematical region complementary to that of ATLAS and CMS, adding precious information.

The precise determination of the W and Z production and decay kinematics has also allowed new measurements of fundamental parameters of the weak interaction: the W mass (m_W) and the weak mixing angle (sinθ_W). The measurement of sinθ_W is now approaching the precision inherited from the LEP experiments and SLD, and will soon improve to shed light on the outstanding discrepancy between those two measurements. The m_W precision obtained by the ATLAS experiment, Δm_W = 19 MeV, is the best worldwide, and further improvements are certain. The combination with the ATLAS and CMS measurements of the Higgs boson mass (Δm_H ≅ 200 MeV) and of the top quark mass (Δm_top ≲ 500 MeV), provides a strong validation of the SM predictions (see figure 3). For both m_W and sinθ_W the limiting source of systematic uncertainty is the knowledge of the PDFs, which future data will improve, underscoring the profound interplay among the different components of the LHC programme.

QCD matters

The understanding of the forms and phases that QCD matter can acquire is a fascinating, broad and theoretically challenging research topic, which has witnessed great progress in recent years. Exotic multi-quark bound states, beyond the usual mesons (qq) and baryons (qqq), were initially discovered at e⁺e^– colliders. The LHCb experiment, with its large rates of identified charm and bottom final states, is at the forefront of these studies, notably with the first discovery of heavy pentaquarks (qqqcc) and with discoveries of tetraquark candidates in the charm sector (qccq), accompanied by determinations of their quantum numbers and properties. These findings have opened a new playground for theoretical research, stimulating work in lattice QCD, and forcing a rethinking of established lore.

The study of QCD matter at high density is the core task of the heavy-ion programme. While initially tailored to the ALICE experiment, all active LHC experiments have since joined the effort. The creation of a quark–gluon plasma (QGP) led to astonishing visual evidence for jet quenching, with 1 TeV jets shattered into fragments as they struggle their way out of the dense QGP volume. The thermodynamics and fluctuations of the QGP have been probed in multiple ways, indicating that the QGP behaves as an almost perfect fluid, the least viscous fluid known in nature. The ability to explore the plasma interactions of charm and bottom quarks is a unique asset of the LHC, thanks to the large production rates, which unveiled new phenomena such as  the recombination of charm quarks, and the sequential melting of bb bound states.

While several of the qualitative features of high-density QCD were anticipated, the quantitative accuracy, multitude and range of the LHC measurements have no match. Examples include ALICE’s precise determination of dynamical parameters such as the QGP shear-viscosity-to-entropy-density ratio, or the higher harmonics of particles’ azimuthal correlations. A revolution ensued in the sophistication of the required theoretical modelling. Unexpected surprises were also discovered, particularly in the comparison of high-density states in PbPb collisions with those occasionally generated by smaller systems such as pp and pPb. The presence in the latter of long-range correlations, various collective phenomena and an increased strange baryon abundance (figure 4), resemble behaviour typical of the QGP. Their deep origin is a mysterious property of QCD, still lacking an explanation. The number of new challenging questions raised by the LHC data is almost as large as the number of new answers obtained!

Flavour physics

Understanding the structure and the origin of flavour phenomena in the quark sector is one of the big open challenges of particle physics. The search for new sources of CP violation, beyond those present in the CKM mixing matrix, underlies the efforts to explain the baryon asymmetry of the universe. In addition to flavour studies with Higgs bosons and top quarks, more than 10¹⁴ charm and bottom quarks have been produced so far by the LHC, and the recorded subset has led to landmark discoveries and measurements. The rare B_s → μμ decay, with a minuscule rate of approximately 3 × 10^–9, has been discovered by the LHCb, CMS and ATLAS experiments. The rarer B_d → μμ decay is still unobserved, but its expected ~10^–10 rate is within reach. These two results alone had a big impact on constraining the parameter space of several BSM theories, notably supersymmetry, and their precision and BSM sensitivity will continue improving. LHCb has discovered DD mixing and the long-elusive CP violation in D-meson decays, a first for up-type quarks (figure 5). Large hadronic non-perturbative uncertainties make the interpretation of these results particularly challenging, leaving under debate whether the measured properties are consistent with the SM, or signal new physics. But the experimental findings are a textbook milestone in the worldwide flavour physics programme.

LHCb produced hundreds more measurements of heavy-hadron properties and flavour-mixing parameters. Examples include the most precise measurement of the CKM angle γ = (74.0^+5.0_–5.8)^o and, with ATLAS and CMS, the first measurement of φ_s, the tiny CP-violation phase of B_s → J/ψϕ, whose precisely predicted SM value is very sensitive to new physics. With a few notable exceptions, all results confirm the CKM picture of flavour phenomena. Those exceptions, however, underscore the power of LHC data to expose new unexpected phenomena: B → D^(*) ℓν (ℓ = μ,τ) and B → K^(*)ℓ⁺ℓ^– (ℓ = e,μ) decays hint at possible deviations from the expected lepton flavour universality. The community is eagerly waiting for further developments.

Beyond the Standard Model

Years of model building, stimulated before and after the LHC start-up by the conceptual and experimental shortcomings of the SM (e.g. the hierarchy problem and the existence of DM), have generated scores of BSM scenarios to be tested by the LHC. Evidence has so far escaped hundreds of dedicated searches, setting limits on new particles up to several TeV (figure 6). Nevertheless, much was learned. While none of the proposed BSM scenarios can be conclusively ruled out, for many of them survival is only guaranteed at the cost of greater fine-tuning of the parameters, reducing their appeal. In turn, this led to rethinking the principles that implicitly guided model building. Simplicity, or the ability to explain at once several open problems, have lost some drive. The simplest realisations of BSM models relying on supersymmetry, for example, were candidates to at once solve the hierarchy problem, provide DM candidates and set the stage for the grand unification of all forces. If true, the LHC should have piled up evidence by now. Supersymmetry remains a preferred candidate to achieve that, but at the price of more Byzantine constructions. Solving the hierarchy problem remains the outstanding theoretical challenge. New ideas have come to the forefront, ranging from the Higgs potential being determined by the early-universe evolution of an axion field, to dark sectors connected to the SM via a Higgs portal. These latter scenarios could also provide DM candidates alternative to the weakly-interacting massive particles, which so far have eluded searches at the LHC and elsewhere.

With such rapid evolution of theoretical ideas taking place as the LHC data runs progressed, the experimental analyses underwent a major shift, relying on “simplified models”: a novel model-independent way to represent the results of searches, allowing published results to be later reinterpreted in view of new BSM models. This amplified the impact of experimental searches, with a surge of phenomenological activity and the proliferation of new ideas. The cooperation and synergy between experiments and theorists have never been so intense.

Having explored the more obvious search channels, the LHC experiments refocused on more elusive signatures. Great efforts are now invested in searching corners of parameter space, extracting possible subtle signals from large backgrounds, thanks to data-driven techniques, and to the more reliable theoretical modelling that has emerged from new calculations and many SM measurements. The possible existence of new long-lived particles opened a new frontier of search techniques and of BSM models, triggering proposals for new dedicated detectors (Mathusla, CODEX-b and FASER, the last of which was recently approved for construction and operation in Run 3). Exotic BSM states, like the milli-charged particles present in some theories of dark sectors, could be revealed by MilliQan, a recently proposed detector. Highly ionising particles, like the esoteric magnetic monopoles, have been searched for by the MoEDAL detector, which places plastic tracking films cleverly in the LHCb detector hall.

While new physics is still eluding the LHC, the immense progress of the past 10 years has changed forever our perspective on searches and on BSM model building.

Final considerations

Most of the results only parenthetically cited, like the precision on the mass of the top quark, and others not even quoted, are the outcome of hundreds of years of person-power work, and would have certainly deserved more attention here. Their intrinsic value goes well beyond what was outlined, and they will remain long-lasting textbook material, until future work at the LHC and beyond improves them.

Theoretical progress has played a key role in the LHC’s progress, enhancing the scope and reliability of the data interpretation. Further to the developments already mentioned, a deeper understanding of jet structure has spawned techniques to tag high-p_T gauge and Higgs bosons, or top quarks, now indispensable in many BSM searches. Innovative machine-learning ideas have become powerful and ubiquitous. This article has concentrated only on what has already been achieved, but the LHC and its experiments have a long journey of exploration ahead.

The terms precision and discovery, applied to concrete results rather than projections, well characterise the LHC 10-year legacy. Precision is the keystone to consolidate our description of nature, increase the sensitivity to SM deviations, give credibility to discovery claims, and to constrain models when evaluating different microscopic origins of possible anomalies. The LHC has already fully succeeded in these goals. The LHC has also proven to be a discovery machine, and in a context broader than just Higgs and BSM phenomena. Altogether, it delivered results that could not have been obtained otherwise, immensely enriching our understanding of nature.

The post LHC at 10: the physics legacy appeared first on CERN Courier.

There is a longstanding puzzle concerning the value of the Cabibbo–Kobayashi–Maskawa matrix element |V_cb|, which describes the coupling between charm and beauty quarks in W^± interactions. This fundamental parameter of the Standard Model has been measured with two complementary methods. One uses the inclusive rate of b-hadron decays into final states containing a c hadron and a charged lepton; the other measures the rate of a specific (exclusive) semileptonic B decay, e.g. B⁰ → D^*–μ⁺ν_μ. The world average of results using the inclusive approach, |V_cb|^incl = (42.19 ± 0.78) × 10^–3, differs from the average of results using the exclusive approach, |V_cb|^excl = (39.25 ± 0.56) × 10^–3, by approximately three standard deviations.

So far, exclusive determinations have been carried out only at e⁺e^– colliders, using B⁰ and B⁺ decays. Operating at the ϒ(4S) resonance, the full decay kinematics can be determined, despite the undetected neutrino, and the total number of B mesons produced, needed to measure |V_cb|, is known precisely. The situation is more challenging in a hadron collider – but the LHCb collaboration has just completed an exclusive measurement of |V_cb| based, for the first time, on B_s⁰ decays.

The exclusive determination of |V_cb| relies on the description of strong-interaction effects for the b and c quarks bound in mesons, the so-called form factors (FF). These are functions of the recoil momentum of the c meson in the b-meson rest frame, and are calculated using non-perturbative QCD techniques, such as lattice QCD or QCD sum rules. A key advantage of semileptonic B_s⁰ decays, compared to B^0/+ decays, is that their FF can be more precisely computed. Recently, the FF parametrisation used in the exclusive determination has been considered to be a possible origin of the inclusive–exclusive discrepancy, and comparisons between the results for |V_cb| obtained using different parametrisations, such as that by Caprini, Lellouch and Neubert (CLN) and that by Boyd, Grinstein and Lebed (BGL), are considered a key check.

Both parametrisations are employed by LHCb in a new analysis of B_s⁰ → D_s^(*)–μ⁺ν_μ decays, using a novel method that does not require the momentum of particles other than D_s^– and μ⁺ to be estimated. The analysis also uses B⁰ → D^(*)–μ⁺ν_μ as a normalisation mode, which has the key advantage that many systematic effects cancel in the ratio. With the form factors and relative efficiency-corrected yields in hand, obtaining |V_cb| requires only a few more inputs: branching fractions that were well measured at the B-factories, and the ratio of B_s⁰ and B⁰ production fractions measured at LHCb.

The values of |V_cb| obtained are (41.4 ± 1.6) × 10^–3 and (42.3 ± 1.7) × 10^–3 in the CLN and BGL parametrisations, respectively. These results are compatible with each other and agree with previous measurements with exclusive decays, as well as the inclusive determination (figure 1).  This new technique can also be applied to B⁰ decays, giving excellent prospects for new |V_cb| measurements at LHCb. They will also benefit from expected improvements at Belle II to a key external input, the B⁰ → D^(*)–μ⁺ν_μ branching fraction. Belle II’s own measurement of |V_cb| is also expected to have reduced systematic uncertainties. In addition, new lattice QCD calculations for the full range of the D^*– recoil momentum are expected soon and should give valuable constraints on the form factors. This synergy between theoretical advances, Belle II and LHCb (and its upgrade, due to start in 2021) will very likely say the final word on the |V_cb| puzzle.

The post Opening gambit for LHCb in the V_cb puzzle appeared first on CERN Courier.

The origin of the three families of quarks and leptons and their extreme range of masses is a central mystery of particle physics. According to the Standard Model (SM), quarks and leptons come in complete families that interact identically with the gauge forces, leading to a remarkably successful quantitative theory describing practically all data at the quantum level. The various quark and lepton masses are described by having different interaction strengths with the Higgs doublet (figure 1, left), also leading to quark mixing and charge-parity (CP) violating transitions involving strange, bottom and charm quarks. However, the SM provides no understanding of the bizarre pattern of quark and lepton masses, quark mixing or CP violation.

In 1998 the SM suffered its strongest challenge to date with the decisive discovery of neutrino oscillations resolving the atmospheric neutrino anomaly and the long-standing problem of the low flux of electron neutrinos from the Sun. The observed neutrino oscillations require at least two non-zero but extremely small neutrino masses, around one ten millionth of the electron mass or so, and three sizeable mixing angles. However, since the minimal SM assumes massless neutrinos, the origin and nature of neutrino masses (i.e. whether they are Dirac or Majorana particles, the latter requiring the neutrino and antineutrino to be related by CP conjugation) and mixing is unclear, and many possible SM extensions have been proposed.

The discovery of neutrino mass and mixing makes the flavour puzzle hard to ignore, with the fermion mass hierarchy now spanning at least 12 orders of magnitude, from the neutrino to the top quark. However, it is not only the fermion mass hierarchy that is unsettling. There are now 28 free parameters in a Majorana-extended SM, including a whopping 22 associated with flavour, surely too many for a fundamental theory of nature. To restate Isidor Isaac Rabi’s famous question following the discovery of the muon in 1936: who ordered all of that?