STAR hunts QCD critical point

Just as water takes the form of ice, liquid or vapour, QCD matter exhibits distinct phases. But while the phase diagram of water is well established, the QCD phase diagram remains largely conjectural. The STAR collaboration at Brookhaven National Laboratory’s Relativistic Heavy Ion Collider (RHIC) recently completed a new beam-energy scan (BES-II) of gold–gold collisions. The results narrow the search for a long-sought-after “critical point” in the QCD phase diagram.

“BES-II precision measurements rule out the existence of a critical point in the regions of the QCD phase diagram accessed at LHC and top RHIC energies, while still allowing the possibility at lower collision energies,” says Bedangadas Mohanty of the National Institute of Science Education and Research in India, who co-led the analysis. “The results refine earlier BES-I indications, now with much reduced uncertainties.”

At low temperatures and densities, quarks and gluons are confined within hadrons. Heating QCD matter leads to the formation of a deconfined quark–gluon plasma (QGP), while increasing the density at low temperatures is expected to give rise to more exotic states such as colour superconductors. Above a certain threshold in baryon density, the transition from hadron gas to QGP is expected to be first-order – a sharp, discontinuous change akin to water boiling. As density decreases, this boundary gives way to a smooth crossover where the two phases blend. A hypothetical critical point marks the shift between these regimes, much like the endpoint of the liquid–gas coexistence line in the phase diagram of water (see “Phases of QCD” figure).

Heavy-ion collisions offer a way to observe this phase transition directly. At the Large Hadron Collider, the QGP created in heavy-ion collisions transitions smoothly to a hadronic gas as it cools, but the lower energies explored by RHIC probe the region of phase space where the critical point may lie.

To search for possible signatures of a critical point, the STAR collaboration measured gold–gold collisions at centre-of-mass energies between 7.7 and 27 GeV per nucleon pair. The collaboration reports that their data deviate from frameworks that do not include a critical point, including the hadronic transport model, thermal models with canonical ensemble treatment, and hydrodynamic approaches with excluded-volume effects. Depending on the choice of observable and non-critical baseline model, the significance of the deviations ranges from two to five standard deviations, with the largest effects seen in head-on collisions when using peripheral collisions as a reference.

“None of the existing theoretical models fully reproduce the features observed in the data,” explains Mohanty. “To interpret these precision measurements, it is essential that dynamical model calculations that include critical-point physics be developed.” The STAR collaboration is now mapping lower energies and higher baryon densities using a fixed target (FXT) mode, wherein a 1 mm gold foil sits 2 cm below the beam axis.

“The FXT data are a valuable opportunity to explore QCD matter at high baryon density,” says Mohanty. “Data taking will conclude later this year when RHIC transitions to the Electron–Ion Collider. The Compressed Baryonic Matter experiment at FAIR in Germany will then pick up the study of the QCD critical point towards the end of the 2020s.”