Proton Number Fluctuations At, 27 GeV Enable Insights into Au+Au Collision Dynamics

Scientists at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory investigate the fundamental properties of matter under extreme conditions, and recent work by the STAR Collaboration, led by researchers including those at Brookhaven, explores fluctuations in proton and antiproton production during heavy ion collisions. This research presents high-precision measurements of these fluctuations, specifically examining fifth- and sixth-order variations in the number of protons and antiprotons created, and offers new constraints on our understanding of the strong force described by Quantum Chromodynamics (QCD). The team’s findings, which demonstrate compatibility with both theoretical predictions from lattice QCD and model calculations like Ultra-relativistic Molecular Dynamics, provide valuable insights into the behaviour of matter at extremely high temperatures and densities, and help to refine our understanding of the QCD phase diagram. These measurements place important limits on baryon number fluctuations, contributing to a more complete picture of the fundamental structure of matter.

These measurements, sensitive to the nature of the QCD critical point and the equation of state of strongly interacting matter, utilize higher-order cumulants of net baryon number, net electric charge, and net strangeness. Data from the Beam Energy Scan program at RHIC, covering a range of collision energies, were analyzed to reveal non-monotonic behavior in the skewness and kurtosis of net proton number, potentially indicating the proximity of the QCD critical point.

Baryon Number Fluctuations at RHIC Energies

High-precision measurements of fifth- and sixth-order factorial cumulants and cumulant ratios of net-proton multiplicity distributions in gold-gold collisions at approximately 27 GeV provide valuable insights into the quantum chromodynamics (QCD) phase structure and baryon number fluctuations. Researchers observed that these values increase with order, but exhibit no sign alternation within current uncertainties, offering no evidence for a two-component structure in the proton multiplicity distribution, which might be expected near a first-order phase transition. The results, compatible with predictions from lattice QCD, functional renormalization group, and hadron resonance gas models, place constraints on baryon number fluctuations and offer valuable insights into the QCD phase structure.

Baryonic Matter Fluctuations Constrain QCD Phase Diagram

Measurements of proton and antiproton distributions resulting from heavy-ion collisions at RHIC reveal insights into the fundamental properties of quantum chromodynamics, the theory describing the strong nuclear force. Researchers achieved high-precision measurements of factorial cumulants and their ratios, characterizing fluctuations in proton multiplicity across a range of collision energies. These results, consistent with both lattice QCD calculations and expectations from the UrQMD model, constrain the behaviour of baryonic matter under extreme conditions and offer valuable tests of theoretical predictions.

These ongoing investigations promise to further refine our understanding of the QCD phase diagram and the properties of matter at extreme temperatures and densities.