Probing bulk dynamics of the QGP with correlations and fluctuations

TL;DR

This work surveys how fluctuations and correlations of conserved charges in ultrarelativistic heavy-ion collisions illuminate the QCD phase diagram and the search for the critical end point (CEP). It links experimental cumulants $\kappa_n$ to lattice QCD susceptibilities $\chi_n$ and discusses the Skellam baseline as a diagnostic for dynamical correlations, while detailing experimental strategies to control volume fluctuations and misidentification using the mixed-event technique and Fuzzy Logic PID. The status near the crossover shows limited evidence for critical behavior in pp, with intriguing but inconclusive signals in A–A at higher cumulant orders, highlighting the need for robust baselines and baryon-number conservation corrections. Looking forward, higher-statistics runs and next-generation detectors (e.g., ALICE-3, CBM) will extend the explored region of $\mu_B$ and enable more precise CEP searches and cross-checks with alternative observables such as intermittency.

Abstract

Ultrarelativistic heavy-ion collisions are considered ideal environments for exploring the QCD phase diagram and probing the properties of the QGP as functions of temperature and baryon chemical potential. At the highest energies, such as those reached at the Large Hadron Collider (LHC), and near vanishing baryon chemical potential, the transition from hadronic matter to the QGP is expected to occur as a smooth crossover. At larger baryon chemical potentials, this crossover may end at a critical point, beyond which the transition becomes first order. Locating this critical point remains a central goal of current and future beam energy scan programs at RHIC, the CERN SPS, and FAIR/GSI. Fluctuation and correlation measurements are widely used to probe the QCD phase structure, as they provide information on the system's dynamical evolution and the nature of the phase transition at different regions of the phase diagram. This report presents an overview of recent experimental results across a broad energy range and discusses their implications for our current understanding of the QCD phase diagram.

Figures (7)

• Figure 1: (Color online) Left: HIJING hijing calculations of the rapidity dependence of second-order cumulants of net baryons (blue, green, red) and net electric charge (black), normalized to Skellam expectations. The grey band indicates the ALICE acceptance. Right: Centrality dependence of the $\kappa_{4}/\kappa_{2}$ ratio of net protons from HIJING hijing and EPOS-LHC epos for two momentum acceptances.
• Figure 2: (Color online) HIJING model calculations of the centrality dependence of net-proton cumulants for different bin widths, shown for same-event and mixed-event samples. The top left panel displays the normalized second-order cumulants relative to the Skellam expectation, while the top right and bottom panels show the ratio $\kappa_{4}/\kappa_{2}$ before and after volume-fluctuation corrections for 5–10% and 1% bins, respectively ilya.
• Figure 3: (Color online) Left: Mass spectrum measured by the HADES collaboration with Gaussian fits to individual particle species; dashed lines indicate the $2\sigma$ selection window around the proton mean. Middle and right: second- and fourth-order proton cumulants from the particle-counting and Fuzzy Logic methods in full simulation of the SMASH transport model smash, compared to the true cumulants obtained from MC truth.
• Figure 4: Sixth- to second-order cumulant ratios of the net-proton number in pp collisions measured by ALICE ilya (left) and in pp and Pb--Pb collisions measured by STAR starpp (right). ALICE results are compared to PYTHIA and Thermal-FIST model calculations, while STAR results are compared to PYTHIA only.
• Figure 5: Left: Effect of local (dashed lines) and global (solid red line) baryon number conservation on the ratio of fourth- to second-order cumulants of the net-baryon number as a function of rapidity metropolis. Right: Centrality dependence of fourth- to second-order cumulants of the net-proton number measured by the ALICE Collaboration in Pb--Pb collisions ilya. Results are compared to HIJING model calculations. Both data and model results are corrected for volume fluctuation effects using the mixed-event technique mixedevent.