Centrality fluctuations in heavy-ion collisions

Volume or centrality fluctuations (CF) are one of the main uncertainties for interpreting the centrality dependence of many experimental observables. The CF is constrained by centrality selection based on particle multiplicity in a reference subevent and contributes to observables measured in another subevent. Using a Glauber-based independent source model, we study the influence of CF on several distributions of multiplicity N and eccentricities ϵn: p(N),p(ϵn),p(ϵn,ϵm), and p(N,ϵn), where the effects of CF are quantified using multiparticle cumulants of these distributions. In midcentral collisions, a general relation is established between the multiplicity fluctuation and resulting CF in the reference subevent. In ultracentral collisions, where distribution of particle production sources is strongly distorted, we find these cumulants exhibit rich sign-change patterns, due to observable-dependent non-Gaussianity in the underlying distributions. The details of sign-change pattern change with the size of the collision systems. Simultaneous comparison of these different types of cumulants between model prediction and experimental data can be used to constrain the CF and particle production mechanism in heavy-ion collisions. Since the concepts of centrality and CF are expected to fluctuate in the longitudinal direction within a single event, we propose to use the pseudorapidity-separated subevent cumulant method to explore the nature of intraevent fluctuations of centrality and collective dynamics. The subevent method can be applied for any bulk observable that is sensitive to centrality and has the potential to separate different mechanisms for multiplicity and flow fluctuations happening at different timescales. The forward detector upgrades at the Relativistic Heavy Ion Collider and the Large Hadron Collider will greatly enhance such studies in the future.