Centrality fluctuations in heavy-ion collisions

TL;DR

This paper investigates how volume or centrality fluctuations affect multiplicity and eccentricity distributions in heavy-ion collisions, proposing methods to constrain these fluctuations and explore intra-event dynamics using subevent cumulants.

Contribution

It introduces a Glauber-based model to analyze centrality fluctuations and proposes a subevent cumulant method to study intra-event fluctuations in heavy-ion collisions.

Findings

Cumulants exhibit sign-change patterns in ultra-central collisions.

Model predictions align with experimental data to constrain fluctuations.

Subevent methods can distinguish fluctuation mechanisms at different time scales.

Abstract

Volume or centrality fluctuations (CF) is 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 $\epsilon_n$: $p(N)$, $p(\epsilon_n)$, $p(\epsilon_n,\epsilon_m)$ and $p(N,\epsilon_n)$, where the effects of CF is quantified using multi-particle cumulants of these distributions. In mid-central collisions, a general relation is established between the multiplicity fluctuation and resulting CF in the reference subevent. In ultra-central 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 cumulants between model prediction and experimental data can be used to constrain the CF and particle production mechanism in heavy-ion collisions. Since the concept of centrality and CF are expected to fluctuate in the longitudinal direction within a single event, we propose to use pseudorapidity-separated subevent cumulant method to explore the nature of intra-event 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 time scales. The forward detector upgrades at RHIC and LHC will greatly enhance such studies in the future.