paper

Elucidating the event-by-event flow fluctuations in heavy-ion collisions via the event shape selection technique

arXiv:1311.7091 · doi:10.1103/PhysRevC.90.024910

Abstract

The presence of large event-by-event flow fluctuations in heavy ion collisions at RHIC and the LHC provides an opportunity to study a broad class of flow observables. This paper explores the correlations among harmonic flow coefficients $v_{n}$ and their phases $I^¦_{n}$ , and the rapidity fluctuation of $v_{n}$ . The study is carried out usin Pb+Pb events generated by the AMPT model with fixed impact parameter. The overall ellipticity/triangularity of events is varied by selecting on the eccentricities $I^µ_{n}$ or the magnitudes of the flow vector $q_{n}$ for n=2 and 3, respectively. The responses of the $v_{n}$ , the event-plane correlations, and the rapidity fluctuations, to the change in $I^µ_{n}$ and $q_{n}$ are then systematized. Strong positive correlations are observed among all even harmonics $v_{2}, v_{4}$ , and $v_{6}$ (all increase with $q_{2}$ ), between $v_{2}$ and $v_{5}$ (both increase with $q_{2}$ ) and between $v_{3}$ and $v_{5}$ (both increase with $q_{3}$ ), consistent with the effects of nonlinear collective response. In contrast, an anti-correlation is observed between $v_{2}$ and $v_{3}$ similar to that seen between $I^µ_{2}$ and $I^µ_{3}$ . These correlation patterns are found to be independent of whether selecting on $q_{n}$ or $I^µ_{n}$ , validating the ability of $q_{n}$ in selecting the initial geometry. A forward/backward asymmetry of $v_{n} (I^\cdot)$ is observed for events selected on $q_{n}$ but not on $I^µ_{n}$ , reflecting dynamical fluctuations exposed by the $q_{n}$ selection. Many event-plane correlators show good agreement between $q_{n}$ and $I^µ_{n}$ selections, suggesting that their variations with $q_{n}$ are controlled by the change of $I^µ_{n}$ in the initial geometry. Hence these correlators may serve as promising observables for disentangling the fluctuations generated in various stages of the evolution of the matter created in heavy ion collisions.

14 pages, 20 figures