Flow-plane decorrelations in heavy-ion collisions with multiple-plane cumulants

TL;DR

This paper introduces a new cumulant observable, $T_2$, to better measure longitudinal flow-plane decorrelation in heavy-ion collisions by suppressing nonflow effects, tested through simulations and applied to AMPT model data.

Contribution

A novel cumulant observable, $T_2$, is proposed to distinguish flow decorrelation from nonflow effects in heavy-ion collision data.

Findings

$T_2$ effectively suppresses nonflow effects in simulations.

The method reveals decorrelation signals consistent with initial-state scenarios.

Application to AMPT data demonstrates the observable's practical utility.

Abstract

The azimuthal correlations between local flow planes at different (pseudo)rapidities ($\eta$) may reveal important details of the initial nuclear matter density distributions in heavy-ion collisions. Extensive experimental measurements of a factorization ratio ($r_2$) and its derivative ($F_2$) have shown evidence of the longitudinal flow-plane decorrelation. However, nonflow effects also affect this observable and prevent a quantitative understanding of the phenomenon. In this paper, to distinguish decorrelation and nonflow effects, we propose a new cumulant observable, $T_2$, which largely suppresses nonflow. The technique sensitivity to different initial-state scenarios and nonflow effects are tested with a simple Monte Carlo model, and in the end, the method is applied to events simulated by a multiphase transport model (AMPT) for Au+Au collisions at $\sqrt{s_{\rm NN}} =200$ GeV. We also emphasize that a distinct decorrelation signal requires not only the right sign of an observable, but also its proper dependence on the $\eta$-window of the reference flow plane, to be consistent with the pertinent decorrelation picture.