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

Abstract

The azimuthal correlations between local flow planes at different (pseudo)rapidities ($\ensuremath{\eta}$) 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 $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{{s}_{NN}}=200$ 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 $\ensuremath{\eta}$ window of the reference flow plane to be consistent with the pertinent decorrelation picture.