Abstract
Measurements of the variation of anisotropic flow-plane angles (psi(n)) with rapidity, commonly known as the flow-plane decorrelation, provide important insights into the initial conditions of the matter produced in heavy-ion collisions. In this paper, using data collected by the STAR experiment, we report the first measurement of the four-plane correlator observable T-n{ba; dc} = << sin[n(Psi(b)(n) - Psi n(a))] sin[n(Psi(d)(n) - Psi n(c))]>>, where superscripts a, b, c, and d denote sequential pseudorapidity (eta) regions with a corresponding to the most backward region, b and c close to midrapidity with eta(b) < 0 and eta(c) > 0, and d being the most forward. The measurement is performed for the elliptic and triangular flow (i.e., n = 2 and 3) in Au + Au and isobar (Ru + Ru, Zr + Zr) collisions at root s(NN) = 200 GeV. The goal of calculating the correlation of the flow-plane angle variations from backward to midcentral, and from midcentral to forward regions, is to probe the systematic variation of flow angle over a wide eta range. In midcentral collisions (10-30% centrality), we find T-2 {ba; dc} = -0.004 +/- 0.001(stat) +/- 0.002(syst) independent of the collision system. Such a small value of T-2 favors a "random-walk" variation of the flow-plane angles, where the rapidity correlation length is smaller than the entire region under study. These measurements provide new information on the decorrelation patterns in the system and offer a quantitative estimate of possible systematic variations in anisotropic flow angles such as "twist" between forward and backward regions. This opens new opportunities for understanding the three-dimensional structure and the time evolution of the quark-gluon plasma created in heavy-ion collisions.