Importance of non-flow background on the chiral magnetic wave search

Abstract

An observable sensitive to the chiral magnetic wave (CMW) is the charge asymmetry dependence of the $\pi^{-}$ and $\pi^{+}$ anisotropic flow difference, $\Delta v_{n}(A_{\rm ch})$. We show that, due to non-flow correlations, the flow measurements by the Q-cumulant method using all charged particles as reference introduce a trivial linear term to $\Delta v_{n}(A_{\rm ch})$. The trivial slope contribution to the triangle flow difference $\Delta v_{3}(A_{\rm ch})$ can be negative if the non-flow is dominated by back-to-back pairs. This can explain the observed negative $\Delta v_{3}(A_{\rm ch})$ slope in the preliminary STAR data. We further find that the non-flow correlations give rise to additional backgrounds to the slope of $\Delta v_{2}(A_{\rm ch})$ from the competition among different pion sources and from the larger multiplicity dilution to $\pi^{+}$ ($\pi^{-}$) at positive (negative) $A_{\rm ch}$.