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, $\Delta v_n(A_{\mathrm{ch}})$. 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 $\Delta v_n(A_{\mathrm{ch}})$. The trivial slope contribution to the triangle flow difference $\Delta v_3(A_{\mathrm{ch}})$ can be negative if the non-flow is dominated by back-to-back pairs. This can explain the observed negative $\Delta v_3(A_{\mathrm{ch}})$ slope in the preliminary STAR data. We further find that the non-flow correlations give rise to additional backgrounds to the slope of $\Delta v_2(A_{\mathrm{ch}})$ from the competition among different pion sources and from the larger multiplicity dilution to ${\pi }^{+}$ (${\pi }^{-}$) at positive (negative) $A_{\mathrm{ch}}$.

Comments: 4 pages, 2 figures, contribution to the proceedings of the 28th international conference on ultrarelativistic nucleus-nucleus collisions (Quark Matter 2019)