Responses of the chiral-magnetic-effect-sensitive sine observable to resonance backgrounds in heavy-ion collisions

Abstract

A new sine observable, $R_{Ψ_2}(ΔS)$, has been proposed to measure the chiral magnetic effect (CME) in heavy-ion collisions; $ΔS = \left \langle \sin φ_+ \right \rangle - \left \langle \sin φ_- \right \rangle$, where $φ_\pm$ are azimuthal angles of positively and negatively charged particles relative to the reaction plane and averages are event-wise, and $R_{Ψ_2}(ΔS)$ is a normalized event probability distribution. Preliminary STAR data reveal concave $R_{Ψ_2}(ΔS)$ distributions in 200 GeV Au+Au collisions. Studies with a multiphase transport (AMPT) and anomalous-viscous Fluid Dynamics (AVFD) models show concave $R_{Ψ_2}(ΔS)$ distributions for CME signals and convex ones for typical resonance backgrounds. A recent hydrodynamic study, however, indicates concave shapes for backgrounds as well. To better understand these results, we report a systematic study of the elliptic flow ($v_{2}$) and transverse momentum ($p_{T}$) dependences of resonance backgrounds with toy-model simulations and central limit theorem (CLT) calculations. It is found that the concavity or convexity of $R_{Ψ_2}(ΔS)$ depends sensitively on the resonance $v_2$ (which yields different numbers of decay $π^+π^-$ pairs in the in-plane and out-of-plane directions) and $p_T$ (which affects the opening angle of the decay $π^+π^-$ pair). Qualitatively, low $p_{T}$ resonances decay into large opening-angle pairs and result in more `back-to-back' pairs out-of-plane, mimicking a CME signal, or a concave $R_{Ψ_2}(ΔS)$. Supplemental studies of $R_{Ψ_3}(ΔS)$ in terms of the triangular flow ($v_3$), where only backgrounds exist but any CME would average to zero, are also presented.