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

TL;DR

This paper introduces a new sine observable to detect the chiral magnetic effect in heavy-ion collisions and investigates how resonance backgrounds influence its shape, revealing that flow parameters and resonance properties critically affect the observable's concavity or convexity.

Contribution

The study systematically analyzes how resonance backgrounds and flow parameters impact the new sine observable used to measure the CME, clarifying background effects.

Findings

Concavity or convexity of the observable depends on resonance $v_2$ and $p_T$.

Low $p_T$ resonances produce large opening-angle pairs mimicking CME signals.

Resonance backgrounds can produce concave shapes similar to CME signals.

Abstract

A new sine observable, $R_{\Psi_2}(\Delta S)$, has been proposed to measure the chiral magnetic effect (CME) in heavy-ion collisions; $\Delta S = \left \langle \sin \varphi_+ \right \rangle - \left \langle \sin \varphi_- \right \rangle$, where $\varphi_\pm$ are azimuthal angles of positively and negatively charged particles relative to the reaction plane and averages are event-wise, and $R_{\Psi_2}(\Delta S)$ is a normalized event probability distribution. Preliminary STAR data reveal concave $R_{\Psi_2}(\Delta S)$ distributions in 200 GeV Au+Au collisions. Studies with a multiphase transport (AMPT) and anomalous-viscous Fluid Dynamics (AVFD) models show concave $R_{\Psi_2}(\Delta S)$ 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 $R_{\Psi_2}(\Delta S)$ depends sensitively on the resonance $v_2$ (which yields different numbers of decay $\pi^+\pi^-$ pairs in the in-plane and out-of-plane directions) and $p_T$ (which affects the opening angle of the decay $\pi^+\pi^-$ 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 $R_{\Psi_2}(\Delta S)$. Supplemental studies of $R_{\Psi_3}(\Delta S)$ in terms of the triangular flow ($v_3$), where only backgrounds exist but any CME would average to zero, are also presented.