Speaker
Description
A new sine observable, $R(\Delta s)$, has been proposed [1] to measure the Chiral Magnetic Effect (CME) in heavy ion collisions, where $\Delta s$ is the $\langle\sin\phi\rangle$ difference between positive and negative charges ($\phi$ is the particle azimuth) and $R(\Delta s)$ is the ratio of the out-of-plane to in-plane $\Delta s$ distributions. Studies with A Multi-Phase Transport (AMPT) and Anomalous Viscous Fluid Dynamics (AVFD) models show concave R($\Delta s$) distributions for CME signals and convex ones for typical resonance backgrounds [1]. A recent hydrodynamic study, however, indicates concave shapes for backgrounds as well [2]. Preliminary STAR data, on the other hand, reveal concave $R(\Delta s)$ distributions in 200 GeV Au+Au collisions.
To better understand these results, we present a systematic study of the $v_{2}$ and $p_{T}$ dependences of resonance backgrounds by toy-model simulations, based on the toy model used in our previous study [3]. The resonance $v_{2}$ introduces different numbers of decay $\pi^+\pi^-$ pairs in the in-plane and out-of-plane directions. The resonance $p_{T}$ affects the opening angle of the decay $\pi^+\pi^-$ pair. Low $p_{T}$ resonances decay into large opening-angle pairs, and result in more ``back-to-back'' pairs out-of-plane because of the more in-plane resonances, mimicking a CME charge separation signal perpendicular to the reaction plane, or a concave $R(\Delta s)$. High $p_{T}$ resonances, on the other hand, decay into small opening-angle pairs, and result in a background behavior of convex $R(\Delta s)$. With this toy-model insight, we further investigate the responses of the $R(\Delta s)$ observable to AMPT backgrounds and AVFD CME signals,and the possible implications of the preliminary STAR data.
[1] N. Magdy, S. Shi, J. Liao, N. Ajitanand and R. A. Lacey, arXiv:1710.01717
[2] P. Bozek, arXiv:1711.02563
[3] F. Wang, J. Zhao, Phys. Rev. C95, 051901 (R) (2017)
| Content type | Theory |
|---|---|
| Centralised submission by Collaboration | Presenter name already specified |
