Quark Matter 2018

Response studies of the CME-sensitive sine observable to heavy ion backgrounds

15 May 2018, 17:00

2h 40m

First floor and third floor (Palazzo del Casinò)

Poster Chirality, vorticity and polarisation effects Poster Session

Speaker

Yicheng Feng (Purdue University)

Description

A new sine observable, $R(\mathrm{\Delta }s)$, has been proposed [1] to measure the Chiral Magnetic Effect (CME) in heavy ion collisions, where $\mathrm{\Delta }s$ is the $⟨\sin \varphi ⟩$ difference between positive and negative charges ($\varphi$ is the particle azimuth) and $R(\mathrm{\Delta }s)$ is the ratio of the out-of-plane to in-plane $\mathrm{\Delta }s$ distributions. Studies with A Multi-Phase Transport (AMPT) and Anomalous Viscous Fluid Dynamics (AVFD) models show concave R($\mathrm{\Delta }s$) 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 $R(\mathrm{\Delta }s)$ 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 $R(\mathrm{\Delta }s)$. High $p_T$ resonances, on the other hand, decay into small opening-angle pairs, and result in a background behavior of convex $R(\mathrm{\Delta }s)$. With this toy-model insight, we further investigate the responses of the $R(\mathrm{\Delta }s)$ 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)