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Responses of the chiral-magnetic-effect-sensitive sine observable to resonance backgrounds in heavy-ion collisions
- Source :
- Phys. Rev. C 98, 034904 (2018)
- Publication Year :
- 2018
-
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.<br />Comment: 11 pages, 8 figures
- Subjects :
- Nuclear Theory
Nuclear Experiment
Subjects
Details
- Database :
- arXiv
- Journal :
- Phys. Rev. C 98, 034904 (2018)
- Publication Type :
- Report
- Accession number :
- edsarx.1803.02860
- Document Type :
- Working Paper
- Full Text :
- https://doi.org/10.1103/PhysRevC.98.034904