Speaker
Description
Using two novel methods, pair invariant mass ($m_{inv}$) [1] and comparative measurements with respect to reaction plane ($\psi_{\rm RP}$) and participant plane ($\psi_{\rm PP}$) [2], we isolate the chiral magnetic effect (CME) from backgrounds in 200 GeV Au+Au collisions at STAR.
The invariant mass method identifies the resonance background contributions, coupled with the elliptic flow ($v_{2}$), to the charge correlator CME observable ($\Delta\gamma$). At high mass ($m_{inv}>1.5$ GeV/$c^{2}$) where resonance contribution is small, $\Delta\gamma$ is found to be consistent with zero within uncertainty. In the low mass region ($m_{inv}<1.5$ GeV/$c^{2}$), resonance peaks are observed in $\Delta\gamma$ as function of $m_{inv}$. A two-component model fit is devised to extract the CME signal, assumed smooth in $m_{inv}$.
In the comparative method, the $\psi_{\rm RP}$ is assessed by spectator neutrons measured by the ZDC, and the $\psi_{\rm PP}$ by the 2nd harmonic event plane measured by TPC. The $v_{2}$ is stronger along $\psi_{\rm PP}$ and weaker along $\psi_{\rm RP}$; in contrast, the magnetic field, being from spectator protons, is weaker along $\psi_{\rm PP}$ and stronger along $\psi_{\rm RP}$. As a result the $\Delta\gamma$ measured with respect to $\psi_{\rm RP}$ and $\psi_{\rm PP}$ contain different amounts of CME and background, and can thus determine these two contributions. We report the results from this determination.
References
[1] J. Zhao, H. Li, F. Wang, arXiv:1705.05410 (2017).
[2] H. Xu, J. Zhao, X. Wang, H. Li, Z. Lin, C. Shen, F. Wang, arXiv:1710.07265 (2017).
| Content type | Experiment |
|---|---|
| Collaboration | STAR |
| Centralised submission by Collaboration | Presenter name already specified |
