May 13 – 19, 2018
Venice, Italy
Europe/Zurich timezone
The organisers warmly thank all participants for such a lively QM2018! See you in China in 2019!

Measurements of the Chiral Magnetic Effect with Background Isolation in 200 GeV Au+Au Collisions at STAR

May 16, 2018, 9:40 AM
Sala Mosaici-2, 3rd Floor (Palazzo del Casinò)

Sala Mosaici-2, 3rd Floor

Palazzo del Casinò

Parallel Talk Chirality, vorticity and polarisation effects Chirality, vorticity and polarisation effects


Jie Zhao (Purdue University)


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.

[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

Primary author

Zhenyu Ye (University of Illinois at Chicago)

Presentation materials