Metastable domains of fluctuating topological charges can change the chirality of quarks and induce local parity violation in quantum chromodynamics. This can lead to observable charge separation along the direction of the strong magnetic field produced in relativistic heavy-ion collisions, a phenomenon called the chiral magnetic effect (CME). A major background source for CME measurements is the intrinsic particle correlations (such as resonances/jets decay) coupled with the azimuthal elliptical anisotropy $v_{2}$. In heavy-ion collisions, the magnetic field direction and event plane azimuthal angle $\Psi_{2}$ are correlated, thus the CME and the $v_{2}$-induced background are entangled. In small system p+Au and d+Au collisions, the $\Psi_{2}$ is mostly due to geometry fluctuations, and thus magnetic field direction and $\Psi_{2}$ are uncorrelated. The correlation measurements in small system collisions with respect to $\Psi_{2}$ are only sensitive to $v_{2}$-induced background while any CME is averaged to zero.

In this talk, we will present the STAR measurements of two-particle correlations with respect to $\Psi_{2}$ in p+Au, d+Au and Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 200 GeV. These results are analyzed as a function of particle multiplicity to shed light on the background contaminations of the CME measurements in heavy-ion collisions. We will also report results from a new analysis approach as a function of the particle pair invariant mass in order to suppress non-CME related physics backgrounds [1]. Data-model comparisons will also be shown wherever available.

[1] Jie Zhao, Hanlin Li, Fuqiang Wang, arXiv:1705.05410 [nucl-th].

Collectivity in high energy collisions