Disentangling flow and signals of Chiral Magnetic Effect in U+U, Au+Au and p+Au collisions

Abstract

We present STAR measurements of the charge-dependent three-particle correlator $\gamma^{a,b}=\langle \cos(\phi_1^{a}+\phi_2^{b}-2\phi_3)\rangle/v_{2}\{2\}$ and elliptic flow $v_{2}\{2\}$ in U+U, Au+Au and p+Au collisions at RHIC. The difference $\Delta \gamma = \gamma(\rm{opposite\!-\!sign})\!-\!\gamma(\rm{same\!-\!sign})$ measures charge separation across the reaction plane, a predicted signal of the Chiral Magnetic Effect (CME). Although charge separation has been observed, it has been argued that the measured separation can also be explained by elliptic flow related backgrounds. In order to separate the two effects, we perform measurements of the $\gamma$-correlator where background expectations differ from magnetic field driven effects. A differential measurement of $\gamma$ with the relative pseudorapidity ($\Delta\eta$) between the first and second particles indicate that $\Delta \gamma$ in peripheral A+A and p+A collisions are dominated by short-range correlations in $\Delta\eta$. However, a relatively wider component of the correlation in $\Delta\eta$ tends to vanish the same way as projected magnetic field as predicted by MC-Glauber simulations.