Collision-system Dependence of the Charge Separation Relative to the Event Plane; Implications for Chiral Magnetic Effect Search in STAR

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Unambiguous validation of the Chiral Magnetic Effect (CME) in the quark-gluon plasma (QGP) produced in heavy-ion collisions, can provide key insights into anomalous transport in the QGP and the connections between chiral symmetry restoration, axial anomaly and gluonic topology in QCD. Charge separation measurements play a pivotal role in ongoing searches for the Chiral Magnetic Effect (CME). A charge-sensitive correlator $\mathrm{R_{\Psi_m}(\Delta S)}$~[1] is used to carry out a detailed set of charge separation measurements, relative to the $2^{\mathrm{nd}}$- ($\Psi_2$) and $3^{\mathrm{rd}}$-order ($\Psi_3$) event planes which is used to discern between background- and CME-driven charge separation.
The measurements indicate nearly flat $\mathrm{R_{\Psi_m}(\Delta S)}$ distributions for the measurements relative to $\Psi_3$ and those relative to $\Psi_2$ for the p(d)+Au systems, consistent with the essentially random $\vec{B}$-field orientations for these measurements. By contrast, the A+A measurements relative to $\Psi_2$, show concave-shaped $\mathrm{R_{\Psi_2}(\Delta S)}$ distributions suggestive of a CME-driven charge separation, quantified by widths inversely proportional to the Fourier dipole coefficient $a_1$. Results for U+U collisions at $\sqrt{s_{NN}}$= 193~GeV, and p(d)+Au, Cu+Au, Cu+Cu and Au+Au collisions at $\sqrt{s_{NN}}=$~200~GeV will be presented and discussed.\\

[1] N. Magdy, S. Shi, J. Liao, N. Ajitanand, and R. A. Lacey, Phys. Rev. C 97, 061901 (2018)

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