Observation of charge-dependent azimuthal correlations and possible local strong parity violation in heavy ion collisions

TL;DR

The paper investigates local strong parity violation in heavy-ion collisions by measuring a parity-even three-particle azimuthal correlator that is sensitive to charge separation along the system's angular momentum. Using STAR data from Au+Au and Cu+Cu at $\sqrt{s_{NN}}=200$ and 62 GeV, the analysis employs a third particle to proxy the reaction plane and applies corrections to control detector effects, reporting centrality, $\Delta\eta$, and $p_T$ dependencies. The results show same-charge correlations larger than opposite-charge ones and trends that align with some CME predictions, while extensive studies of backgrounds with HIJING, UrQMD, MEVSIM, and other tests indicate backgrounds alone cannot account for the signal. A zero first-order parity signal is observed with ZDC-SMD, and the authors call for energy scans and improved theory to establish whether the effect is indeed local parity violation via the CME, highlighting the need for further experimental and theoretical work. Overall, the work provides a rigorous framework for disentangling potential CME signals from backgrounds in heavy-ion collisions and outlines clear directions for future investigations.

Abstract

Parity-odd domains, corresponding to non-trivial topological solutions of the QCD vacuum, might be created during relativistic heavy-ion collisions. These domains are predicted to lead to charge separation of quarks along the orbital momentum of the system created in non-central collisions. To study this effect, we investigate a three particle mixed harmonics azimuthal correlator which is a ¶-even observable, but directly sensitive to the charge separation effect. We report measurements of this observable using the STAR detector in Au+Au and Cu+Cu collisions at $\sqrt{s_{NN}}$=200 and 62~GeV. The results are presented as a function of collision centrality, particle separation in rapidity, and particle transverse momentum. A signal consistent with several of the theoretical expectations is detected in all four data sets. We compare our results to the predictions of existing event generators, and discuss in detail possible contributions from other effects that are not related to parity violation.

Figures (14)

• Figure 1: (Color online) Schematic view of the charge separation along the system orbital momentum. The orientation of the charge separation fluctuates in accord with the sign of the topological charge. The direction of the orbital momentum $\bf L$, and that of the magnetic field $\bf B$, is indicated by an arrow.
• Figure 2: (Color online) Schematic view of the transverse plane indicating the orientation of the reaction plane and particle azimuths relative to that plane. The colliding nuclei are traveling into and out of the figure.
• Figure 3: (Color) $\left\langle \cos(\phi_{\alpha} +\phi_{\beta} -2\phi_c) \right\rangle$ as a function of reference multiplicity for different charge combinations, before corrections for acceptance effects. In the legend the signs indicate the charge of particles $\alpha$, $\beta$, and $c$. The results shown are for Au+Au collisions at 200 GeV obtained in (a) the Reversed Full Field, and (b) the Full Field configurations.
• Figure 4: (Color) Same as Fig. \ref{['fig:no_recenter']} after correction for acceptance effects.
• Figure 5: (Color) $\left\langle \cos(\phi_{\alpha} -\phi_{\beta}) \right\rangle$ as a function of centrality for different charge combinations and FF and RFF configurations. The data points corresponding to different charge and field configurations are slightly shifted in the horizontal direction with respect to each other for clarity. The error bars are statistical. Also shown are model predictions described in Section \ref{['sec:simulations']}.