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

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 \P-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.

• The paper identifies charge-dependent azimuthal correlations as a potential signature of local strong parity violation in heavy-ion collisions.
• The methodology utilizes three-particle mixed harmonic correlators to evaluate charge separation in Au+Au and Cu+Cu systems.
• The results highlight dependencies on collision centrality and system size, underscoring the need for refined theoretical models and further experiments.

Observation of Charge-Dependent Azimuthal Correlations and Possible Local Strong Parity Violation in Heavy-Ion Collisions: An Overview

This paper examines the intriguing possibility of local strong parity ($\mathcal{P}$) violation in the context of quantum chromodynamics (QCD) through the analysis of heavy-ion collisions. It presents results from the STAR Collaboration's investigation using the RHIC accelerator, specifically focusing on the collision systems of Au+Au and Cu+Cu at center-of-mass energy $\sqrt{s_{NN}}$ of 200 and 62 GeV. The core interest is the identification of charge-dependent azimuthal particle correlations, which could suggest parity-odd domains in the QCD vacuum.

Core Concepts and Methodology

The study of $\mathcal{P}$-odd effects in QCD is premised on the hypothesis that, at high temperatures or energy densities, domains with non-zero topological charges may form. These domains have the potential to cause charge separation along the angular momentum direction of the colliding system, a phenomenon theoretically linked to the Chiral Magnetic Effect (CME). Such azimuthal charge separation could provide indirect evidence of local parity violation, otherwise unobservable due to the negligible magnitude of the QCD vacuum angle $\theta$.

To empirically explore these predictions, the authors employ a multi-particle correlation technique using a three-particle mixed harmonics azimuthal correlator. This correlator, a $\mathcal{P}$-even observable derived from differences in azimuthal angles, was chosen for its ability to indirectly gauge charge separation effects.

Experimental Observations

Key experimental observations include:

• Charge-Dependent Correlations: Same-sign charged particle correlations exhibit signals of the predicted magnitude and differentiate themselves from the correlations of opposite-sign pairs. This charge asymmetry aligns with theoretical expectations.
• Centrality and System Size: The correlator's behavior varies with the centrality of the collision, displaying a reduction in more central collisions. Differences also emerge between Au+Au and Cu+Cu systems, indicating that the system size influences the magnitude of these correlations.
• Rapidity and Transverse Momentum Dependence: The results include a rapid fall-off of the signal with increasing pseudorapidity separation, consistent with local effects. However, contrary to CME predictions, the signal extends into higher transverse momentum ranges.

Implications and Future Directions

The findings hint at phenomena consistent with local $\mathcal{P}$ violation, although alternative non-parity-violating explanations that can generate such correlated signals are acknowledged and discussed. The paper underscores the need for developing more accurate theoretical models for the CME and related backgrounds. Future experimental efforts, proposed to occur across a broader range of collision energies, could clarify the extent of such effects' dependence on various QCD phase transitions. This might help pinpoint the energy levels at which strong $\mathcal{P}$ violation becomes pronounced, potentially offering insights into the conditions necessary for quark-gluon plasma formation.

Conclusion

The study provides compelling empirical data that contribute to the ongoing discourse on the interplay between topological effects in QCD and the observable phenomena at high-energy nuclear collisions. While the charge-dependent azimuthal correlations observed in these experiments do not yield definitive proof of local $\mathcal{P}$ violation, they offer a promising avenue of investigation into the non-trivial structures of QCD at extreme states. Enhanced theoretical frameworks and experimental strategies are crucial next steps in this quest to understand the subatomic regime's complex phenomena.