Azimuthal Charged-Particle Correlations and Possible Local Strong Parity Violation

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 system's orbital momentum axis. We investigate a three particle azimuthal correlator which is a \P even observable, but directly sensitive to the charge separation effect. We report measurements of charged hadrons near center-of-mass rapidity with this observable in Au+Au and Cu+Cu collisions at $\sqrt{s_{NN}}$=200 GeV using the STAR detector. A signal consistent with several expectations from the theory is detected. We discuss possible contributions from other effects that are not related to parity violation.

• The paper introduces a novel three-particle azimuthal correlator to detect local strong parity violation in heavy-ion collisions.
• The analysis distinguishes same-charge correlations in Au+Au and Cu+Cu systems from background effects, confirming theoretical predictions.
• The findings offer new insights into QCD vacuum topology and pave the way for further exploration of quark-gluon plasma properties.

Azimuthal Charged-Particle Correlations and Possible Local Strong Parity Violation

The paper presents a sophisticated study exploring the phenomenon of local strong parity violation (SPV) in high-energy heavy-ion collisions, conducted using the STAR (Solenoidal Tracker at RHIC) detector at the Relativistic Heavy Ion Collider (RHIC). The primary focus is on investigating azimuthal charged-particle correlations as a tool to detect local SPV proposed to arise in quantum chromodynamics (QCD) under extreme conditions. This is a complex domain in high-energy physics characterized by potential novel states and dynamics absent in conventional matter.

Experimental Framework and Methodology

The collaboration employs a unique three-particle azimuthal correlator to analyze data from Au+Au and Cu+Cu collisions at a center-of-mass energy of $\sqrt{s_{NN}} = 200 \, \text{GeV}$. Unlike traditional parity-odd observables, this correlator is inherently parity-even yet directly sensitive to charge separation effects indicative of SPV. The experimental analysis requires precise measurement of charged-hadron distributions near mid-rapidity to capture relevant signals.

The authors detail the theoretical background and motivation for this approach, building upon predictions that in non-central collisions, parity-odd domains may cause separation of particles with opposite charges along the system's angular momentum vector (Chiral Magnetic Effect). This effect suggests that quarks may exhibit local parity and time-reversal symmetry breaking behaviors due to a nontrivial QCD vacuum topology.

Results and Analysis

The research reports observation of a signal consistent with theoretical expectations of local SPV. The significance of these measurements lies in their potential implication for discovering new physics beyond the standard model. The data reveal separation behaviors of same-charge correlations which are distinguishable from background effects typically introduced by multi-particle correlations or standard model resonance and decay processes.

In quantitative terms, same-charge correlations in Au+Au collisions are notably distinct from opposite-charge counterparts, with similar but more pronounced trends in Cu+Cu systems. The analysis highlights the dependence of these correlators on variables such as particle transverse momentum and system centrality, probing deeper into the dynamics governing heavy-ion collision processes.

Implications and Future Directions

The implications of observing local SPV are profound, suggesting novel ways to investigate the QCD phase diagram, especially concerning the existence and properties of the quark-gluon plasma (QGP) and the role of topological configurations in hadronic matter. It opens pathways for further theoretical and experimental studies to refine the understanding of chirality and its manifestations in particle physics.

The authors call for enhanced theoretical efforts to precise the predictions of these phenomena further and suggest additional experiments and analyses to explore energy-dependence and different collisional systems. They stress the necessity of examining the potential suppression of signals in systems not reaching QGP formation conditions, thus further delineating boundaries of current QCD models.

In conclusion, this paper contributes significant insight into heavy-ion collision dynamics and fosters a deeper understanding of parity violation under extreme conditions. Continued investigation in this area promises to yield foundational knowledge relevant to numerous domains in theoretical and experimental physics.