Beam-energy dependence of charge separation along the magnetic field in Au+Au collisions at RHIC (1404.1433v2)

Abstract: Local parity-odd domains are theorized to form inside a Quark-Gluon-Plasma (QGP) which has been produced in high-energy heavy-ion collisions. The local parity-odd domains manifest themselves as charge separation along the magnetic field axis via the chiral magnetic effect (CME). The experimental observation of charge separation has previously been reported for heavy-ion collisions at the top RHIC energies. In this paper, we present the results of the beam-energy dependence of the charge correlations in Au+Au collisions at midrapidity for center-of-mass energies of 7.7, 11.5, 19.6, 27, 39 and 62.4 GeV from the STAR experiment. After background subtraction, the signal gradually reduces with decreased beam energy, and tends to vanish by 7.7 GeV. The implications of these results for the CME will be discussed.

• The paper investigates the beam-energy dependence of charge separation in Au+Au collisions at RHIC using STAR experiment data and the three-point correlator method.
• A key finding shows that charge separation decreases significantly with lower beam energies, suggesting a transition from partonic to hadronic interaction dominance.
• This energy-dependent trend provides insights into Quark-Gluon Plasma properties and supports the theoretical framework of the chiral magnetic effect.

Beam-Energy Dependence of Charge Separation in Au+Au Collisions

The paper under consideration investigates the beam-energy dependence of charge separation attributed to the chiral magnetic effect (CME) in Au+Au collisions at the Relativistic Heavy Ion Collider (RHIC). This effect, a manifestation of local parity-odd domains, is theoretically predicted to emerge in a Quark-Gluon Plasma (QGP) formed during high-energy heavy-ion collisions. The charge separation induced along the magnetic field axis provides a crucial experimental observable for understanding the underlying phenomena in QCD under extreme conditions.

Key Findings and Methodology

• Experimental Setup: The paper employs data from the STAR experiment across a range of center-of-mass energies from 7.7 to 62.4 GeV. Charge correlations are measured using the three-point correlator, denoted $\gamma$, which quantifies charge separation along the magnetic field and is sensitive to parity-odd effects.
• Beam-Energy Dependence: A core observation is that the charge separation signal decreases with lower beam energies, becoming nearly negligible at 7.7 GeV. This trend suggests that at lower energies, the hadronic interactions dominate, diminishing the prevalence of partonic interactions necessary for QGP formation, subsequently reducing the CME.
• Systematics and Interpretation: Rigorous corrections for background effects, such as momentum conservation and elliptic flow, are applied. Results are presented in terms of the difference between opposite-charge and same-charge correlators $(\gamma_{\rm OS} - \gamma_{\rm SS})$, which indicates charge separation. The observed trends provide insights into the QGP properties and the local parity violation hypothesis, following the theoretical predictions within the framework of CME.
• Theoretical Considerations: The analysis models the charge separation effect using parameters that incorporate contributions from both CME and background sources. The paper derives an expression for the CME contribution, considering unknown factors such as the proportionality parameter $\kappa$, which remains an area for potential refinement.

Implications and Further Research

The reduction of charge separation with decreasing energy implies a shift from partonic to hadronic interactions, illuminating the transition phase between QGP dominance and hadronic gas in nuclear matter. The paper effectively tests the limits of CME at different energies, contributing evidence to the discourse on parity violation in high-energy QCD.

Future advancements could focus on refining the theoretical models, particularly in determining the $\kappa$ parameter, to enhance the accuracy of CME extraction from experimental data. Increasing the statistical dataset for lower-energy collisions can further validate these observations. Ultimately, these efforts will bolster the understanding of QGP and the fundamental QCD symmetries under extreme conditions, potentially impacting both theoretical foundations and experimental methodologies in high-energy nuclear physics.