Electric Charge Separation in Strong Transient Magnetic Fields

TL;DR

This work investigates event-by-event electric charge fluctuations aligned with the reaction plane in relativistic heavy-ion collisions, attributing them to the chiral magnetic effect in strong transient magnetic fields. It shows that CME-like charge separation can occur both in the quark-gluon plasma (via topological winding fluctuations) and in hadronic matter (via magnetically induced transitions such as $\pi^0\to\rho^0$), without requiring explicit parity or CP violation. The paper develops a unified framework linking local anomalous currents to observable momentum-space charge asymmetries through freeze-out dynamics and collective flow, and it analyzes five scenarios (CGC, QGP, glasma, corona, HG) with semi-quantitative estimates. The results indicate that the hadronic and CGC mechanisms yield fluctuations smaller than observed by STAR, highlighting possible enhancements from in-medium effects and underscoring the need for more detailed modeling of transport and mass-shifts during the evolution. Overall, the study provides a structured, multi-scenario treatment of CME-driven charge asymmetries and clarifies the roles of early-time dynamics and freeze-out geometry in translating microscopic currents into measurable signals.

Abstract

We discuss various mechanisms for the creation of an asymmetric charge fluctuation with respect to the reaction plane among hadrons emitted in relativistic heavy-ion collisions. We show that such mechanisms exist in both, the hadronic gas and the partonic phases of QCD. The mechanisms considered here all require the presence of a strong magnetic field (the ``chiral magnetic effect''), but they do not involve parity or charge-parity violations. We analyze how a transient local electric current fluctuation generated by the chiral magnetic effect can dynamically evolve into an asymmetric charge distribution among the final-state hadrons in momentum space. We estimate the magnitude of the event-by-event fluctuations of the final-state charge asymmetry due to the partonic and hadronic mechanisms.

Figures (12)

• Figure 1: Feynman diagram describing the anomalous coupling between two photons and a neutral pseudoscalar meson via a triangular quark loop.
• Figure 2: Feynman diagram describing the effective pseudoscalar coupling between two gluons and two photons via a quark loop.
• Figure 3: Feynman diagram describing the effective pseudoscalar coupling between two gluons and two photons via a virtual $\eta$- or $\eta'$-meson.
• Figure 4: Temperature dependence of the coefficient $\kappa$ in the effective pseudoscalar QED--QCD interaction (\ref{['eq-Leff-QCD']}).
• Figure 5: Quark triangle diagram for the electromagnetic current ${\bf j}$ induced by the presence of a magnetic field $B$ in a neutral pion. The cross at the upper corner of the triangle indicates the insertion of the electromagnetic current operator $e_f\gamma^\mu$.