Charge separation induced by P-odd bubbles in QCD matter

TL;DR

The paper analyzes how P- and CP-odd fluctuations in QCD matter, governed by a nonzero $\theta$, can induce an electric field along the angular-momentum axis in rotating, off-central heavy-ion collisions. Using an anomalous Lagrangian, it derives $${\mathbf E} \propto \theta \boldsymbol{\Omega}$$ and a surface charge density $${\sigma_{xy}} \propto \theta \Omega_z$$, establishing a topological mechanism for charge separation observable via event-by-event correlations. Numerical estimates suggest a few percent quark–antiquark asymmetry for semi-central RHIC events, vanishing for central collisions, in qualitative agreement with preliminary STAR results. The authors also extrapolate these CP-odd effects to cosmology, proposing that nonzero $\theta$ during the QCD phase transition could create CP-odd domains and quark nuggets as dark matter, potentially connecting baryogenesis to dark matter in the early Universe. Overall, the work links anomalous QCD topological effects to measurable heavy-ion observables and explores intriguing cosmological implications of charge separation.

Abstract

We examine the recent suggestion that P- and CP-odd effects in QCD matter can induce electric charge asymmetry with respect to reaction plane in relativistic heavy ion collisions. General arguments are given which confirm that the angular momentum of QCD matter in the presence of non-zero topological charge should induce an electric field aligned along the axis of the angular momentum. A simple formula relating the magnitude of charge asymmetry to the angular momentum and topological charge is derived. The expected asymmetry is amenable to experimental observation at RHIC and LHC; we discuss the recent preliminary STAR result in light of our findings.

Figures (1)

• Figure 1: Preliminary STAR data from Selyuzhenkov:2005xa. Charged particle asymmetry parameters as a a function of centrality bins selected on the basis of charged particle multiplicity in $|\eta| < 0.5$ region. Points are STAR preliminary data for Au+Au at $\sqrt{s_{NN}} = 62$ GeV: circles are $a_{+}^2$, triangles are $a_{-}^2$ and squares are $a_{+}a_{-}$, see text for details. Black lines are theoretical prediction Kharzeev:2004ey corresponding to the topological charge $|Q|=1$.