Dilepton excess from local parity breaking in baryon matter

TL;DR

This paper suggests that local parity breaking in baryon-rich matter during heavy ion collisions could explain the observed excess of low-mass dileptons, through modified photon and meson dispersion and new decay channels.

Contribution

It introduces a novel mechanism involving local parity breaking to account for dilepton excesses in heavy ion collisions, linking topological charge fluctuations and pseudoscalar meson condensation.

Findings

Dilepton excess can be explained by local parity breaking effects.

Photon and vector meson dispersion relations are modified in the medium.

New decay channels emerge due to parity violation.

Abstract

We propose that local parity breaking induced by a large-scale fluctuation of topological charge at large temperatures and/or condensation of pseudoscalar mesons in the isotriplet channel for large baryon densities may be responsible for the substantial dilepton excess that is found for low invariant masses and moderate values of $p_T$ in central heavy ions collisions. This insofar unexplained enhancement could be understood by a combination of two effects leading both to an excess of $e^+e^-$ and $\mu^+ \mu^-$ pairs with respect to theoretical predictions based on conventional hadronic processes: (a) a modification of the dispersion relation of photons and vector mesons propagating in such a nuclear medium due to local parity breaking; (b) the appearance of new decay channels, forbidden by parity conservation in QCD in the usual vacuum. Possible signatures of this effect and perspectives for its detection are discussed.