Chiral magnetic wave at finite baryon density and the electric quadrupole moment of quark-gluon plasma in heavy ion collisions

TL;DR

This paper investigates how the Chiral Magnetic Wave at finite baryon density induces an electric quadrupole moment in quark-gluon plasma, affecting pion elliptic flow differences in heavy ion collisions.

Contribution

It introduces the effect of finite baryon density on the Chiral Magnetic Wave and predicts resulting electric quadrupole deformation in quark-gluon plasma.

Findings

Electric quadrupole moment induced in quark-gluon plasma.

Deformation causes difference in elliptic flow of positive and negative pions.

Estimated magnitude of the effect on heavy ion collision observables.

Abstract

Chiral Magnetic Wave (CMW) is a gapless collective excitation of quark-gluon plasma in the presence of external magnetic field that stems from the interplay of Chiral Magnetic (CME) and Chiral Separation Effects (CSE); it is composed by the waves of the electric and chiral charge densities coupled by the axial anomaly. We consider CMW at finite baryon density and find that it induces the electric quadrupole moment of the quark-gluon plasma produced in heavy ion collisions: the "poles" of the produced fireball (pointing outside of the reaction plane) acquire additional positive electric charge, and the "equator" acquires additional negative charge. We point out that this electric quadrupole deformation lifts the degeneracy between the elliptic flows of positive and negative pions leading to $v_2(\pi^+) < v_2(\pi^-)$, and estimate the magnitude of the effect.