# Novel mechanism for electric quadrupole moment generation in relativistic heavy-ion collisions

## TL;DR

This paper proposes a new mechanism for generating electric quadrupole moments in relativistic heavy-ion collisions, linking electromagnetic field distributions to charge-dependent flow differences observed experimentally.

## Contribution

It introduces a novel mechanism involving the coupling of electromagnetic anomaly and magnetic fields to explain charge-dependent elliptic flow without chiral magnetic wave formation.

## Key findings

- Dipolar distribution of E·B observed in non-central collisions
- Electric quadrupole moment induced by E·B dipole and magnetic field coupling
- Centrality dependence of E·B density matches experimental flow slope trends

## Abstract

We present the spatial distributions of electromagnetic fields ($\bf E$ and $\bf B$) and electromagnetic anomaly $ \bf E \cdot B$ in Au+Au collisions at the RHIC energy $\sqrt{s}$=200 GeV based on a multi-phase transport model. A dipolar distribution of $\bf E \cdot B$ is observed in non-central collisions. We find that the coupling of the $\bf E \cdot B$ dipole and magnetic field $\bf B$ can induce an electric quadrupole moment which can further lead to the difference in elliptic flows between positive charged particles and negative charged particles through final interactions. The centrality dependence of the density of $\bf E \cdot B$ is similar to the trend of the slope parameter $r$ measured from the difference in elliptic flows between positive pions and negative pions by the STAR collaboration. Therefore, the novel mechanism for electric quadrupole moment generation can offer a new interpretation of the observed charge-dependent elliptic flow of pions, but without the formation of chiral magnetic wave.