Observation of an energy-dependent difference in elliptic flow between particles and anti-particles in relativistic heavy ion collisions

TL;DR

This study reports an energy-dependent difference in elliptic flow between particles and anti-particles in heavy ion collisions, revealing deviations from universal scaling at lower energies.

Contribution

It provides the first detailed measurement of particle-antiparticle elliptic flow differences across a range of collision energies at RHIC.

Findings

Difference in $v_{2}$ increases as beam energy decreases.

Baryons show larger $v_{2}$ differences than mesons.

Universal NCQ scaling breaks down at lower energies.

Abstract

Elliptic flow ($v_{2}$) values for identified particles at mid-rapidity in Au+Au collisions, measured by the STAR experiment in the Beam Energy Scan at RHIC at $\sqrt{s_{NN}}=$ 7.7--62.4 GeV, are presented. A beam-energy dependent difference of the values of $v_{2}$ between particles and corresponding anti-particles was observed. The difference increases with decreasing beam energy and is larger for baryons compared to mesons. This implies that, at lower energies, particles and anti-particles are not consistent with the universal number-of-constituent-quark (NCQ) scaling of $v_{2}$ that was observed at $\sqrt{s_{NN}}=$ 200 GeV.