Flow and interferometry results from Au+Au collisions at $\sqrt{\textit{s}_{NN}}$ = 4.5 GeV

TL;DR

This paper reports the first measurements of flow and femtoscopy in Au+Au collisions at 4.5 GeV using STAR's fixed-target mode, validating the approach and providing new insights into low-energy heavy-ion collisions.

Contribution

First presentation of flow and femtoscopy results at 4.5 GeV in fixed-target mode, demonstrating STAR's capability and extending the Beam Energy Scan program to lower energies.

Findings

Proton flow and pion femtoscopy results agree with previous experiments.

First measurements of pion directed and elliptic flow at this energy.

Hints of constituent quark scaling in elliptic flow, but with large uncertainties.

Abstract

The Beam Energy Scan (BES) program at the Relativistic Heavy Ion Collider (RHIC) was extended to energies below $\sqrt{\textit{s}_{NN}}$ = 7.7 GeV in 2015 by successful implementation of the fixed-target mode of operation in the STAR (Solenoidal Track At RHIC) experiment. In the fixed-target mode, ions circulate in one ring of the collider and interact with a stationary target at the entrance of the STAR Time Projection Chamber. The first results for Au+Au collisions at $\sqrt{\textit{s}_{NN}}$ = 4.5 GeV are presented, including directed and elliptic flow of identified hadrons, and radii from pion femtoscopy. The proton flow and pion femtoscopy results agree quantitatively with earlier measurements by Alternating Gradient Synchrotron experiments at similar energies. This validates running the STAR experiment in the fixed-target configuration. Pion directed and elliptic flow are presented for the first time at this beam energy. Pion and proton elliptic flow show behavior which hints at constituent quark scaling, but large error bars preclude reliable conclusions. The ongoing second phase of BES (BES-II) will provide fixed-target data sets with 100 times more events at each of several energies down to $\sqrt{\textit{s}_{NN}}$ = 3.0 GeV.