Azimuthal anisotropy in U+U and Au+Au collisions at RHIC

TL;DR

This study investigates how initial geometric configurations in heavy-ion collisions influence azimuthal anisotropies, using U+U and Au+Au collisions at RHIC energies, and compares experimental results with theoretical models.

Contribution

It provides new measurements of azimuthal anisotropy cumulants in U+U and Au+Au collisions and demonstrates the effectiveness of ZDC-based selection in probing initial geometry effects.

Findings

ZDC and multiplicity can select different overlap configurations.

Gluon saturation model better describes $v_2 ext{ }$ dependence than Glauber model.

Initial geometry impacts azimuthal anisotropy in heavy-ion collisions.

Abstract

Collisions between prolate uranium nuclei are used to study how particle production and azimuthal anisotropies depend on initial geometry in heavy-ion collisions. We report the two- and four-particle cumulants, $v_2\{2\}$ and $v_2\{4\}$, for charged hadrons from U+U collisions at $\sqrt{s_{\rm NN}}$ = 193 GeV and Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 200 GeV. Nearly fully overlapping collisions are selected based on the amount of energy deposited by spectators in the STAR Zero Degree Calorimeters (ZDCs). Within this sample, the observed dependence of $v_2\{2\}$ on multiplicity demonstrates that ZDC information combined with multiplicity can preferentially select different overlap configurations in U+U collisions. An initial-state model with gluon saturation describes the slope of $v_2\{2\}$ as a function of multiplicity in central collisions better than one based on Glauber with a two-component multiplicity model.