Azimuthal anisotropy in U+U collisions at STAR

TL;DR

This paper investigates how the shape of uranium nuclei influences azimuthal anisotropy in high-energy collisions, revealing discrepancies with existing models and providing insights into entropy production in heavy ion collisions.

Contribution

It presents new measurements of azimuthal anisotropy in U+U collisions at STAR, highlighting the impact of nuclear geometry and challenging current theoretical models.

Findings

Differences in $v_2$ between U+U and Au+Au collisions.

Multiplicity dependence of $v_2$ not explained by Monte Carlo Glauber model.

Central collision data show unique anisotropy behavior.

Abstract

The azimuthal anisotropy of particle production is commonly used in high-energy nuclear collisions to study the early evolution of the expanding system. The prolate shape of uranium nuclei makes it possible to study how the geometry of the colliding nuclei affects final state anisotropies. It also provides a unique opportunity to understand how entropy is produced in heavy ion collisions. In this paper, the two- and four- particle cumulant $v_2$ ($v_{2}\{2\}$ and $v_{2}\{4\}$) from U+U collisions at $\sqrt{s_{NN}}$ = 193 GeV and Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV for inclusive charged hadrons will be presented. The STAR Zero Degree Calorimeters are used to select very central collisions. Differences were observed between the multiplicity dependence of $v_{2}\{2\}$ for most central Au+Au and U+U collisions. The multiplicity dependence of $v_{2}\{2\}$ in central collisions were compared to Monte Carlo Glauber model predictions and it was seen that this model cannot explain the present results.