  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.

Nuclear Experiment