Flow measurements and selection of body-body and tip-tip enhanced samples in UÂ +Â U collisions at STAR

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 provides the possibility to study how the initial geometry of the nuclei affects the azimuthal distributions. In this paper, the two- and four-particle cumulants, v2{2} and v2{4}, from U + U collisions at sNN=193 GeV and Au + Au collisions at sNN=200 GeV for inclusive charged hadrons will be presented. The STAR Zero Degree Calorimeter is used to subdivide the 0-1% centrality bin into even finer centralities. Differences were observed between the multiplicity dependence of v2{2} for most central Au + Au and U + U collisions. It has also been demonstrated that ZDC and multiplicity in combination provide a way to select body-body or tip-tip enhanced samples of central U + U collisions. Comparisons to models show that an IP-Glasma model based on gluon saturation for the initial state better describes the slope of v2{2} vs. multiplicity than the Glauber model with a standard two-component model for multiplicity.