Azimuthal Anisotropy in $\mathrm{U}+\mathrm{U}$ and $\mathrm{Au}+\mathrm{Au}$ Collisions at RHIC

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 $\mathrm{U}+\mathrm{U}$ collisions at $\sqrt{s_{\mathrm{NN}}}=193\mathrm{GeV}$ and $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{\mathrm{NN}}}=200\mathrm{GeV}$. Nearly fully overlapping collisions are selected based on the energy deposited by spectators in 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 $\mathrm{U}+\mathrm{U}$ collisions. We also show that $v_2$ vs multiplicity can be better described by models, such as gluon saturation or quark participant models, that eliminate the dependence of the multiplicity on the number of binary nucleon-nucleon collisions.

Figure 1

The two- and four-particle cumulant $v_2\{2\}$ and $v_2\{4\}$ within $|\eta |<1$ vs $d{N}_{\text{ch}}/d\eta$ from 200 GeV $\mathrm{Au}+\mathrm{Au}$ and 193 GeV $\mathrm{U}+\mathrm{U}$ collisions. Dashed lines show $\mathrm{U}+\mathrm{U}$ centralities based on $d{N}_{\text{ch}}/d\eta$ measured in $|\eta |<0.5$. $v_2^4\{4\}$ (the experimentally observed quantity) is shown in the inset without taking the fourth root in the range where it is near zero or negative.

Figure 2

$v_2$ scaled by participant eccentricity from 200 GeV $\mathrm{Au}+\mathrm{Au}$ and 193 GeV $\mathrm{U}+\mathrm{U}$ collisions. The eccentricity distributions are calculated in a Monte Carlo Glauber model [27, 28, 29, 30]. Both $\mathrm{U}+\mathrm{U}$ and $\mathrm{Au}+\mathrm{Au}$ follow a similar trend for $v_2/{ϵ}_2$ and a turnover is observed in central collisions. The inset shows the same quantity but with the eccentricity calculated in a constituent quark Glauber model [18, 19] with the Woods-Saxon parameters proposed in Ref. [30].

Figure 3

Top panels: charged particle $v_2\{2\}$ vs normalized multiplicity within $|\eta |<1.0$. The upper panel is for the top 1% most central events based on the smallness of the ZDC signal, while the middle panel is for the top 0.125%. Small boxes indicate the possible range of variation of $v_2$ from uncertainties in the efficiency corrections on the $x$ axis. Model comparisons are described in the text. Bottom panel: The slopes as a function of increasingly tighter ZDC centrality selections. The systematic uncertainties are shown as bands.