Azimuthal Anisotropy in U plus U and Au plus Au Collisions at RHIC

Collisionsbetween prolateuranium 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 collisionsat root s(NN) = 193 GeV and Au + Au collisionsat root s(NN) = 200 GeV. Nearly fully overlapping collisionsare 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 U + 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.