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Abstract: Universal centrality and collision-energy trends for v2 measurementsfrom two-dimensional angular correlations at STAR

We have measured the pt-integrated quadrupole component of two-particle azimuth correlations (related to quantity v2,
denoted in this case by via two-dimensional (2D) angular autocorrelations on (eta,phi) for unidentified hadrons
in Au-Au collisions at 62 and 200 GeV. The 2D autocorrelation provides a method to remove
non-quadrupole contributions to v2 (conventionally termed "nonflow") under the assumption that such processes produce significant dependence
on pair-wise relative eta within the detector acceptance. We hypothesize, based on empirical observations, that non-quadrupole
contributions are dominated by minijets or minimum-bias jets. Using the optical Glauber eccentricity model for initial-state
geometry we find simple and accurate universal energy and centrality trends for the quadrupole component. Centrality
trends are determined only by the initial collision geometry (impact parameter or n_binary). The observed energy
trend (linear with log is the same as observed for mean-pt fluctuations and correlations. There
is no apparent dependence on evolving system dyn!
amics (e.g., equation of state or number of secondary collisions). Our results also have implications
for conventional v2 measurements. Observed systematic differences between or and correspond to measured
properties of minijets and impose a small upper limit on v2 fluctuations (v2 fluctuations are now
consistent with zero). Any measurement of v2 fluctuations would require measuring minijet correlations at the same
level of accuracy.