Universal Centrality and Collision Energy Trends for v2 Measurements From 2D Angular Correlations

We have measured the pt-integrated quadrupole component of two-particle azimuth correlations (related to quantity v2, denoted in this case by v2{2D}) via two-dimensional (2D) angular autocorrelations on (η, φ) 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 η 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 state (impact parameter b and center-of-mass energy √ sNN ). There is no apparent dependence on evolving system dynamics (e.g., equation of state or number of secondary collisions). Our measurements of the quadrupole and non-quadrupole components have implications for the contributions to v2. They suggest that the main source of the difference between v2{2} and v2{4} (or v2{2D}) is measured properties of minijets. PACS. PACS-key discribing text of that key – PACS-key discribing text of that key