Two-particle correlations in ultra relativistic heavy ion collisions

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Daugherity, Michael Scott, 1979-

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The Relativistic Heavy Ion Collider (RHIC) accelerates gold nuclei to nearly the speed of light and smashes them together, forming the most extreme conditions of high energy and density ever produced in a laboratory. The first detailed study of the energy and centrality (collision overlap) dependence of two-particle autocorrelations is presented for charged hadrons produced in √ sNN = 62 and 200 GeV Au+Au collisions and measured by the STAR detector at RHIC. This analysis is unique in using a large momentum acceptance of pt > 0.15 GeV/c, |η| ≤ 1.0, and full 2π azimuth to form all possible two-particle pairs to measure minimum-bias correlations. Protonproton collisions at 200 GeV are studied as a reference, where correlation structure in these collisions is dominated by a peak centered at zero relative opening angles vii on η and φ due to minimum-bias jets (minijets) from semi-hard parton scattering. Correlations in heavy ion collisions show significant deviations from this reference revealing new interactions. A sudden and dramatic increase of the minijet peak amplitude and η width is observed relative to binary-collision scaling which occurs at an energy-dependent centrality point. These results confirm a rapid transition of minijet correlation properties suggested in previous studies at 130 GeV. There is a possible scaling of the transition point with transverse particle density. This transition leads to a large excess of minijet correlations in more-central Au-Au collisions relative to binary-collision scaling. Additional studies of charge-dependence and transverse correlations reveal important distinctions between correlations from the originating minijets and the additional correlations emerging above the transition point. When considered with similar systematic trends from studies of transverse momentum in single-particle spectra and two-particle correlations, these results appear to be strongly inconsistent with often made assumptions of rapid thermalization in RHIC heavy ion collisions.