Probing the bulk medium in relativistic heavy ion collisions using two-particle correlations

Measurements of charge-independent and charge-dependent two-particle correlations for Au-Au collisions at √sNN = 130 GeV from the STAR experiment are presented as functions of transverse momentum pt and difference variables η1 - η2 (pseudorapidity) and φ1 - φ2 (azimuth) for final state hadrons with 0.15 ≤ pt ≤ 2 GeV/c, η ≤ 1.3 and full 2π azimuth. significant large-momentum-scale correlation structures are observed which were not seen in similar studies at lower energy and are not predicted by theoretical collision models. Charge-independent correlations at pt < 0.5 GeV/c and small η, φ differences are observed which are consistent with a scenario in which initial-state semi-hard (low-Q2) parton scattering and fragmentation are modified by in-medium dissipation resulting in event-wise mean-pt fluctuations and significant broadening of the same-side correlation peak on η1 - η2 by a factor 2.3 from peripheral to central collisions. Charge-dependent correlations on (η, φ) difference variables indicate the evolution of hadron emission geometry with increasing centrality from one-dimensional fragmentation of longitudinal color strings to two-dimensional (η, φ) fragmentation of a hadron-opaque bulk medium. The new correlation methods presented here permit quantitative study of the nonperturbative interactions between low-Q2 partons and the bulk medium produced in relativistic heavy ion collisions