High Transverse Momentum Results from the STAR Collaboration

The STAR collaboration presents new measurements of high pT hadron production in Au+Au and p+p collisions at RHIC. We extend the previously reported suppression of inclusive hadrons and large azimuthal anisotropies to much higher transverse momentum, decisively establishing the existence of strong medium effects on hadron production well into the perturbative regime. Near-angle two-particle correlations show directly that hadrons at pT > 4 GeV/c result from the fragmentation of jets. Additional evidence for the onset of perturbative QCD in this region comes from the flavor dependence of the inclusive yields and elliptic flow. Finally, comparison of back-to-back hadron pairs at high pT in p+p and in Au+Au collisions at various centralities reveals a striking suppression of high pT back-to-back pairs in the most central Au+Au collisions. All of these phenomena suggest a picture in which partons or their hadronic fragments are strongly absorbed in the bulk matter, with the observed hadrons resulting from jets produced on the periphery of the collision zone and directed outwards.