Jet Quenching at Rhic

We present high transverse momentum (p T) measurements made at the Relativis-tic Heavy Ion Collider (RHIC) for Au+Au, d+Au, and p+p collisions at √ s N N = 200 GeV, as well as for Au+Au collisions at √ s N N = 62 GeV. We observe a suppression in the yield of high p T hadrons measured in central Au+Au collisions, relative to the yield in p+p collisions scaled by the number of binary nucleon-nucleon collisions. This observation, together with the absence of such a suppression in d+Au collisions, leads to the conclusion that a dense medium is formed in central Au+Au collisions. Numerical simulations of quantum chromodynamics (QCD) on a lattice predict a phase transition (or crossover) from hadronic matter to deconfined, chirally symmetric matter at sufficiently large energy densities [1]. The critical energy density calculated is typically ∼ 0.7 GeV/fm 3 , approximately 5 times the density of normal nuclear matter. The primary goal of high energy heavy ion physics is to achieve such a transition and to study this new state of matter, the Quark Gluon Plasma (QGP), in order to better understand fundamental properties of QCD. With the new collider RHIC, the field has recently reached a new energy regime, which will be a milestone in the endeavor to attain this goal. The Relativistic Heavy Ion Collider (RHIC) is located at Brookhaven National Laboratory (BNL) on Long Island, New York. RHIC has provided Au+Au collisions at √ s N N = 130 GeV; Au+Au collisions at √ s N N = 62 GeV; and Au+Au, d+Au, and p+p collisions at √ s N N = 200 GeV. At these energies, hard scattering is expected to contribute significantly to particle production. 1.1 Hard Scattering in Heavy Ion Collisions In order to understand the complicated dynamics of a heavy ion collision, one needs a calibrated, understood probe. This can be provided by hard-scattered partons, for which the fragmentation dominates particle production at high p T. In particular, it is known that the fragmentation of partons into jets dominates the production of hadrons above p T ∼ 2 GeV/c [2] in p+p collisions. For the hard-scattered partons to serve as a probe of heavy ion collisions, a baseline for high p T particle production, at the same √ s N N , must first be established from p+p collisions. Figure 1 shows the neutral pion production cross section measured …