Energy loss of partons traversing a QGP fluid

Measurements of high pT hadrons at the Relativistic Heavy Ion Collider (RHIC) may provide insight into a quark gluon plasma (QGP) [1]. Since jets have to traverse the excited matter, their spectra should be changed in comparison with the elementary hadron-hadron data. The energy loss of jets has been proposed as a possible signal of the QGP phase in relativistic heavy ion collisions [2]. Over the past year, a lot of work have been devoted to study the propagation of jets through QCD matter [3]. Recently, hadronic transverse momentum distributions in Au + Au collisions at √ sNN = 130 and 200 GeV have been measured at RHIC and found that the yield of hadrons is suppressed at high transverse momentum [4,5]. These data are consistent with the picture in which jets considerably lose their energy in hot/dense medium. In addition, the magnitude of the away-side jets is found to be significantly suppressed in comparison with the near-side jets in central collisions from the azimuthal correlation measurements at high pT [6]. The disappearance of the away-side peaks in central collisions implies that the one jet can escape from medium and the other correlated jet is absorbed by medium [7]. On the other hand, a novel hydrodynamical calculation, in which the early chemical freeze-out is taken into account, show that the pT slope for pions is insensitive to thermal freeze-out temperature T th [8] and that the hydrodynamical result starts to deviate from the experimental data at pT = 1.5-2.0 GeV/c. See Fig. 1. This result naturally leads us to include the mini-jets component in addition to the hydrodynamic component. Thus, we developed the hydro+jet model in which a full three dimensional space-time evolution of a fluid is combined with explicit propagation of jets [9]. The hydro+jet model enables us to estimate the dynamical effect of hot matter on the parton energy loss in relativistic heavy ion collisions. By using this model, we study the azimuthal correlation functions at the RHIC energy.