Phenomenological Study In Heavy-Ion Collisions With Jet Quenching

Relativistic heavy-ion collision is a significant experimental tool to study physics at high energy and high density environment and then to explore quark-gluon plasma(QGP). Enough high temperature environment is expected to create in the collision and then realize the phase transition from hadron matter to QGP matter. In the beginning of this century, the relativistic heavy-ion collider (RHIC) was built and run in Brookheaven National Laboratory in U.S. with colliding energy up to 200GeV/n. With the running of experiment, a large amount of interesting data have been accumulated. Some most striking evidences of the formation of QGP are:1) The anisotropic collective behavior of particles has been observed at low pT in non-central collisions; 2) The degree of freedom for constituent quark in hadronization at medium pT;3) Jet quenching at high pT. Combining all the experimental results, people believe that parton (quark and gluon) degree of freedom in a volume about thousands times that of hadrons has been observed, a strongly coupled quark gluon plasma(sQGP) has been formed.Physics at high transverse momentum is becoming more and more important in RHIC and LHC((?)= 2.76TeV) experiment with higher energy. Among the experimental evidences for the formation of sQGP are the jet quenching phenomena that include the strong suppression of hadron spectra with large transverse momentum in central Au+Au collisions as compared to p+p collisions and the disappearance of back-to-back correlations of large transverse momentum hadrons. In this paper, we will study some physical issues with next-to-leading order (NLO) perturbative QCD (PQCD) parton model at high pT in heavy-ion collisions. The NLO PQCD parton model has been very successful in describing the high pT region physics in p+p collisions. Later by including the nuclear effects it is extended to consider the single hadron and di-hadron productions in Au+Au collisions and has given a very good description on the high pT pion and charged hadron productions at RHIC.The ratios of p/πat large transverse momentum in central Au+Au collisions at RHIC are studied in the framework of jet quenching based on a NLO pQCD parton model. It is shown that theoretical calculations with a gluon energy loss larger than the quark energy loss will naturally lead to a smaller p/πratios at large transverse momentum in Au+Au collisions than those in p+p collisions at the same energy. Scenarios with equal energy losses for gluons and quarks and a strong jet conversion are both explored and it is demonstrated in both scenarios p/πratios at high pT in central Au+Au collisions are enhanced and the calculated ratios of protons over pions approach to the experimental measurements. However, p/p in the latter scenario is found to fit data better than that in the former scenario.Within the picture of jet quenching induced by multiple parton scattering and gluon bremsstrahlung, medium modification of parton fragmentation functions and therefore the suppression of large transverse momentum hadron spectra are controlled by both the value and the space-time pro-file of the jet transport parameter along the jet propagation path. Experimental data on single hadron suppression in high-energy heavy-ion collisions at the RHIC energy are analyzed within the higher-twist (HT) approach to the medium modified fragmentation functions and the NLO PQCD parton model. Assuming that the jet transport parameter q is proportional to the particle number density in both QGP and hadronic phase, experimental data on jet quenching in deeply inelastic scattering (DIS) off nuclear targets can provide guidance on qh in the hot hadronic matter. One can then study the dependence of extracted initial value of jet quenching parameter q0 at initial timeτ0 on the bulk medium evolution. Effects of transverse expansion, radial flow, phase transition and non-equilibrium evolution are examined. The extracted values are found to vary from q0τ0= 0.54 GeV2 in the 1+3D ideal hydrodynamic model to 0.96 GeV2 in a cascade model, with the main differences coming from the initial non-equilibrium evolution and the later hadronic evolution. The overall contribution to jet quenching from the hadronic phase, about 22-44%, is found to be significant. Therefore, realistic description of the early no-equilibrium parton evolution and later hadronic interaction will be critical for accurate extraction of the jet transport parameter in the strongly interacting QGP phase in high-energy heavy-ion collisions.In the last, nuclear modification factor RAA(pT) for large transverse momentum pion spectra in Pb+Pb collisions at (?)= 2.76 TeV is predicted within the NLO PQCD parton model. Data on charged hadron multiplicity dNch/dη=1584±80 in central Pb+Pb collisions from the ALICE Experiment at the LHC are used to constrain the initial parton density both for determining the jet transport parameter and the 3+1D ideal hydrodynamic evolution of the bulk matter that is employed for the calculation of RpbPb(pT) for neutral pions.