Modified Fragmentation Function and Jet Quenching at RHIC

Medium modification of jet fragmentation functions and parton energy loss in cold and hot matter are reviewed. The predicted nuclear modification of the jet fragmentation function agrees well with the recent HERMES data with a resultant energy loss dE/dx approximately 0.5 GeV/fm. From the the recent PHENIX data of high p_T pi_0 spectra in central Au+Au collisions at sqrt(s)=130 GeV, one extracts an energy loss for a 10 GeV parton that is equivalent to dE/dx=7.3 GeV/fm in a static medium with the same gluon density as in the initial stage of the collision at tau_0=0.2 fm/c. Constraints on jet quenching by the central rapidity density of charged hadrons is also discussed.Comment: Invited talk at International Symposium on Statistical QCD, Bielefeld, Germany, 26-30 Aug 2001; 11 pages with 6 postscript figure

Jet Tomography of Quark Gluon Plasma

Recent experimental measurements of high $p_T$ hadron spectra and jet correlation at RHIC are analyzed within a parton model which incoporates initial jet production and final propagation in heavy-ion collisions. The suppresion of single hadron spectra, back-to-back correlation, their centrality dependence and azimuthal anisotropy point to a dense matter with an initial parton density about 30 times of that in a cold heavy nucleus.Comment: 7 pages in RevTex, 4 figures. Invited talk at Confinement 2003-International Symposium on ``Color Confinement and Hadrons in Quantum Chromodynamics", RIKEN, Japan, July 21-24, 2003 and XXIV Brazilian Meeting of Particle and Field Physics, Caxambu, Brazil, Oct. 1-3, 200

Hard Probes in High-energy Heavy-ion Collisions

Hard QCD processes in ultrarelativistic heavy-ion collisions become increasingly relevant and they can be used as probes of the dense matter formed during the violent scatterings. We will discuss how one can use these hard probes to study the properties of the dense matter and the associated phenomenologies. In particular, we study the effect of jet quenching due to medium-induced energy loss on inclusive particle $p_T$ distributions and investigate how one can improve the measurement of parton energy loss in direct photon eventsComment: Talk given at the International School on Physics of the Quark Gluon Plasma, Hiroshima, June 3-6, 1997 and the 5th International Workshop on Relativistic Aspects of Nuclear Physics, Rio de Janeiro, August 27-29. 17 pages with 9 ps figure

Monte Carlo Models: Quo Vadimus?

Coherence, multiple scattering and the interplay between soft and hard processes are discussed. These physics phenomena are essential for understanding the nuclear dependences of rapidity density and $p_T$ spectra in high-energy heavy-ion collisions. The RHIC data have shown the onset of hard processes and indications of high $p_T$ spectra suppression due to parton energy loss. Within the pQCD parton model, the combination of azimuthal anisotropy ($v_2$) and hadron spectra suppression at large $p_T$ can help one to determine the initial gluon density in heavy-ion collisions at RHIC.Comment: 10 Pages with 8 figures, Plenary talk at 15th International Conference on Ultrarelativistic Nucleus-Nucleus Collisions (QM2001), Stony Brook, New York, 15-20 Jan 200

Particle Production in High-energy Heavy-ion Collisions

Particle production mechanisms in high-energy heavy-ion collisions are reviewed in connection with recent experimental data from RHIC. Implications on mini-jet production, parton saturation and jet quenching are discussed.Comment: 14 pages, 5 figures in eps, talk given at XXXI International Symposium on Multiparticle Dynamics, Sept 1-7, 2001, Datong China. URL http://ismd31.ccnu.edu.cn

Rapidity Asymmetry in High-energy d+Ad+A Collisions

In contrast to the recent prediction of high $p_T$ hadron suppression within the parton saturation model, it is shown that multiple parton scattering suffered by the projectile will enhance high $p_T$ hadron spectra in $d+A$ collisions relative to a superposition of binary $p+p$ collisions at RHIC. A stronger enhancement in the forward rapidity region of the projectile is also predicted, resulting in a unique rapidity asymmetry of the hadron spectra at high $p_T$. The shape of the rapidity asymmetry should be reversed for low $p_T$ hadrons that are dominated by soft and coherent interactions which suppress hadron spectra in the projectile rapidity region. The phenomenon at the LHC energies is shown to be qualitatively different because of parton shadowing.Comment: 13 pages in RevTex, 5 figures. The numerical data files of the figures can be downloaded from http://www-hpc.lbl.gov/datacode_files/hadrons.ht