Particle production in relativistic heavy-ion collisions with perturbative QCD

The commissioning of the Relativistic Heavy Ion Collider (RHIC) opened new era in nuclear collision physics, with the study of excited strongly-interacting matter becoming a reality. A primary motivation for studying high-p _T hadron production in ultrarelativistic heavy ion collisions is to gain insight into the gluon density of the quark-gluon medium via jet energy loss. The sensitivity of high-p_T hadron spectra to initial gluon density may be a probe of the formation of quark-gluon-plasma (QGP). However, a thorough understanding of ultrarelativistic nuclear (AA ) collisions requires the accurate description of proton-proton ( pp) and proton-nucleus (pA) collisions in the same framework. In the present dissertation we follow the evolution of high-p _T hadron production in relativistic collisions from pp to pA to AA reactions.; The perturbative Quantum Chromodynamics (pQCD) improved parton model is used for the study. We apply leading-order (LO) pQCD throughout, and augment the standard one-dimensional cross section calculation by the intrinsic transverse momentum distribution of partons. We use abundant pion production data from pp collisions at c.m. energies $s\lesssim$ 60 GeV to extract the width of the transverse momentum distribution of partons in the nucleon. This gives a satisfactory fit of pion and kaon production data in pp collisions in the 2 ≤ p_T ≤ 6 GeV window.; For the treatment of nuclear systems, we developed a model based on the enhancement of the width of the transverse momentum distribution of partons in the nuclear medium. An additional parameter is fitted to describe the Cronin effect (cross section enhancement in pA collisions relative to pp collisions) at these energies. Shadowing and the isospin asymmetry of heavy nuclei are taken into account. We tested the model on charged pion and kaon production. In AA collisions at SPS energies we find an indication of a need for a mechanism to decrease the calculated cross section of neutral pion production. We speculate that jet energy loss in dense nuclear medium (“jet quenching”) may provide that mechanism, beginning to appear already at SPS energies.; Using proton-antiproton (pp¯) data at higher energies, the domain of the extracted transverse momentum width was extended to cover RHIC energies. Keeping in mind the rather large uncertainties, predictions are made for pA and AA collisions at RHIC. The over prediction of the preliminary PHENIX data on central π 0 production at $s$ = 130 GeV indicates that additional physics needs to be incorporated in the model. Since the disagreement with the central data is much stronger at these energies than at SPS, and jet quenching is expected to increase with energy, we suggest jet quenching as a mechanism to bring the calculated cross sections in agreement with the data. The jet quenching implied by our work is used by others to point to very high initial gluon densities, well in the quark-gluon plasma domain.