Hadron Production In High-energy Hadronic And Nuclear Collisions

Quantum Chromodynamics (QCD) predicts a transition in the structure of nuclear matter at high temperature and/or high baryon density, which frees quarks and gluons from their nor-mal confinement in hadrons. This exotic form of nuclear matter, called Quark-Gluon Plasma (QGP), is well established by lattice field theory simulations and is thought to have permeated the entire universe for a few microseconds after the Big Bang. To recreate similar extreme con-ditions in the laboratory, experiments of high-energy heavy-ion collisions are carried out at the Relativistic Heavy-Ion Collider (RHIC) at Brookhaven National Laboratory (BNL) and Large Hadron Collider at CERN. Collective phenomena and jet quenching are two important probes that are used to detect the formation of QGP and study its properties in high-energy heavy-ion collisions. Data from these experiments provided convincing evidences for the formation of a strongly interacting QGP that exhibits very small shear viscosity in its hydrodynamic expansion and is also opaque to high-energy jets.Jet quenching as a probe of QGP is based on the principle that the interaction between hard partons and the medium can lead to parton energy loss and suppression of large transverse momentum (pΥ) hadrons. Through measurement and phenomenological study of medium mod-ification of final large transverse momentum hadron spectra one can extract medium properties such as the jet transport coefficient. On the other hand, the high initial pressure of the formed QGP causes collective expansion of the bulk medium and in noncentral collisions with initial asymmetric geometry leads to the azimuthal anisotropy in the final hadron spectra or anisolrop-ic flow. Comparisons between experimental data and viscous hydrodynamic calculation of the anisotropic flow can provide indirect constraints on the transport properties of the QCD matter such as shear viscosity. One common condition for both of these phenomenological studies is the knowledge of initial particle production that determines the initial energy density distribution and the later space-time evolution of the bulk medium. It is therefore crucial to study particle production in high-energy hadronic and nuclear collisions.HIJING Monte Carlo model is based on a two-component model for hadron production in high-energy p+p, p+A, and A+A collisions. The soft and hard components are separated by a cut-off in the transverse momentum exchange. Hard parton scatterings are assumed to be described by the perturbative QCD, while soft interactions are approximated by string excitation with an effective cross section. Nuclear effects in HIJING include the nuclear shadowing of parton distributions inside cold nuclei through a parameterization and jet quenching in the final state through a simple model of parton energy loss. In this thesis, HIJING is updated with the latest parton distributions and a new set of the parameters in the two-component model that control total p+p cross section and the central pseudorapity density. Cold nuclear effects in high-energy p+A collisions are included with a new set of parameterization of parton shadowing and implementation of pΤ broadening through multiple parton scatterings. Within the updated HIJTNG model, we study hadron spectra and multiplicity distributions and compare to recent experimental data from p+p collisions at the LHC energies. Nuclear modification of final hadron spectra in p+A collisions relative to that in p+p collisions are also studied. Parton shadowing andpr broadening are found to influence final hadron spectra at intermediate pΤ.Modification of parton flavor composition due lo multiple scattering and hadronization of many-parton jet systems are shown to lead to suppression of final hadrons at large pΤ in p+A collisions at the Large Hadron Collider. For A+A collisions, the new data on the central rapidity density of charged hadrons in the most central Pb+Pb collisions at LHC are used to carry out a combined fit together with the RHIC data to reduce the uncertainty in the gluon shadowing parameter that controls the overall magnitude of gluon shadowing at small fractional momentum x. Predictions on the centrality dependence of charged hadron multiplicity density at mid-rapidity with reduced uncertainties are given for Pb+Pb collisions at LHC.In addition to hadron production, we also studied transverse momentum correlation in az-imuthal angle of produced hadrons due to mini-jets within the HIJING model in high-energy heavy-ion collisions. Quenching of mini-jets during the thermalization is shown to lead to sig-nificant diffusion (broadening) of the correlation. Evolution of the transverse momentum density fluctuation that gives rise to such correlation in azimuthal angle in the later stage of heavy-ion collisions is further investigated within a linearized diffusion-like equation and is shown to be determined by the shear viscosity of the evolving dense matter. Such a diffusion equation for the transverse momentum fluctuation is solved with initial values given by HIJING and together with the hydrodynamic equation for the bulk medium. The final transverse momentum corre-lation in azimuthal angle is calculated along the freeze-out hyper-surface and is found further diffused for larger values of shear viscosity to entropy density ratio η/s～0.2－0.4.