Jet Productions At Next-to-leading Order In High-energy Heavy Ion Collisions

Quantum Chromodynamics (QCD), a fundamental theory describing the strong force inside nucleus in terms of interactions of quarks and gluons, has two important proper-ties:asymptotic freedom and quark confinement. Asymptotic freedom tells that at large momentum transfer the strong coupling constant of quark and gluon interactions becomes weaker, which makes perturbative expansion according to the strong coupling constant applicably. Quark confinement suggests that there are no free quarks or gluons in nature and usually quarks and gluons are bounded inside of hadrons. However, scientists expected that there would be huge energy deposited with great energy density in the center of the extremely high-energy heavy ion collisions and the confined quarks and gluons may be liberated to form a new kind of matter, the Quark-Gluon-Plasma(QGP). The QGP only exists for a very short time in the heavy ion collisions. How to estimate the formation of the QGP and investigate its properties with the information of the final hadrons has been a very challenging and an extremely important research frontier in high-energy nu-clear physics. Since the run of the Relativistic Heavy Ion Collider (RHIC) of Brookhaven National Laboratory(BNL) in USA in2000, they have carried out a large number of ex-periments, especially the Au+Au collisions at center-of-mass energy1/2s=200GeV, and obtained many results manifesting they have found the QGP. The operation of heavy-ion programs at the Large Hadron Collider(LHC) of CERN has provided another excellent platform for us to explore the properties and intrinsic physics of the QGP.Energetic parton produced in the early time of heavy ion collisions, is scattered by particles in the QGP and radiates gluons in the propagation. That causes energy loss of parton jets, which is called as the jet quenching. Jet quenching is an optimal probe to study the quark-gluon-plasma in heavy ion collisions and will leave its fingerprint not only in leading particles production but also reconstructed jet transverse energy distribution. Reconstructed jet, the collimated showers of energetic particles in the final state, can reflect the intrinsic physics of parton interactions more accurately, test the predictions of QCD and new physics beyond Standard Model better. Jet observables as new tools to explore the properties of the QGP have recently attracted intensive attentions from both experimentalists and theorists.This thesis investigates the inclusive jet and dijet productions at next-to-leading or- der(NLO) in heavy ion collisions within the framework of perturbative QCD. We start from the jet productions at NLO in hadron-hadron collisions as the inputs, and then in-corporate with nuclear matter effects to study jet productions in heavy ion collisions. Our investigations are involving not only the initial-state cold nuclear matter(CNM) effects, but also the final-state hot nuclear matter effects.The nuclear modifications for inclusive jet transverse momentum spectra, dijet an-gular distributions, dijet invariant mass spectra, dijet transverse momentum spectra and dijet momentum imbalance due to CNM effects are calculated by employing different parametrization sets of nuclear parton distribution functions(nPDFs). It is found that di-jet angular distributions and dijet momentum imbalance are insensitive to the initial-state CNM effects and thus provide optimal tools to study the final-state hot QGP effects. On the other hand, inclusive jet transverse momentum spectra, dijet invariant mass spectra and dijet transverse momentum spectra are generally enhanced in a wide region due to CNM effects with a feature opposite to the expected suppression because of the final-state parton energy loss effect in the QGP. The deviation between different parametrization sets of nPDFs is appreciable for these three observables.The hot and dense nuclear effects are evolved with the GLV theory mechanism for jet quenching. When jet is propagating in the QGP medium, it radiates gluons induced by medium. However, parts of the radiative gluons carrying energy would fall in the jet cone. The inelastic interactions between jet and medium make the final jet yields shift towards smaller energy regime, which leads some interesting phenomena about the medium-modified jet productions. We focus on the suppression for the inclusive jet and dijet cross sections. Our analysis includes not only inelastic parton interactions, but also an estimate of the non-perturbative hadronization corrections. We demonstrate how an enhanced di-jet asymmetry in central Pb+Pb reactions at the LHC, recently measured by the ATLAS and CMS collaborations, can be derived from these results. We show quantitatively that a fraction of this enhancement may be related to the ambiguity in the separation between the jet and the soft background medium and/or the diffusion of the parton shower energy away from the jet axis through collisional processes.