Study On Direct Photon Production In Relativistic Heavy Ion Collisions And Quark Virtuality In QCD Vacuum

In 1970s, T.D. Lee and G.C.Wick predicted that a new state of matter—Quark Gluon Plasma(QGP) could be formed in relativistic heavy ion collisions. Since then, great progress has been made in the experimental and theoretical fields of heavy ion collisions. In 1980s, several experiments performed at AGS in the Brookheaven National Laboratory (BNL) in U.S. and at SPS in CERN in Europe. In the new century, the relativistic heavy ion collider (RHIC) in BNL with energy about 200 GeV has started taking data and obtained a lot of new results. Combining all of the experimental results, people believe that a strongly coupled QGP has been formed in heavy ion collisions. A giant accelerator known as Large Hadron Collider (LHC) at CERN has been in operation and will provide many new insights about the properties of QGP and the theory of strong interactions.The QGP is a state of matter consisting of deconfined quarks and gluons. It is hypothesized to exist at high baryon densities and high temperature. These circumstances are available for experiments in heavy ion collisions, and even there the presence of the QGP cannot be measured directly. So far many signals have been proposed to probe the formation and the properties of QGP, such as:heavy quark enhancement, J/ψsuppression, collective flow, and so on. Among them, high pΤphoton production is a good signal to study the properties of QGP. Unlike hadrons, photons are gauge bosons in electromagnetic interaction in nature, (αe=(?)), they interact weakly with hadrons and their mean free path is large. As a result, once produced, the photons do not suffer further interaction with the medium and carry the information about the circumstances of their production to the detector.In nucleon-nucleon collisions, photons can be categorized into the soft photons which come from soft processes, such as:decay photons, and the hard photons which come from hard pro-cesses, such as:direct photons and fragmentation photons. In nucleus-nucleus collisions, due to the effect of the medium, photons can be categorized into thermal photons which emitted from a thermalized plasma, decay photons which decay from the hadrons such as pions and etas, and hard photons which come from hard processes. Usually hard photons also come from several sources in nucleus-nucleus collisions:direct photons from the initial hard scattering, such as:Compton scattering or annihilation process, the fragmentation photons produced by jet fragmentation, and photons converted from a fast jet, which happens when energetic partons traveling through the hot medium and interacting with the hot medium, this process can also produce hard photons. We are mainly interested in the hard photons in this thesis. In p+p collisions, the photon spectrum chiefly consists of direct photons and fragmentation photons. In A+A collisions, due to the effect of the medium, besides the fragmentation photons, we have to consider other photon sources:such as jet-photon conversion. So that the photon spectrum is complicated by the presence of a number of other photon sources during A+A collisions.In this thesis, we study the influence of the final state energy loss in the production of the direct photons and estimate the transverse momentum dependence of the nuclear modification factor RAA-We use two different models for the bulk matter evolution in central Au+Au collisions at the RHIC energy:(1+1)d Bjorken model and (3+1) d ideal hydrodynamical model by Hirano to investigate the parton energy loss and discuss direct photon production coming from different sources. Our results show that at very low pΤ, thermal photon production is important in the photon production, and the thermal photon spectrum decrease very fast with pΤincrease; at high pΤregion, the direct photon production dominates photon production. The calculating results also show that the influence of energy loss is important for investigation the direct photon production at RHIC with (?)= 200 GeV, and jet-photon conversion plays an important role to explain experiment data in mid pr region besides the contribution from the direct photons and fragmentation photons.Nuclear shadowing effects are investigated in productions of direct photons and neutral pions in heavy ion collisions at RHIC/LHC energy within a next-to-leading order perturbative QCD parton model. The transverse momentum dependence of the nuclear modification factors is presented in p+A and A+A collisions to distinguish different parameterizations for nuclear shadowing effects. Our numerical results show that in central A+A collisions the nuclear modification factor for direct photons is more sensitive to shadowing effects than the factor for pion hadrons, and it is a powerful observable to discriminate different parameterizations for parton distributions.The nonlocal vacuum condensates of QCD describe the distributions of quarks and gluons in the nonperturbative QCD vacuum. Physically, this means that vacuum quarks and gluons have nonzero mean squared momentum, called virtuality. In this thesis we study the quark virtuality which is given by the ratio of the local quark-gluon mixed vacuum condensate to the quark local vacuum condensate. The two vacuum condensates are obtained by solving Dyson-Schwinger equations of a fully dressed quark propagator with an effective gluon propagator. Using our calculated condensates, we obtain the virtuality of quarks in the QCD vacuum state. Our numerical predictions are consistent with other theoretical model calculations such as QCD sum rules, lattice QCD and instanton models.