The QGP Phase Transition And Production Of Photons And Dileptons In Relativistic Heavy Ion Collisions

In this work, we discuss the production of large transverse momentum dilep-tons and photons produced by direct and resolved photoproduction processes in pp (proton-proton), dA (deuteron-nucleus) and AA (nucleus-nucleus) collisions at RHIC (Relativistic Heavy Ion Collider) and LHC (Large Hadron Collider) en-ergies. Photoproduction processes play a fundamental role in the ep (electron-proton) deep inelastic scattering at HERA (Hadron-Electron Ring Accelerator). In the ep deep inelastic scattering, a high energy photon emitted from the inci-dent electron directly interacts with the proton by the interaction ofγp→jets (direct photoproduction processes). The Heisenberg's uncertainty principle allows the high energy photon for a short time to fluctuate into a quark-antiquark pair, then the quark or antiquark interacts with the parton of the proton. In such in-teractions the resolved photon can be regarded as an extended object consisting of quarks and also gluons. The interactions are the so-called resolved photopro-duction processes.Charged partons of the incident nucleon also can emit high energy photons (and resolved photons) in relativistic heavy ion collisions. In direct photopro-duction processes, the high energy photon emitted from the charged parton of the incident nucleon interacts with the parton of another incident nucleon by the QED (quantum electrodynamics) Compton scattering. In resolved photoproduc-tion processes, the hadron-like photon interacts with the parton of the nucleon by the Compton scattering and annihilation. Numerical results show that the modi- fication of photoproduction processes is not prominent at RHIC energies, but the modification becomes evident in the large PT (transverse momentum) region for p+p, d+Pb and Pb+Pb collisions at LHC energies. Resolved photoproduction processes are more important than direct photoproduction processes in the small PT region at HERA. Since the energy of nucleons in the center-of-mass system at RHIC ((?)SNN=200 GeV) is smaller than the HERA collision energy ((?)S=300 GeV), the contribution of direct photoproduction processes is more prominent than resolved photoproduction processes at RHIC. The contribution of resolved photoproduction processes becomes evident at LHC energies ((?)SNN=5.5 TeV) by compared with direct photoproduction processes.Because of the abundance of gluons at small x the QCD (quantum chromody-namics) induced gluon-photon contributions (gg→gγ,gg→γγ(orγ*)) are not negligible in nucleon-nucleon collisions, gluon-photon processes may correct the leading-order contribution of the Compton scattering and annihilation. We find that the correction of gluon-photon processes is not negligible in the low PT region at RHIC and LHC energies. In AA collisions the low PT spectrum is dominated by the thermal information, therefore the correction of gluon-photon processes is covered by the thermal contribution. However, the correction is important for pp and dA collisions due to the absence of the QGP (quark-gluon plasma) in such col-lisions. We also investigate the thermal contribution of gluon-photon processes. Since thermal gluons and quarks satisfy the same thermal distribution in the medium, the spectra of thermal photons and dileptons produced by gluon-photon processes are suppressed by the higher-order coupling parameter.Furthermore, the production of photons and dileptons originating from the annihilation and Compton scattering of jets passing through the plasma is also calculated. The contribution of the dilepton yield due to the jet+plasma interac-tion is evident in the intermediate mass region at RHIC. The spectrum of thermal productions and Drell-Yan processes is enhanced by the jet+plasma mechanism. We compare these with PHENIX data. The contribution of the jet-dilepton con-version is remarkable at LHC energies. We find that the jet-dilepton spectra fall off with the invariant mass and transverse momentum faster than the spectra of Drell-Yan dileptons due to the attenuation function. The rate of the jet-photon conversion is Rjet-γ∝fjet(p),the spectra of the jet-photon conversion do not decrease quickly with the transverse momentum.