Electromagnetic Probes From Relativistic Heavy Ion Collisions

Since the last century, relativistic heavy ion collisions have been used to study the properties of QCD matter under the condition of high density and high temperature in laboratory. Recent years, large number of experimental results at RHIC and LHC show that a kind of high density and high temperature matter, strong coupled Quark Gluon Plasma （sQGP）, was created in the high energy heavy ion collisions. The study of the properties of sQGP is an important subject, not only because of high energy nuclear physics itself, but also because of the understanding of the early universe. The study of the properties of sQGP mainly depends on the experimental measurement of hadrons, such as transverse momentum spectrum, anisotropic flows etc. However, because of the influence of secondary collisions in the late stage during the system evolution, the information carried by those hadrons is mixed with the hadron gas phase. Electromagnetic probes, like photons and dileptons, could be produced in QGP. They do not participant the subsequent strong interaction but penetrate the whole system, offering us the cleanest and the most direct information of sQGP.There are several sources of photons in heavy ion collisions. In this thesis, we mainly focus on the direct photons （not from decay）. They are mainly prompt photons and thermal photons. At parton level, they are produced via Compton scattering and quark-antiquark annihilation, or high order processes. The colliding partons come from the initial nucleus for the former and from the thermalized QGP for the latter. Thermal photons can also be emitted from hadronic gas phase, due to the collisions between the thermalized hadrons.The elliptic flows of direct photons measured by experiments are very large, which is a puzzle now in heavy ion physics. The traditional methods make no distinction between the thermal equilibrium time and QGP formation time, so the calculated results are much lower than experimental data. Based on a delayed QGP formation time, we explained not only the measured transverse momentum spectra but also the elliptic flow of direct photons, with hadron data constrained hydrodynamics. The experimental data, especially the elliptic flow of direct photons, are used to extract the QGP formation time, which is about 2 fm/c. Then the high order harmonic coefficients vn （n= 3,4,5） of direct photons were predicated.Similar to direct photons, dilepton is also a kind of penetrating probe. To check the delayed QGP formation mechanism, we constructed the calcution framework of dileptons, and fulfilled the preliminary calcution of dileptons from 200GeV Au+Au collisions using EPOS hydrodynamic model.Dileptons are produced in all stages of the heavy ion collisions, withifferent kinematic characteristics. Dileptons at the low invariant mss region are dominated by the vector mesons decay, where the medium modifications of these vector mesons spectral functions were considered to be the signal of the chiral symmetry restoration. In the intermediate invariant mass region, it’s often said that the emission of QGP is the main contribution. However, heavy flavor quarks decay, such as cc â†'l+l-X, is also an important source. In high invariant mass region, dileptons are mainly from the initial hard scattering Drell-Yan pair and the heavy quarkonia contribution.In this thesis, we studied the dielectrons invariant mass spectra from Au+Au collisions at (S_NN)^1/2= 200GeV:for QGP phase, we compared the perterbative QCD and thermal Lattice QCD methods, and found that they only differ in low invariant mass region; for Hadron Gas phase, we usd the forward scattering amplitude method to calculate the emission rate. After doing the space-time integral with （3+1） dimensional EPOS hydrodynamic, we got the thermal contribution of the dielectrons invariant mass spectra. Then with STAR Cocktail data （including Ï€0,η,η’,ω,ψ,J/φ,cc,bb decay or Dalitz decay and Drell-Yan contribution） for the Cocktail contribution, we successfully explained the measured dielectrons invariant mass spectra at different collision centralities. At last, we also showed the anisotropic flows vn（n= 2,3,4,5） of thermal dielectrons from 200GeV Au+Au collisions. We studied the characteristics of high order flows and their dependences on the collision centralities and order n.The framework of dilepton study is just made for thermal emission. Further work is on-going to constrct the cocktail description.