In the paper we discuss the role of the axial U(1)A symmetry in the chiral phase transition using the U(Nf)R×U(Nf)L linear sigma model with two massless quark flavors.It is expected that above a certain temperature the axial U(1)A symmetry will be restored.A string-like static solution,the η string can be formed and detected in the ultrarelativistic heavy-ion collision process.
We calculate the production of large transverse momentum dileptons and photons by using direct and resolved photoproduction processes in relativistic heavy ion collisions. Considering the central collisions of heavy nuclei at Relativistic Heavy Ion Collider (RHIC) and Large Hadron Collider (LHC) energies, we find that the photoproduction processes modify the dilepton and photon production in the large transverse momentum region.
The cold component of large transverse momentum dilepton production via semi-coherent two-photon interaction is calculated. The cold contribution is essential to the dilepton spectra in the soft region for dilerent mass bins. The results are compared with the PHENIX experimental data at RHIC, and we find that the modification of semi-coherent two-photon processes is more evident with the rising dilepton mass bins.
We consider the production sources of prompt and thermal photons which include the contribution of gluons in relativistic heavy ion collisions.Considering in our calculation the shadowing and iso-spin effects of the nucleus we can properly estimate the prompt photon production.We develop a new thermal jet-photon conversion mechanism which plays a vital role in the low transverse momentum region.The effect of the non-equilibrium quark-gluon plasma enhances the contribution of thermal photons.
