Phase Transition And Thermodynamics In Heavy-ion Collisions At High Energies

Studying quantum chromodynamics (QCD) phase transition and properties of hot quark matter at high temperature has been the main target of heavy ion collision experiments at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC), which has just started running. By fitting the elliptic flow at. RHIC, a very small shear viscosity is obtained. It was expected that deconfined quark matter formed at high temperature should behave like a gas of weakly interacting quark-gluon plasma(wQGP). It is now believed that the system created at RHIC is a strongly coupled quark-gluon plasma (sQGP) and behaves like a nearly "perfect" fluid. We have investigated the thermodynamic properties including the equation of state, the trace anomaly, the sound velocity, and the specific heat, as well as transport properties like bulk viscosity, Meanwhile, the transverse momentum distributions and elliptic flows of final-state particles produced in high-energy collisions have been investigated, and the system evolution in the collisions has been discussed.In the Cornwall-Jackiw-Tomboulis formalism, we investigate the phase transition and entropy in a scalar O(N) model with a spontaneously broken symmetry. In the strongly and weakly coupling cases, the sunset diagrams lead to nonzero decay widths. It shows that the sunset diagrams contribution may be neglected for the values of the critical temperature and the entropy.Based on the above results of calculating the effective potential, in the Z(2) and O(4) models in the Cornwall-Jackiw-Tomboulis formalism, we have studied the thermodynamic properties and the transport properties in different cases, e.g. the first-order phase transition, the second-order phase transition, the crossover and the case without phase transition, and discussed the correlation between the bulk viscosity and the thermodynamic properties of the system. We have found that at phase transition, the system either in weak coupling or strong coupling shows some common properties. The bulk viscosity to entropy density ratio shows upward cusp behavior at Tc. The cusp behavior in the first-order phase transition is sharper and narrower than that in the second-order phase transition. In the case of crossover, the cusp behavior is washed out. Because the behavior of the bulk viscosity over entropy density ratio is so different in the cases of the first-order phase transition and the crossover, our results support the idea that the bulk viscosity over entropy density ratio is a better quantity than the shear viscosity over entropy density ratio to locate the critical endpoint (CEP).We have studied the dependence of elliptic flows on the transverse momentum for the final-state hadrons produced in Cu-Cu collisions at high energy by using a multi-source ideal gas model which includes the interaction contribution of the emission sources. It is found that the model can describe the elliptic flow and the magnitude of the expansion and anisotropy are indicated fully by fitting the experimental data of the elliptic flow. Furthermore, the transverse momentum distributions of strange particles produced in high-energy collisions have been analyzed. It is found that the calculated results are in good agreement with the experimental data. The excitation degree of emission sources on the central axis of the cylinder increases obviously with increasing the collision centrality and incident energy. The thermodynamic properties can be obtained from these statistical properties.