Early Equilibration And Dissipative Property In Au+Au Collisions At RHIC Energy

The main goal of relativistic heavy-ion collisions at RHIC energy is to study the properties of quark–gluon plasma(QGP). RHIC experiments have demonstrated that the deconfined QCD phase matter is created in relativistic heavy-ion collisions, via the observation to the quark number scaling of elliptic flow and the Jet-quenching phenomenon.The success of the ideal hydrodynamic model at RHIC suggests the strongly coupled QGP which behaves as an almost perfect liquid with a very small ratio of viscosity to entropy density. The early stage dynamics can be investigated through the Ad S/CFT correspondence, which indicates the system starts out being weakly-coupled after a time interval of the order of tstop≈ 1fm/c at RHIC. The result of the Ad S/CFT correspondence is inconsistent with the ideal hydrodynamic model, thus it is necessary to study the mechanism of system equilibration at the early stages. The ideal hydrodynamic model can not give a good description of the ratio of HBT radii Ro/Rsand the elliptic flow in the medium transverse momentum range which suggests the importance of non-equilibration dissipative property to the system evolution. The AMPT model is a hybrid model based on non-equilibration transport dynamics and contains partonic transport process and nonequilibration dissipative property. Using the AMPT model, we study the equilibration process of the early collision stages and non-equilibration dissipative property.The effect of free streaming and sudden equilibration(FS+SE) process at early collision stages on the observation of final state are studied. The results of the transverse momentum spectra, elliptic flow and HBT radii from the AMPT model with the FS+SE approximation are close to the experimental data, which indicate a delay of the equilibration time from τ = 0.6fm/c to τ = 1fm/c and support the Ads/CFT results of a weaklycoupled time interval. FS+SE approximation lead to a slow drop of the eccentricity with increasing time and the slow decrease can be reflected on the freeze-out eccentricity.The effect of resonance decays on the structure of particle emission source and the HBT radii are studied. The pions from the resonance decays lead to a core–halo source.The HBT parameters of the core–halo source can be extracted by the double-Gaussian parametrization with a two-scale approach. The spherical harmonic analysis result indicates that the double-Gaussian fit is more appropriate for the pion emission source, and the core–halo structure is closer to the source distribution that obtained long-lived resonance decays in the AMPT model. The evolution time as a function of the partonic cross section σpfor Au+Au and p+p collisions at√sN N= 200 Ge V shows that the drop of the source evolution time with increasing σpis due to the lifetime of the core.The effect of shear viscosity to entropy density ratio η/s on HBT radii are studied.Based on the kinetic theory and assumption of baryon-free, the temperature dependence of η/s can be estimated by the QCD coupling constant and the screening mass. Neglect the spatial location influence on the medium effect, a approach of constant shear viscosity to entropy density ratio can be simulated in the AMPT model with the partonic cross section depended on the energy density in the central cell. The results of the HBT radii with energy density cut cut= 0.89 Ge V/fm~3 at partonic stage are close to the STAR data and qualitative similar with the dissipative hydrodynamics. The shear viscosity accelerates the transverse expansion, which increases the radial flow, and suppresses the longitudinal expansion. η/s gives smaller values of Ro, Rland Ro/Rs, but increases value of Rs,which indicates that Roand Rlare sensitive to the transverse and longitudinal expansion,respectively, and Rsis most related to the source size.Effect of the non-equilibration dissipative property on the system eccentricity and observation of final state are studied. Using the AMPT model, we investigate the effect of shear viscosity to entropy density ratio on the transverse momentum spectra, elliptic flow, HBT radii and freeze-out eccentricity with the partonic cross section depended on the energy density distribution which contains the spatial location influence on the medium effect. η/s accelerates the transverse expansion, which increases the radial flow, and raise the transverse momentum spectra. Elliptic flow and freeze-out eccentricity are sensitive to the shear viscosity. The larger η/s leads to a larger suppression of elliptic flow and an earlier saturation of differential elliptic flow. The shear viscosity accelerates the drop of spatial eccentricity, but the decreased spatial eccentricity can not transition to the momentum space eccentricity. η/s decrease the freeze-out eccentricity and elliptic flow.