he medium temperature dependence of jet transport coefficient in high-energy nucleus-nucleus collisions

he medium temperature $T$ dependence of jet transport coefficient $ hat q$ is studied via nuclear modification factor $R_{AA}(p_{ rm T})$ and elliptical flow parameter $v_2(p_{ rm T})$ for large transverse momentum $p_{ rm T}$ hadrons in high-energy nucleus-nucleus collisions. Within a next-to-leading order perturbative QCD parton model for hard scatterings with modified fragmentation functions due to jet quenching controlled by $ hat q$, we check the suppression as well as the azimuthal anisotropy for large $p_{ rm T}$ hadrons, and extract $ hat q$ by global fits to $R_{AA}(p_{ rm T})$ and $v_2(p_{ rm T})$ data in $A+A$ collisions at RHIC and the LHC, respectively. Numerical results from the best fits show that $ hat q/T^3$ goes down with the local medium temperature $T$ in the parton jet trajectory. Compared with the case of a constant $ hat{q}/T^3$, the going-down $T$ dependence of $ hat{q}/T^3$ makes a hard parton jet to lose more energy near $T_c$ and therefore strengthens the azimuthal anisotropy for large $p_{ rm T}$ hadrons. As a result, $v_2(p_{ rm T})$ for large $p_{ rm T}$ hadrons is enhanced by about 10 % to fit data better at RHIC/LHC. Considering the first-order phase transition from QGP to the hadron phase and additional energy loss in the hadron phase, $v_2(p_{ rm T})$ is again enhanced by 5-10 % at RHIC/LHC.