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

The medium-temperature T dependence of the jet transport coefficient \(\hat{q}\) $\widehat{q}$ was studied via the nuclear modification factor \(R_{AA}(p_\textrm{T})\) $R_{\mathit{AA}}(p_{\text{T}})$ and elliptical flow parameter \(v_2(p_\textrm{T})\) $v_2(p_{\text{T}})$ for large transverse momentum \(p_\textrm{T}\) $p_{\text{T}}$ 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 \(\hat{q}\) $\widehat{q}$ , we check the suppression and azimuthal anisotropy for large \(p_\textrm{T}\) $p_{\text{T}}$ hadrons, and extract \(\hat{q}\) $\widehat{q}$ by global fits to \(R_{AA}(p_\textrm{T})\) $R_{\mathit{AA}}(p_{\text{T}})$ and \(v_2(p_\textrm{T})\) $v_2(p_{\text{T}})$ data in A + A collisions at RHIC and LHC, respectively. The numerical results from the best fits show that \(\hat{q}/T^3\) $\widehat{q}/T^3$ goes down with local medium-temperature T in the parton jet trajectory. Compared with the case of a constant \(\hat{q}/T^3\) $\widehat{q}/T^3$ , the going-down T dependence of \(\hat{q}/T^3\) $\widehat{q}/T^3$ makes a hard parton jet to lose more energy near \(T_\textrm{c}\) $T_{\text{c}}$ and therefore strengthens the azimuthal anisotropy for large \(p_\textrm{T}\) $p_{\text{T}}$ hadrons. As a result, \(v_2(p_\textrm{T})\) $v_2(p_{\text{T}})$ for large \(p_\textrm{T}\) $p_{\text{T}}$ hadrons was enhanced by approximately 10% to better fit the data at RHIC/LHC. Considering the first-order phase transition from QGP to the hadron phase and the additional energy loss in the hadron phase, \(v_2(p_\textrm{T})\) $v_2(p_{\text{T}})$ is again enhanced by 5–10% at RHIC/LHC.