Speaker
Description
Jet transport coefficients computed in Hard-Thermal-Loop (HTL) effective theory are applicable at high temperature, while the medium created in heavy-ion collisions, at RHIC and LHC, predominantly samples the region near the QCD transition temperature. The dimensionless transport coefficient $\hat{q}/T^3$, calculated in leading order HTL theory, shows a monotonic rise with decreasing temperature, in spite of all scaled thermodynamic quantities ($s/T^3, \varepsilon/T^4$) dropping with temperature, and is thus invalid at temperatures near the transition.
We introduce a simple temperature-dependent modification of the in-medium parton distribution through a multiplicative $(1 + a/T)$ correction in the dispersion relation, which leads to a transport coefficient $\hat{q}/T^3$ exhibiting a plateau near the transition and a suppression at lower temperatures, consistent with lattice QCD calculations. At high-temperatures, the distribution asymptotically approaches the HTL limit. At lower temperatures, the correction functions as a fugacity, gradually reducing parton populations from the Boltzmann limit. Implemented within the multi-stage MATTER+LBT generators of the JETSCAPE framework, this correction allows the energy loss simulations to be extended beyond the transition temperature, and into the hadronic phase. In this way, we include partonic energy loss in the hadronic phase, where hard partons from the jet, scatter off partons within the excited hadrons of the hadronic phase. The form of the correction, automatically diminishes $\hat{q}/T^3$ at lower temperatures.
The modified distribution increases the contribution from late-time, low-temperature interactions, allowing partonic energy loss to be extended deep into the hadronic phase. When combined with initial state shadowing effects, this framework provides the first simultaneous description of the nuclear modification factor and elliptic anisotropy of jets and leading hadrons over a wide range of centralities from top RHIC to LHC energies, highlighting the role of low-temperature dynamics in hard-probe observables.
