Speaker
Description
We study the phenomenon of transverse momentum broadening for a high-$p_T$ parton propagating through a weakly-coupled quark-gluon plasma undergoing boost-invariant longitudinal expansion. We propose a boost-invariant description for this phenomenon, in which the broadening refers to the angular variables $\eta$ (the pseudo-rapidity) and $\phi$ (the azimuthal angle) --- the same variables as generally used to describe particle distributions in the experiments. The jet quenching parameter $\hat{q}$, which is the only property of the medium to enter this description, depends upon the proper time alone: it decreases with time due the dilution of the medium via expansion.
We furthermore consider radiative corrections to $\hat q$. As in the case of a static medium, we find potentially large corrections enhanced by a double logarithm. But unlike for the static medium, these corrections are now local in time: they depend upon the local (proper) time characterizing the expansion, and not upon the overall path length. We identify and resum such corrections to all orders into a renormalized jet quenching parameter. The main effect of this resummation is to slow down the decrease of $\hat q$ with increasing time. We argue that the same (renormalized) value for $\hat q$ should also enter the calculation of medium-induced radiation in the expanding medium.
