Jet quenching parameter in an expanding QCD plasma

We study the phenomenon of transverse momentum broadening for a high-pT 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 η (the pseudo-rapidity) and ϕ (the azimuthal angle). The jet quenching parameter qˆ which enters this description depends upon the proper time alone. We furthermore consider radiative corrections to qˆ. 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 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 qˆ with increasing proper time.