Momentum broadening in weakly coupled quark-gluon plasma (with a view to finding the quasiparticles within liquid quark-gluon plasma)

Abstract

We calculate P (k [subscript ⊥]), the probability distribution for an energetic parton that propagates for a distance L through a medium without radiating to pick up transverse momentum k [subscript ⊥], for a medium consisting of weakly coupled quark-gluon plasma. We use full or HTL self-energies in appropriate regimes, resumming each in order to find the leading large-L behavior. The jet quenching parameter [ˆ over q] is the second moment of P (k [subscript ⊥]), and we compare our results to other determinations of this quantity in the literature, although we emphasize the importance of looking at P (k [subscript ⊥]) in its entirety. We compare our results for P (k [subscript ⊥]) in weakly coupled quark-gluon plasma to expectations from holographic calculations that assume a plasma that is strongly coupled at all length scales. We find that the shape of P (k [subscript ⊥]) at modest k [subscript ⊥] may not be very different in weakly coupled and strongly coupled plasmas, but we find that P (k [subscript ⊥]) must be parametrically larger in a weakly coupled plasma than in a strongly coupled plasma — at large enough k [subscript ⊥]. This means that by looking for rare (but not exponentially rare) large-angle deflections of the jet resulting from a parton produced initially back-to-back with a hard photon, experimentalists can find the weakly coupled short-distance quark and gluon quasiparticles within the strongly coupled liquid quark-gluon plasma produced in heavy ion collisions, much as Rutherford found nuclei within atoms or Friedman, Kendall and Taylor found quarks within nucleons.