Seminar in Hadronic Physics

Jet Propagation in Hot and Dense Media from Effective Field Theory

Francesco d'Eramo

MIT

The propagation of energetic partons through the matter created in relativistic heavy-ion collisions involves widely different scales, the parton energy and the scale characteristic of the medium (e.g. its temperature). Motivated by this scale separation, we discuss how to formulate this problem within the Effective Field Theory framework. We start by analyzing transverse momentum broadening for an energetic parton propagating through a medium, and we give a field theoretical definition of the probability distribution to pick up a given transverse momentum. The final result is independent on the parton energy, and is given in terms of the expectation value of two transversely separated light-like path-ordered Wilson lines of gluon fields from the medium. The nature of the medium does not enter this calculation, and any of its property becomes relevant only when we seek to evaluate the expectation value. We explicitly evaluate the thermal expectation value for the strongly coupled plasma of N=4 SYM theory by using gauge/gravity duality, and for a weakly-coupled QCD plasma by using perturbation theory. In the weak-coupling case, we resum an infinite class of diagrams which are enhanced by the medium length, and we use full or HTL gluon self-energies in the appropriate regimes. We then compare the two results, and we find that the weak coupling and strong coupling results need not differ greatly at modest transverse momentum, but we find that the probability distribution must be parametrically larger in a weakly coupled plasma than in a strongly coupled plasma at large enough transverse momentum. By looking for 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 quark and gluon short-distance constituents of the strongly coupled liquid quark-gluon plasma. Finally, we present the preliminary results on the calculation of the spectrum of the gluons radiated by the hard parton, where the radiated gluons are collinear with the incoming hard parton and with an arbitrary energy fraction.