Transverse momentum broadening of a fast quark in a N=4 Yang- ills plasma

We compute the momentum broadening of a heavy fundamental charge propagating through a N = 4 Yang Mills plasma at large t' Hooft coupling. We do this by expressing the medium modification of the probe's density matrix in terms of a Wilson loop averaged over the plasma. We then use the AdS/CFT correspondence to evaluate this loop, by identifying the dual semi-classical string solution. The calculation introduces the type "1" and type "2" fields of the thermal field theory and associates the corresponding sources with the two boundaries of the AdS space containing a black hole. The transverse fluctuations of the endpoints of the string determine kappa(T) = root gamma lambda T-3 pi - the mean squared momentum transfer per unit time. (gamma is the Lorentz gamma factor of the quark.) The result reproduces previous results for the diffusion coefficient of a heavy quark. We compare our results with previous AdS/CFT calculations of (q) over cap.