  We calculate P(k_\perp), the probability distribution for an energetic parton
that propagates for a distance L through a medium without radiating to pick up
transverse momentum k_\perp, 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 \hat q is the second moment of P(k_\perp), and we compare our results
to other determinations of this quantity in the literature, although we
emphasize the importance of looking at P(k_\perp) in its entirety. We compare
our results for P(k_\perp) 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_\perp) at modest k_\perp may
not be very different in weakly coupled and strongly coupled plasmas, but we
find that P(k_\perp) must be parametrically larger in a weakly coupled plasma
than in a strongly coupled plasma at large enough k_\perp. 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.
