  Transverse momentum broadening and energy loss of a propagating parton are
dictated by the space-time profile of the jet transport coefficient $\hat q$ in
a dense QCD medium. The spatial gradient of $\hat q$ perpendicular to the
propagation direction can lead to a drift and asymmetry in parton transverse
momentum distribution. Such an asymmetry depends on both the spatial position
along the transverse gradient and path length of a propagating parton as shown
by numerical solutions of the Boltzmann transport in the simplified form of a
drift-diffusion equation. In high-energy heavy-ion collisions, this asymmetry
with respect to a plane defined by the beam and trigger particle (photon,
hadron or jet) with a given orientation relative to the event plane is shown to
be closely related to the transverse position of the initial jet production in
full event-by-event simulations within the linear Boltzmann transport model.
Such a gradient tomography can be used to localize the initial jet production
position for more detailed study of jet quenching and properties of the
quark-gluon plasma along a given propagation path in heavy-ion collisions.
