  We compute holographically the energy loss of a moving quark in various
states of the D1-D5 CFT. In the dual bulk geometries, the quark is the end of a
trailing string, and the profile of this string determines the drag force
exerted by the medium on the quark. We find no drag force when the CFT state
has no momentum, and a nontrivial force for any value of the velocity (even at
rest) when the string extends in the supersymmetric D1-D5-P black-hole
geometry, or a horizonless microstate geometry thereof. As the length of the
throat of the microstate geometry increases, the drag force approaches the
thermal BTZ expression, confirming the ability of these microstate geometries
to capture typical black-hole physics. We also find that when the D1-D5-P black
hole is non-extremal, there is a special value of the velocity at which a
moving quark feels no force. We compute this velocity holographically and we
compare it to the velocity computed in the CFT.

High Energy Physics - Theory