  Single quarks moving in the vacuum of confining gauge theories are stopped by
a drag force. The holographic description relates the confining scale in the
bulk geometry with a range of physical values for the drag force in the vacuum.
The vacuum drag force acting on the isolated quark directly manifests quark
confinement since it prevents the quark from walking freely in the vacuum.
However, analytical expressions for the drag force as a function of the quark
velocity were lacking. In the present work, we propose that the vacuum drag
force is given by the regularized zero-temperature limit of the corresponding
thermal drag force. Within this approach, we obtain the desired analytic
expressions in two different holographic models: the quadratic dilaton and the
D-instanton. In both cases, we find well-behaved functions belonging to their
physical range of values.
