  We study Brownian motion of a heavy quark in field theory plasma in the
AdS/CFT setup and discuss the time scales characterizing the interaction
between the Brownian particle and plasma constituents. In particular, the
mean-free-path time is related to the connected 4-point function of the random
force felt by the Brownian particle. By holographically computing the 4-point
function and regularizing the IR divergence appearing in the computation, we
write down a general formula for the mean-free-path time, and apply it to the
STU black hole which corresponds to plasma charged under three U(1) R-charges.
The result indicates that the Brownian particle collides with many plasma
constituents simultaneously.
