  Understanding the motion of a tracer particle in a rarefied gas is of
fundamental and practical importance. We report the experimental investigation
of individual Cs atoms impinging on a dilute cloud of ultracold Rb atoms with
variable density. We study the nonequilibrium relaxation of the initial
nonthermal state and detect the effect of single collisions which has eluded
observation so far. We show that after few collisions, the measured spatial
distribution of the light tracer atoms is correctly described by a generalized
Langevin equation with a velocity-dependent friction coefficient, over a large
range of Knudsen numbers.
