  Impurity atoms propagating at variable velocities through a trapped
Bose-Einstein condensate were produced using a stimulated Raman transition. The
redistribution of momentum by collisions between the impurity atoms and the
stationary condensate was observed in a time-of-flight analysis. The
collisional cross section was dramatically reduced when the velocity of the
impurities was reduced below the speed of sound of the condensate, in agreement
with the Landau criterion for superfluidity. For large numbers of impurity
atoms, we observed an enhancement of atomic collisions due to bosonic
stimulation. This enhancement is analogous to optical superradiance.
