Collisional decoherence reexamined

We rederive the quantum master equation for the decoherence of a massive Brownian particle due to collisions with lighter particles from a thermal environment. Our careful treatment avoids the occurrence of squares of Dirac $\delta$ functions. It leads to a decoherence rate that is smaller by a factor of $2\pi$ compared to previous findings. This result, which is in agreement with recent experiments, is confirmed both by a physical analysis of the problem and by a perturbative calculation in the weak-coupling limit.