  Frequency shifts from background gas collisions currently contribute
significantly to the inaccuracy of atomic clocks. Because nearly all collisions
with room-temperature background gases that transfer momentum eject the cold
atoms from the clock, the interference between the scattered and unscattered
waves in the forward direction dominates these frequency shifts. We show they
are ~10 times smaller than in room-temperature clocks and that van der Waals
interactions produce the cold-atom background-gas shift. General considerations
allow the loss of Ramsey fringe amplitude to bound this frequency shift.
