  We analyze collisional decoherence of atoms or molecules prepared in a
coherent superposition of nondegenerate internal states at ultralow
temperatures and placed in an ultracold buffer gas. Our analysis is applicable
for an arbitrary bath particle/tracer particle mass ratio. Both elastic and
inelastic collisions contribute to decoherence. We obtain an expression
relating the observable decoherence rate to pairwise scattering properties,
specifically the low-temperature scattering amplitudes. We consider the
dependence on the bath particle/tracer particle mass ratio for the case of
light bath and heavy tracer particles. The expressions obtained may be useful
in low-temperature applications where accurate estimates of decoherence rates
are needed. The results suggest a method for determining the scattering lengths
of atoms and molecules in different internal states by measuring
decoherence-induced damping of coherent oscillations.
