  We study the dynamics of dark matter (DM) particle-antiparticle oscillations
within the context of asymmetric DM. Oscillations arise due to small DM
number-violating Majorana-type mass terms, and can lead to recoupling of
annihilation after freeze-out and washout of the DM density. We derive the
density matrix equations for DM oscillations and freeze-out from first
principles using nonequilibrium field theory, and our results are qualitatively
different than in previous studies. DM dynamics exhibits
particle-vs-antiparticle "flavor" effects, depending on the interaction type,
analogous to neutrino oscillations in a medium. "Flavor-sensitive" DM
interactions include scattering or annihilation through a new vector boson,
while "flavor-blind" interactions include scattering or s-channel annihilation
through a new scalar boson, or annihilation to pairs of bosons. In particular,
we find that flavor-sensitive annihilation does not recouple when coherent
oscillations begin, and that flavor-blind scattering does not lead to
decoherence.
