Particle-Antiparticle Asymmetries from Annihilations

An extensively studied mechanism to create particle-antiparticle asymmetries is the out-of-equilibrium and CP violating decay of a heavy particle. We, instead, examine how asymmetries can arise purely from 2 -> 2 annihilations rather than from the usual 1 -> 2 decays and inverse decays. We review the general conditions on the reaction rates that arise from S-matrix unitarity and CPT invariance, and show how these are implemented in the context of a simple toy model. We formulate the Boltzmann equations for this model, and present an example solution.