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 $2\to 2$ annihilations rather than from the usual $1\to 2$ 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.

• Received 31 July 2014

DOI:https://doi.org/10.1103/PhysRevLett.113.181601