The "standard" grand-unified-theory scenario for producing the baryon number-to-specific-entropy ratio of the universe involves the out-of-equilibrium decay of superheavy particles (≳ ${10}^{14}$ GeV) whose interactions do not conserve $B$, $C$, or $\mathrm{CP}$ invariance. I discuss a scenario in which the baryon asymmetry is produced by $B$-, $C$-, and $\mathrm{CP}$-nonconserving scatterings of massless fermions. In this model at least one fermion species must initially have a nonthermal distribution; the baryon excess generated can be larger than that produced by out-of-equilibrium decays. In order for this scenario to produce a significant baryon excess, the mass of the superheavy boson which mediates the scatterings must be ≳ 3 × ${10}^{15}$ GeV.

• Received 20 April 1981

DOI:https://doi.org/10.1103/PhysRevD.25.299