Electroweak Baryogenesis Using Baryon Number Carrying Scalars

We describe a new mechanism for the generation of the baryon asymmetry of the universe during a first order electroweak phase transition. The mechanism requires the existence of two (or more) baryon number carrying scalar fields with masses and CP violating mixing which vary with the Higgs field expectation value. This mechanism can be implemented using squarks in supersymmetric theories or using leptoquarks. Our central observation is that reflection of these scalars from a bubble wall can yield a significant net baryon number flux into the symmetric phase, balanced by a flux of opposite sign into the broken phase. For generic parameter choices, scalars with incident energies in a specific, but not narrow, range yield order one reflection asymmetries (between the probability of reflection of the scalars and of their antiparticles). The interesting energies are those for which there are two propagating scalars in the symmetric phase but only one in the broken phase. Electroweak sphaleron processes drive the baryon number in the symmetric phase toward zero, but do not act in the broken phase. Our estimate of the resulting baryon asymmetry is consistent with cosmological observations for a range of mass parameters and CP violating phases in a supersymmetric implementation, as long as the bubble walls are not too fast and not too thick.