Hypercharge and the cosmological baryon asymmetry

Stringent bounds on baryon- and lepton-number-violating interactions have been derived from the requirement that such interactions, together with electroweak instantons, do not destroy a cosmological baryon asymmetry produced at an extremely high temperature in the big bang. While these bounds apply in specific models, we find that they are generically avoided. In particular, the only requirement for a theory to avoid these bounds is that it contain charge particles which, during a certain cosmological epoch, carry a nonzero hypercharge asymmetry. Hypercharge neutrality of the Universe then dictates that the remaining particles must carry a compensating hypercharge density, which is necessarily shared among them so as to give a baryon asymmetry. Hence the generation of a hypercharge density in a sector of the theory forces the Universe to have a baryon asymmetry