The small observed baryon asymmetry from a large lepton asymmetry

Primordial Big-Bang Nucleosynthesis (BBN) tightly constrains the existence ofany additional relativistic degrees of freedom at that epoch. However a largeasymmetry in electron neutrino number shifts the chemical equilibrium betweenthe neutron and proton at neutron freeze-out and allows such additionalparticle species. Moreover, the BBN itself may also prefer such an asymmetry toreconcile predicted element abundances and observations. However, such a largeasymmetry appears to be in conflict with the observed small baryon asymmetry ifthey are in sphaleron mediated equilibrium. In this paper we point out thesurprising fact that in the Standard Model, if the asymmetries in the electronnumber and the muon number are equal (and opposite) and of the size required toreconcile BBN theory with observations, a baryon asymmetry of the Universe ofthe correct magnitude and sign is automatically generated within a factor oftwo. This small remaining discrepancy is naturally remedied in thesupersymmetric Standard Model.Comment: 14 page