The Small Observed Baryon Asymmetry from a Large Lepton Asymmetry

TL;DR

This paper shows that a large lepton asymmetry can explain primordial element abundances without conflicting with the small observed baryon asymmetry, and that the supersymmetric Standard Model naturally resolves residual discrepancies.

Contribution

It reveals that equal and opposite electron and muon lepton asymmetries can generate the correct baryon asymmetry within the Standard Model, with supersymmetry further improving the match.

Findings

Large lepton asymmetry affects Big-Bang Nucleosynthesis predictions.

Equal and opposite electron and muon asymmetries produce the observed baryon asymmetry.

Supersymmetry naturally reduces the remaining discrepancy.

Abstract

Primordial Big-Bang Nucleosynthesis (BBN) tightly constrains the existence of any additional relativistic degrees of freedom at that epoch. However a large asymmetry in electron neutrino number shifts the chemical equilibrium between the neutron and proton at neutron freeze-out and allows such additional particle species. Moreover, the BBN itself may also prefer such an asymmetry to reconcile predicted element abundances and observations. However, such a large asymmetry appears to be in conflict with the observed small baryon asymmetry if they are in sphaleron mediated equilibrium. In this paper we point out the surprising fact that in the Standard Model, if the asymmetries in the electron number and the muon number are equal (and opposite) and of the size required to reconcile BBN theory with observations, a baryon asymmetry of the Universe of the correct magnitude and sign is automatically generated within a factor of two. This small remaining discrepancy is naturally remedied in the supersymmetric Standard Model.