First second of leptons

Abstract

A poorly constrained parameter in the Standard Model of Cosmology is the lepton asymmetry l = Σf lf = Σ f(nf + nnf)/s. Each flavour asymmetry l f with f = e, μ, τ is the sum of the net particle density of the charged leptons nf and their corresponding neutrinos, normalized with the entropy density s. Constraints on If from BBN and CMB allow for lepton flavour asymmetries orders of magnitudes larger than the baryon asymmetry b ≃ 10-10. In this article we show how large lepton (flavour) asymmetries lf ≤ O(0.1) influence the early universe, in particular the freeze out of WIMPs and the cosmic QCD transition.