Constraining the universal lepton asymmetry

The relic cosmic background neutrinos accompanying the cosmic microwave background (CMB) photons may hide a universal lepton asymmetry orders of magnitude larger than the universal baryon asymmetry. At present, the only direct way to probe such an asymmetry is through its effect on the abundances of the light elements produced during primordial nucleosynthesis. The relic light element abundances also depend on the baryon asymmetry, parameterized by the baryon density parameter () and on the early-universe expansion rate, parameterized by the expansion rate factor () or, equivalently, by the effective number of neutrinos (). We use data from the CMB (and large scale structure: LSS) along with the observationally inferred relic abundances of deuterium and helium-4 to provide new bounds on the universal lepton asymmetry, finding for η_L, the analog of η_B,0.072 ± 0.053 if it is assumed that N_ν = 3 and, 0.115 ± 0.095 along with N_ν = 3.3_−0.6^+0.7, if N_ν is free to vary.