Constraining The Universal Lepton Asymmetry

TL;DR

This paper investigates the possibility of a large universal lepton asymmetry by analyzing cosmic background neutrinos, primordial element abundances, and cosmological data, providing new bounds on lepton asymmetry parameters.

Contribution

It introduces new constraints on the universal lepton asymmetry using combined CMB, LSS, and primordial nucleosynthesis data, considering both fixed and variable neutrino numbers.

Findings

Lepton asymmetry eta_L constrained to 0.072 +/- 0.053 with fixed N_nu=3.

Lepton asymmetry eta_L constrained to 0.115 +/- 0.095 with N_nu=3.3^{+0.7}_{-0.6}.

Results suggest possible larger lepton asymmetry than baryon asymmetry.

Abstract

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 (eta_B = n_B/n_gamma = 10^(-10)*eta_10), and on the early-universe expansion rate, parameterized by the expansion rate factor (S = H'/H) or, equivalently by the effective number of neutrinos (N_nu = 3 + 43(S^2 - 1)/7). 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 eta_L, the analog of eta_B, 0.072 +/- 0.053 if it is assumed that N_nu = 3 and, 0.115 +/- 0.095 along with N_nu = 3.3^{+0.7}_{-0.6}, if N_nu is free to vary.