Restrictions on the lifetime of sterile neutrinos from primordial nucleosynthesis

TL;DR

This paper investigates how decaying sterile neutrinos in the 1-140 MeV range affect primordial Helium-4 abundance, deriving constraints on their lifetime and implications for cosmological observations.

Contribution

It provides a detailed analysis of sterile neutrino decay effects on nucleosynthesis, establishing a lifetime-dependent bound and reconciling recent helium abundance measurements.

Findings

Helium abundance depends only on sterile neutrino lifetime.

Sterile neutrinos with lifetime 0.01-2 seconds are consistent with observed helium levels.

Decay impacts neutrino spectra and photon heating, affecting cosmological parameters.

Abstract

We analyze the influence of decaying sterile neutrinos with the masses in the range 1-140 MeV on the primordial Helium-4 abundance, explicitly solving the Boltzmann equations for all particle species, taking into account neutrino flavour oscillations, and paying special attention to systematic uncertainties. We show that the Helium abundance depends only on the sterile neutrino lifetime and not on the way the active-sterile mixing is distributed between flavours, and derive an upper bound on the lifetime. We also demonstrate that the recent results of Izotov & Thuan [arXiv:1001.4440], who find 2sigma higher than predicted by the standard primordial nucleosynthesis value of Helium-4 abundance, are consistent with the presence in the plasma of sterile neutrinos with the lifetime 0.01-2 seconds. The decay of these particles perturbs the spectra of (decoupled) neutrinos and heats photons, changing the ratio of neutrino to photon energy density, that can be interpreted as extra neutrino species at the recombination epoch.