Additional Light Sterile Neutrinos and Cosmology

TL;DR

This paper examines whether two light sterile neutrinos can reconcile cosmological bounds from BBN and CMB measurements with terrestrial neutrino oscillation data, considering partial thermalization and neutrino mass effects.

Contribution

It analyzes scenarios with two sterile neutrinos, exploring partial thermalization and neutrino mass impacts to address tensions between cosmological and terrestrial neutrino data.

Findings

Two sterile neutrinos cannot fit all data at 95% confidence level.

Massive sterile neutrinos reduce $ abla N_{eff}^{CMB}$ by over 5%.

Partial thermalization alone does not resolve the data tension.

Abstract

Tantalizing cosmological and terrestrial evidence suggests the number of light neutrinos may be greater than 3, motivating a careful re-examination of cosmological bounds on extra light species. Big Bang Nucleosynthesis constrains the number of relativistic neutrino species present during nucleosynthesis, $N_{eff}^{BBN}$, while measurements of the CMB angular power spectrum constrain the effective energy density in relativistic neutrinos at the time of matter-radiation equality, $N_{eff}^{CMB}$. There are a number of scenarios where new sterile neutrino species may have different contributions to $\Delta N_{eff}^{BBN}$ and $\Delta N_{eff}^{CMB}$, for masses that may be relevant to reconciling cosmological constraints with various terrestrial claims of neutrino oscillations. We consider a scenario with two sterile neutrinos and explore whether partial thermalization of the sterile states can ease the tension between cosmological constraints on $N_{eff}^{BBN}$ and terrestrial data. We then investigate the effect of a non-zero neutrino mass on their contribution to the radiation abundance, finding reductions in $\Delta N_{eff}^{CMB}$ of more than 5% for neutrinos with masses above 0.5 eV. While the effects we investigate here could play a role, we nevertheless find that two additional light sterile neutrinos species cannot fit all the data at the 95% confidence level.