Revisiting cosmological bounds on sterile neutrinos

TL;DR

This paper uses advanced cosmological data to constrain sterile neutrino properties, showing that decay after BBN is strongly limited by CMB and BAO observations, effectively ruling out certain sterile neutrino models.

Contribution

It provides the first comprehensive cosmological bounds on sterile neutrino mass and mixing angle considering their decay and full time-evolution effects.

Findings

Sterile neutrinos decaying between BBN and today are effectively ruled out by cosmology.

The mixing angle is constrained to be less than approximately 0.026 for a given mass.

Full time-evolution modeling tightens bounds on sterile neutrino parameters.

Abstract

We employ state-of-the art cosmological observables including supernova surveys and BAO information to provide constraints on the mass and mixing angle of a non-resonantly produced sterile neutrino species, showing that cosmology can effectively rule out sterile neutrinos which decay between BBN and the present day. The decoupling of an additional heavy neutrino species can modify the time dependence of the Universe's expansion between BBN and recombination and, in extreme cases, lead to an additional matter-dominated period; while this could naively lead to a younger Universe with a larger Hubble parameter, it could later be compensated by the extra radiation expected in the form of neutrinos from sterile decay. However, recombination-era observables including the Cosmic Microwave Background (CMB), the shift parameter $R_{CMB}$ and the sound horizon $r_s$ from Baryon Acoustic Oscillations (BAO) severely constrain this scenario. We self-consistently include the full time-evolution of the coupled sterile neutrino and standard model sectors in an MCMC, showing that if decay occurs after BBN, the sterile neutrino is essentially bounded by the constraint $\sin^2\theta \lesssim 0.026 (m_s/\mathrm{eV})^{-2}$.