Relaxing Cosmological Neutrino Mass Bounds with Unstable Neutrinos

TL;DR

This paper explores how unstable neutrinos decaying into beyond Standard Model particles can relax cosmological neutrino mass bounds, proposing a model extension that allows for higher neutrino masses without conflicting with existing data.

Contribution

It introduces a new decay mechanism involving sterile neutrinos and a Goldstone boson, demonstrating how this can significantly loosen cosmological neutrino mass constraints.

Findings

Decay into BSM particles can relax mass bounds up to ~1 eV.

A minimal model with sterile neutrinos and a Goldstone boson is viable.

The approach works across a wide range of right-handed neutrino masses.

Abstract

At present, cosmological observations set the most stringent bound on the neutrino mass scale. Within the standard cosmological model ($\Lambda$CDM), the Planck collaboration reports $\sum m_\nu < 0.12\,\text{eV}$ at 95% CL. This bound, taken at face value, excludes many neutrino mass models. However, unstable neutrinos, with lifetimes shorter than the age of the universe $\tau_\nu \lesssim t_U$, represent a particle physics avenue to relax this constraint. Motivated by this fact, we present a taxonomy of neutrino decay modes, categorizing them in terms of particle content and final decay products. Taking into account the relevant phenomenological bounds, our analysis shows that 2-body decaying neutrinos into BSM particles are a promising option to relax cosmological neutrino mass bounds. We then build a simple extension of the type I seesaw scenario by adding one sterile state $\nu_4$ and a Goldstone boson $\phi$, in which $\nu_i \to \nu_4 \, \phi$ decays can loosen the neutrino mass bounds up to $\sum m_\nu \sim 1\,\text{eV}$, without spoiling the light neutrino mass generation mechanism. Remarkably, this is possible for a large range of the right-handed neutrino masses, from the electroweak up to the GUT scale. We successfully implement this idea in the context of minimal neutrino mass models based on a $U(1)_{\mu-\tau}$ flavor symmetry, which are otherwise in tension with the current bound on $\sum m_\nu$.