Unstable Neutrinos can Relax Cosmological Mass Bounds

TL;DR

This paper explores how neutrino decay mechanisms can relax cosmological bounds on neutrino masses, challenging the standard constraints and opening new model possibilities within extended seesaw frameworks.

Contribution

It demonstrates that neutrino decay processes can significantly weaken cosmological mass bounds, providing a novel way to reconcile neutrino models with observational data.

Findings

Neutrino decay can relax the cosmological mass bound to about 1 eV.

A decay mode $ u_i o u_4\, ext{phi}$ is feasible within minimal extended seesaw models.

The decay mechanism is explicitly realized in a $U(1)_{\mu-\tau}$ flavor model.

Abstract

The light neutrino masses are at present most stringently constraint via cosmological probes. In particular the Planck collaboration reports $ \sum m_\nu \leq 0.12\,\mathrm{eV}$ at $95\%$ CL within the standard cosmological model. This is more than one order of magnitude stronger than the one arising from laboratory searches. The cosmological bound taken at face value excludes a plethora of neutrino flavour models which can successfully explain the neutrino oscillation data. The indirect nature of the cosmological bound, however, allows to relax the bound to up to $ \sum m_\nu \sim 1\,\mathrm{eV}$ if neutrinos decay on timescales shorter than the age of the Universe, $\tau_\nu \leq t_U$. We present how a decay of the type $\nu_i\to\nu_4\phi$ can be realized within general models of the minimal extended seesaw framework. The idea is then explicitly realized within the context of a $U(1)_{\mu-\tau}$ flavour model.