Unstable Neutrinos can Relax Cosmological Mass Bounds

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.