Relaxing Cosmological Neutrino Mass Bounds with Unstable Neutrinos
  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 $\sum m_\nu$.
