14405 2007.04994 10.1007/JHEP12(2020)119 119 119 Relaxing cosmological neutrino mass bounds with unstable neutrinos Regular Article - Theoretical Physics 1 44 2020 12 18 2020 7 22 2020 9 23 2020 12 17 2020 12 18 The Author(s) 2020 Open Access. This article is distributed under the terms of the Creative Commons Attribution License ( CC-BY 4.0 ), which permits any use, distribution and reproduction in any medium, provided the original author(s) and source are credited. 13130 2020 2020 12 12 Miguel Escudero miguel.escudero@kcl.ac.uk Jacobo Lopez-Pavon jlpavon@ific.uv.es Nuria Rius nuria.rius@ific.uv.es Stefan Sandner stefan.sandner@ific.uv.es grid.13097.3c 0000 0001 2322 6764 Theoretical Particle Physics and Cosmology Group, Department of Physics King’s College London Strand London WC2R 2LS U.K. grid.5338.d 0000 0001 2173 938X Departamento de Física Teórica and IFIC Universidad de Valencia-CSIC C/ Catedrático José Beltrán, 2 E-46980 Paterna Spain Abstract At present, cosmological observations set the most stringent bound on the neutrino mass scale. Within the standard cosmological model (ΛCDM), the Planck collaboration reports ∑m v < 0.12 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 τ ν ≲ 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 ν 4 and a Goldstone boson ϕ, in which ν i → ν 4 ϕ decays can loosen the neutrino mass bounds up to ∑m v ∼ 1 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) μ−τ flavor symmetry, which are otherwise in tension with the current bound on ∑m v . Keywords Beyond Standard Model Cosmology of Theories beyond the SM Neutrino Physics ArXiv ePrint: 2007.04994