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 ∑mv< 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 τν ≲ tU, 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 ν4 and a Goldstone boson ϕ, in which νi→ ν4ϕ decays can loosen the neutrino mass bounds up to ∑mv ∼ 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 ∑mv.
Highlights
The best laboratory constraint on the absolute neutrino mass scale comes from the KATRIN experiment that reports the following Feldman-Cousins upper limit: mνe ≡
In order for the reader to have a feeling of this matter, we summarize in table 1 a suite of cosmological constraints on mν arising from analyzing various data sets, and by using the same data set but within different cosmological models
To be in accordance with the observed light neutrino mass spectrum and mixing, the contribution to the right-handed neutrino Majorana masses from the coupling to the scalar should be of the same order of the mass terms allowed by the U(1)μ−τ symmetry, MeeNecNe and Mμτ NμcNτ, as we have considered in the above equation
Summary
We classify invisible neutrino decays according to two criteria: i) nature of the decay products and ii) number of particles in the final state. In this case, the constraint on mν can be significantly relaxed to the level of mν 1 eV at 95% CL, provided that the BSM particles are massless, see section 3. I.e. mφ, Z > mνi, any 2-body decay is kinematically closed. We will not consider this possibility here since, as we will show, already the 3-body decay channels are only capable of rendering τν < tU across a narrow window of the parameter space. According to this classification, there are six different possible topologies for the neutrino decays that we show in figure 1.
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