Relaxing Cosmological Neutrino Mass Bounds with Unstable Neutrinos. (arXiv:2007.04994v1 [hep-ph])
<a href="http://arxiv.org/find/hep-ph/1/au:+Escudero_M/0/1/0/all/0/1">Miguel Escudero</a>, <a href="http://arxiv.org/find/hep-ph/1/au:+Lopez_Pavon_J/0/1/0/all/0/1">Jacobo Lopez-Pavon</a>, <a href="http://arxiv.org/find/hep-ph/1/au:+Rius_N/0/1/0/all/0/1">Nuria Rius</a>, <a href="http://arxiv.org/find/hep-ph/1/au:+Sandner_S/0/1/0/all/0/1">Stefan Sandner</a>
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 $sum m_nu$.
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 $sum m_nu$.
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