Regular Article - Theoretical Physics
Weaker yet again: mass spectrum-consistent cosmological constraints on the neutrino lifetime
Sydney Consortium for Particle Physics and Cosmology, School of Physics, The University of New South Wales, 2052, Sydney, NSW, Australia
2 Centre for Cosmology, Particle Physics and Phenomenology, Université catholique de Louvain, Chemin du cyclotron, 2, 1348, Louvain-la-Neuve, Belgium
Accepted: 11 June 2022
Published online: 25 July 2022
We consider invisible neutrino decay in the ultra-relativistic limit and compute the neutrino anisotropy loss rate relevant for the cosmic microwave background (CMB) anisotropies. Improving on our previous work which assumed massless and , we reinstate in this work the daughter neutrino mass in a manner consistent with the experimentally determined neutrino mass splittings. We find that a nonzero introduces a new phase space factor in the loss rate proportional to in the limit of a small squared mass gap between the parent and daughter neutrinos, i.e., , where is the rest-frame lifetime. Using a general form of this result, we update the limit on using the Planck 2018 CMB data. We find that for a parent neutrino of mass eV, the new phase space factor weakens the constraint on its lifetime by up to a factor of 50 if corresponds to the atmospheric mass gap and up to if the solar mass gap, in comparison with naïve estimates that assume . The revised constraints are (i) s and s if only one neutrino decays to a daughter neutrino separated by, respectively, the atmospheric and the solar mass gap, and (ii) s in the case of two decay channels with one near-common atmospheric mass gap. In contrast to previous, naïve limits which scale as , these mass spectrum-consistent constraints are remarkably independent of the parent mass and open up a swath of parameter space within the projected reach of IceCube and other neutrino telescopes in the next two decades.
© The Author(s) 2022
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