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Cosmological Constraints on Invisible Neutrino Decays Revisited

Invisible neutrino decay modes are difficult to target at laboratory experiments, and current bounds on such decays from solar neutrino and neutrino oscillation experiments are somewhat weak. It has been known for some time that Cosmology can serve as a powerful probe of invisible neutrino decays. In this work, we show that in order for Big Bang Nucleosynthesis to be successful, the invisible neutrino decay lifetime is bounded to be $τ_ν> 10^{-3}\,\text{s}$ at 95\% CL. We revisit Cosmic Microwave Background constraints on invisible neutrino decays, and by using Planck2018 observations we find the following bound on the neutrino lifetime: $τ_ν> (1.3-0.3)\times 10^{9}\,\text{s} \, \left({m_ν}/{ 0.05\,\text{eV} }\right)^3$ at $95\%$ CL. We show that this bound is robust to modifications of the cosmological model, in particular that it is independent of the presence of dark radiation. We find that lifetimes relevant for Supernova observations ($τ_ν\sim 10^{5}\,\text{s}\, \left({m_ν}/{ 0.05\,\text{eV} }\right)^3$) are disfavoured at more than $5\,σ$ with respect to $Λ$CDM given the latest Planck CMB observations. Finally, we show that when including high-$\ell$ Planck polarization data, neutrino lifetimes $τ_ν= (2-16)\times 10^{9}\,\text{s} \, \left({m_ν}/{ 0.05\,\text{eV} }\right)^3$ are mildly preferred -- with a 1-2 $σ$ significance -- over neutrinos being stable.