Neutrino decays as a natural explanation of the neutrino mass tension

A new tension is emerging between the tight cosmological upper bounds on the total neutrino mass ($\sum m_{\nu }\lesssim 0.06\mathrm{e}\mathrm{V}$) and the lower limits from oscillation experiments, with potentially far-reaching implications for cosmology and particle physics. Neutrinos decaying into massless BSM particles with lifetimes ${\tau }_{\nu }\sim 0.01-1\mathrm{G}\mathrm{y}\mathrm{r}$ represent a theoretically well-motivated mechanism to reconcile such measurements. Using DESI DR2 and CMB datasets, we show that such decays relax the bound on the total neutrino mass up to $\sum m_{\nu }<0.23\mathrm{e}\mathrm{V}$ (95%), restoring full agreement with oscillation data. We also present the first late-time cosmological analysis of neutrino decays into lighter neutrinos in a manner consistent with the measured mass splittings. In contrast to the decays into massless BSM particles, we find that this scenario only marginally alleviates - or even tightens - the cosmological neutrino mass bounds, depending on the mass ordering.