Neutrino decays as a natural explanation of the neutrino mass tension

Abstract

A new tension is emerging between the tight cosmological upper bounds on the total neutrino mass ($\sum m_ν\lesssim 0.06 \, \rm{eV}$) 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 $τ_ν\sim 0.01-1\, \rm{Gyr}$ 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 $\sum m_ν< 0.23 \, \rm{eV}$ (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.

Figures (7)

• Figure 1: Effects of massive neutrinos on the Hubble rate (left panel) and on the present-day linear matter power spectrum (right panel), relative to the massless case, for fixed cosmological parameters $\{H_0, \omega_{\rm b}, \omega_{\rm cdm}, A_s, n_s, \tau_{\rm reio}, N_{\rm eff}\}$ and total neutrino mass $\sum m_\nu = 0.12 \, \rm{eV}$. We compare the case of three stable degenerate neutrinos (dashed lines) to the decay scenarios A, B1, and B2 introduced in the main text (solid lines), all assuming a lifetime equal to 5% of the age of the Universe .
• Figure 2: Constraints from DESI DR2 BAO and Planck PR3 CMB on the total neutrino mass $\sum m_\nu$ and neutrino lifetime $\log_{10}(\tau_\nu/t_{\rm U})$, assuming that three degenerate neutrinos decay into a $\nu_4+\phi$ dark radiation fluid (solid blue). The stable limit is denoted by the black dashed line. The pink and green shaded regions indicates exclusion from neutrino oscillation experiments, while the scratched area indicates the regime of relativistic decays excluded from the prior.
• Figure 3: Constraints from DESI DR2 BAO and Planck PR3 CMB on the total neutrino mass $\sum m_\nu$ and neutrino lifetime $\log_{10}(\tau_\nu/t_{\rm U})$, assuming that neutrinos decay with the atmospheric mass gap in the degenerate-$\nu_{1,2}$ approximation, for the normal (solid green) and inverted (solid red) orderings. The yellow and purple dashed lines denote the corresponding stable limits.
• Figure 4: Free-streaming scale of the lighter neutrino in scenarios B1 (left panel) and B2 (right panel), for neutrino decays with $\tau_\nu = 0.05 \,t_{\rm U}$ (solid lines) and the corresponding stable limit (dashed lines). In each case, the values of $m_{\nu_L}$ are chosen such that $\sum m_\nu =0.12\,\rm{eV}$. The inset plots display the final phase-space distribution of $\nu_L$ (solid lines), with comoving momentum $q$ given in units of $T_{\nu 0}$. These are compared to a standard Fermi-Dirac distribution (dashed lines).
• Figure 5: Training data and CONNECT posteriors in the $(\log_{10}(\tau_\nu/t_{\rm U}),\, m_{\nu})$--plane, for each of the three neutrino decay scenarios A (left panel), B1 (middle panel) and B2 (right panel). For scenario A, we additionally show the posterior resulting from a standard CLASS-based run. The gray areas mark the regime of relativistic decays, which is excluded during the MCMC analysis.