Neutrino decays as a natural explanation of the neutrino mass tension

TL;DR

This paper investigates how neutrino decays into lighter particles can resolve the tension between cosmological and oscillation experiment bounds on neutrino mass, showing that certain decay scenarios relax the constraints significantly.

Contribution

It introduces a cosmological analysis of neutrino decays into lighter neutrinos, demonstrating their potential to reconcile neutrino mass bounds from different experiments.

Findings

Decays into massless BSM particles relax the neutrino mass bound up to 0.23 eV.

Neutrino decays into lighter neutrinos only marginally affect or tighten mass bounds.

The analysis is consistent with measured neutrino mass splittings.

Abstract

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