Improved cosmological constraints on the neutrino mass and lifetime

TL;DR

This paper derives cosmological constraints on the sum of neutrino masses considering their decay into dark radiation, showing improved bounds thanks to Planck 2018 polarization data, but still less restrictive than stable neutrino scenarios.

Contribution

It provides the first comprehensive cosmological analysis of neutrino mass constraints accounting for neutrino decay into dark radiation.

Findings

Neutrino mass sum constrained to < 0.42 eV at 95% CL with decay considerations.

Planck 2018 polarization data significantly tighten constraints.

Decay of neutrinos relaxes the bounds compared to stable neutrino assumptions.

Abstract

We present cosmological constraints on the sum of neutrino masses as a function of the neutrino lifetime, in a framework in which neutrinos decay into dark radiation after becoming non-relativistic. We find that in this regime the cosmic microwave background (CMB), baryonic acoustic oscillations (BAO) and (uncalibrated) luminosity distance to supernovae from the Pantheon catalog constrain the sum of neutrino masses $\sum m_\nu$ to obey $\sum m_\nu< 0.42$ eV at (95$\%$ C.L.). While the the bound has improved significantly as compared to the limits on the same scenario from Planck 2015, it still represents a significant relaxation of the constraints as compared to the stable neutrino case. We show that most of the improvement can be traced to the more precise measurements of low-$\ell$ polarization data in Planck 2018, which leads to tighter constraints on $\tau_{\rm reio}$ (and thereby on $A_s$), breaking the degeneracy arising from the effect of (large) neutrino masses on the amplitude of the CMB power spectrum.