# Decaying neutrinos: The long way to isotropy

## TL;DR

This paper explores how neutrino decays into pseudoscalars affect their thermal equilibrium, updating bounds on coupling constants and lifetimes, and confirming the delayed equilibrium analytically and numerically.

## Contribution

It provides updated bounds on neutrino-pseudoscalar coupling and lifetime, and analytically and numerically confirms the delayed kinetic equilibrium due to Lorentz factors.

## Key findings

- Updated bound on neutrino-pseudoscalar coupling: g<2.6×10^{-13}
- Updated bound on neutrino lifetime: τ<10^{13} seconds
- Confirmed delay in kinetic equilibrium due to Lorentz factors

## Abstract

We investigate a scenario in which neutrinos are coupled to a pseudoscalar degree of freedom $\ph$ and where decays $\nu_1 \to \nu_2+\ph$ and inverse decays are the responsible mechanism for obtaining equilibrium. In this context we discuss the implication of the invisible neutrino decay on the neutrino-pseudoscalar coupling constant and the neutrino lifetime. Assuming the realistic scenario of a thermal background of neutrinos and pseudoscalar we update the bound on the (off-diagonal) neutrino-pseudoscalar coupling constant to $g<2.6\times10^{-13}$ and the bound on the neutrino lifetime to $\tau<1\times10^{13}\,\rm{s}$. Furthermore we confirm analytically that kinetic equilibrium is delayed by two Lorentz $\ga$--factors -- one for time dilation of the (decaying) neutrino lifetime and one from the opening angle. We have also confirmed this behavior numerically.