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.

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.