Constraining invisible neutrino decays with the cosmic microwave background

TL;DR

The paper uses the cosmic microwave background’s requirement that neutrinos free-stream at photon decoupling to constrain late-time neutrino interactions with light bosons. By analyzing both binary neutrino–boson scatterings and first-order decays/coalescences, the authors derive stringent upper bounds on neutrino–scalar Yukawa couplings: diagonal couplings satisfy $g \lesssim 1.1\times 10^{-7}$, while off-diagonal couplings obey $g \lesssim 0.61\times 10^{-11}\left(\dfrac{50\, {\rm meV}}{m}\right)^2$ (with $m$ the heavier neutrino mass). They further show that decay channels would not significantly modify the flavor content of high-energy cosmic neutrinos within these bounds, and discuss implications for supernova physics and diffuse neutrino fluxes. These results firmly restrict invisible neutrino decays mediated by light bosons and demonstrate the CMB as a sensitive laboratory for beyond-Standard-Model neutrino interactions.

Abstract

Precision measurements of the acoustic peaks of the cosmic microwave background indicate that neutrinos must be freely streaming at the photon decoupling epoch when T ~ 0.3 eV. This requirement implies restrictive limits on ``secret neutrino interactions,'' notably on neutrino Yukawa couplings with hypothetical low-mass (pseudo)scalars φ. For diagonal couplings in the neutrino mass basis we find g < 1 x 10^-7, comparable to limits from supernova 1987A. For the off-diagonal couplings and assuming hierarchical neutrino masses we find g < 1 x 10^-11 (0.05 eV/m)^2 where m is the heavier mass of a given neutrino pair connected by g. This stringent limit excludes that the flavor content of high-energy neutrinos from cosmic-ray sources is modified by ν-> ν' + φdecays on their way to Earth.