Neutrino dynamics below the electroweak crossover

TL;DR

This paper estimates the thermal masses and damping rates of active and sterile neutrinos below the electroweak crossover, analyzing their impact on lepton asymmetry generation and implications for dark matter and baryogenesis.

Contribution

It provides the first comprehensive calculation of neutrino thermal rates below the electroweak crossover, including all leading-order reactions, with results tabulated for scenario-specific applications.

Findings

Washout rates exceed the Hubble rate between 5 GeV and 30 GeV.

Large lepton asymmetry generation is challenging below 5 GeV.

Baryon asymmetry generation remains feasible above 130 GeV.

Abstract

We estimate the thermal masses and damping rates of active (m < eV) and sterile (M ~ GeV) neutrinos with thermal momenta k ~ 3T at temperatures below the electroweak crossover (5 GeV < T < 160 GeV). These quantities fix the equilibration or "washout" rates of Standard Model lepton number densities. Sterile neutrinos interact via direct scatterings mediated by Yukawa couplings, and via their overlap with active neutrinos. Including all leading-order reactions we find that the washout rate generally exceeds the Hubble rate for 5 GeV < T < 30 GeV. Therefore it is challenging to generate a large lepton asymmetry facilitating dark matter computations operating at T < 5 GeV, whereas the generation of a baryon asymmetry at T > 130 GeV remains an option. Our differential rates are tabulated in a form suitable for studies of specific scenarios with given neutrino Yukawa matrices.