We study the equilibration of right-handed electrons in the symmetric phase of the Standard Model. Due to the smallness of the electron Yukawa coupling, it happens relatively late in the history of the Universe. We compute the equilibration rate at leading order in the Standard Model couplings, by including gauge interactions, the top Yukawa-and the Higgs self-interaction. The dominant contribution is due to 2 → 2 particle scattering, even though the rate of (inverse) Higgs decays is strongly enhanced by multiple soft scattering which is included by Landau-Pomeranchuk-Migdal (LPM) resummation. Our numerical result is substantially larger than approximations presented in previous literature. 1

We obtain non-linear kinetic equations for sterile neutrino occupancies and lepton minus baryon numbers by matching real time correlation functions of thermal fluctuations computed in an effective description to those computed in thermal quantum field theory. After expanding in the sterile-neutrino Yukawa couplings, the coefficients in the equations are written as real time correlation functions of Standard Model operators. Our kinetic equations are valid for an arbitrary number of sterile neutrinos of any mass spectrum. They can be used to describe, e.g., low-scale leptogenesis via neutrino oscillations, or sterile neutrino dark matter production in the Higgs phase.

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