Scattering Rates For Leptogenesis: Damping of Lepton Flavour Coherence and Production of Singlet Neutrinos

TL;DR

This paper calculates the lepton flavor coherence relaxation rates and sterile neutrino production rates in the early universe using the Closed-Time-Path approach, providing results applicable to Standard Model extensions.

Contribution

It offers a systematic leading order calculation of flavor relaxation rates and sterile neutrino production, including the impact of t-channel fermion exchange, with results applicable beyond the Standard Model.

Findings

Lepton flavor relaxation rate at T=10^7 GeV is 5.19×10^-3 T.

Relaxation rate at T=10^13 GeV is 4.83×10^-3 T.

Enhanced sterile neutrino production due to t-channel fermion exchange.

Abstract

Using the Closed-Time-Path approach, we perform a systematic leading order calculation of the relaxation rate of flavour correlations of left-handed Standard Model leptons. This quantity is of pivotal relevance for flavoured Leptogenesis in the Early Universe, and we find it to be 5.19*10^-3 T at T=10^7 GeV and 4.83*10^-3 T at T=10^13 GeV. These values apply to the Standard Model with a Higgs-boson mass of 125 GeV. The dependence of the numerical coefficient on the temperature T is due to the renormalisation group running. The leading linear and logarithmic dependencies of the flavour relaxation rate on the gauge and top-quark couplings are extracted, such that the results presented in this work can readily be applied to extensions of the Standard Model. We also derive the production rate of light (compared to the temperature) sterile right-handed neutrinos, a calculation that relies on the same methods. We confirm most details of earlier results, but find a substantially larger contribution from the t-channel exchange of fermions.