  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.
