state may be produced ‘on shell’, save for electrons and positrons. In addition, $-q^2$ does not exceed $300$ MeV$^2$ in this temperature range, a value which is at least seven orders of magnitude smaller than the square of the masses of the two IVBs. This property results in the simplification of the expressions for the various weak-interaction scattering amplitudes, and the subsequent dependence of the observables on only two physical constants, namely $G_F$ and $\xi \coloneqq \sin^2 \theta_W$, known as Fermi coupling constant and (square of the sine of the) weak-mixing angle, respectively. According to the most recent compilation of the physical constants by the Particle-Data Group [@pdg], $G_F=1.1663787(6) \cdot 10^{-5}$ GeV$^{-2}$ and $\xi=0.23129(5)$. In this work, the differential cross sections (DCSs) and the total cross sections (TCSs) will be expressed as multiples of the representative weak-interaction cross section $\sigma_0 \coloneqq G^2_F s/\pi$. The $\sigma_0$ value, corresponding to $T=10^{11}$ K, is about $5 \cdot 10^{-42}$ cm$^2$ or $5 \cdot 10^{-18}$ b.

\[sec:Preliminary\]The weak-interaction scattering amplitude
============================================================

The literature on the weak interaction is vast. For the sake of example, a thorough introduction to the subject may be obtained from Refs. [@ah; @gk]. I generally follow the formalism of Ref. [@ah] 