interchanged final-state particles.

-   For a $4$-vector $a$, ${\ensuremath{\mathrlap{\!\not{\phantom{a}}}a}} \coloneqq \gamma_{\mu} a^{\mu}$.

-   $k$ and $p$ are the $4$-momenta of the incident particles (projectile and target, respectively).

-   $k^\prime$ and $p^\prime$ are the $4$-momenta of the scattered particles.

-   $q \coloneqq k-k^\prime=p^\prime-p$ is the $4$-momentum transfer.

-   $\theta$ and $\Omega$ denote the scattering angle and the solid angle, respectively.

-   $m_e$ stands for the electron (and positron) rest mass.

\[sec:FD\]Feynman graphs relevant to the decoupling of the (anti)neutrini
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It is generally believed that, between the temperatures of one hundred billion K ($T_1=10^{11}$ K), corresponding to a mere one-hundredth of a second after the Big Bang, and about $T_2=2 \cdot 10^{10}$ K, corresponding to the cosmological time of about $250$ ms, when the (anti)neutrini decoupled from the other ingredients of the primordial plasma [^1], our Universe was a superdense mixture of neutrini, antineutrini, electrons, positrons, and photons. In contrast to the huge densities of these particles, a few baryons (protons and neutrons) were also present, about six such particles for every ten billion of photons. Of interest in the context of this work are the interactions of the (anti)neutrini with other (anti)neutrini, as well as with the electrons and the 