positrons, during the temporal interval corresponding to the temperatures $T_1$ and $T_2$.

The permissible tree-level Feynman graphs (henceforth, simply graphs) for the interaction of an electron neutrino (projectile) with the leptons of the plasma (targets), resulting in no more than an electron-positron ($e^+ e^-$) pair in the final state, are shown in Figs. \[fig:NuNu\]-\[fig:NuElPos\]. All these graphs involve the exchange of either of the two intermediate vector bosons (IVBs) which are associated with the weak interaction, namely the $Z^0$ boson, which is associated with the neutral current (NC), and the $W^\pm$ boson, which is associated with the charged current (CC). Given that it yields a cross section several orders of magnitude smaller [@dr], the elastic scattering between the (anti)neutrini and the photons may be safely omitted.

![\[fig:NuNu\]The permissible tree-level Feynman graphs for the interaction of an electron neutrino (projectile) with the neutrini of the electron, muon, and $\tau$-lepton generations of matter (targets). Graphs (a) and (b) relate to the scattering off the electron neutrino. Graph (c) relates to the scattering processes $\nu_e \nu_{\mu,\tau} \to \nu_e \nu_{\mu,\tau}$. In all cases, these $t$-channel exchanges involve the weak neutral current. Similar graphs are applicable in case of the muon and $\tau$-lepton neutrini as 