positron, graph (c) via the $t$-channel exchange of the weak neutral current, graph (d) via the $s$-channel creation of a $W^+$ boson. In the case of the muon and $\tau$-lepton neutrini as projectiles, only graphs (a) and (c) contribute: graph (b) cannot contribute in the energy range explored in this paper, whereas graph (d) does not contribute on the basis of first principles (conservation of the leptonic number within each generation of matter).](NuElPos.eps){width="15cm"}

Ignoring the (presently unknown) (anti)neutrini masses, the interactions (a) of the (anti)neutrini of the electron generation of matter and (b) of the (anti)neutrini of the muon and of the $\tau$-lepton generations of matter are different: three of the graphs in Figs. \[fig:NuNu\]-\[fig:NuElPos\] do not contribute to the collision rates of the muon and $\tau$-lepton (anti)neutrini. As the interactions of the muon and $\tau$-lepton (anti)neutrini are identical in the plasma, the study of the interactions induced only by $\nu_e$ and $\nu_\mu$ suffices for the purposes of this work.

In all cases, only Dirac neutrini are considered herein: therefore, each neutrino and its corresponding antineutrino are assumed to be distinguishable particles. The range of the available energy, corresponding to the temperatures $T_1$ and $T_2$, is such that no massive 