expressions, obtained from the $\lvert \mathscr{T} \rvert^2$ result of this section (e.g., on the basis of interchanges of $4$-momenta) for other processes, a) must involve the original $\lvert \mathscr{T} \rvert^2$ result and b) must correspond to TCSs involving the integration of the corresponding DCSs over $4 \pi$. The restricted solid-angle domain pertains exclusively to processes yielding indistinguishable particles in the final state, which (for Dirac neutrini) is the case only in this section.

\[sec:D4\]Elastic scattering of a neutrino off an antineutrino of the same flavour
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For the elastic scattering of an electron neutrino off an electron antineutrino, the graphs of Fig. \[fig:NuNuBar\](a) and (c) are relevant; the latter graph represents the annihilation to a neutrino-antineutrino pair of the same flavour. The substitutions $p \to - p^\prime$ and $p^\prime \to -p$ enable the speedy extraction of the DCS of this section from the $\lvert \mathscr{T} \rvert^2$ result of Section \[sec:D3\]. Evidently, $$\label{eq:EQ180}
\frac{d\sigma}{d\Omega} = \frac{G^2_F u^2}{2 \pi^2 s} = \frac{\sigma_0 u^2}{2 \pi s^2} \, \, \, ,$$ which, integrated over the solid angle, results in $$\sigma_{\rm tot} = \frac{2 \sigma_0}{3} \, \, \, .$$

\[sec:D5\]Neutrino-antineutrino annihilation to a neutrino-antineutrino pair of different flavour
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For an electron neutrino-antineutrino pair, the 