\, \, \, .$$

\[sec:D2\]Scattering of a neutrino off an antineutrino of different flavour
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For an electron neutrino as projectile, the graph of Fig. \[fig:NuNuBar\](f) is relevant. Feynman’s interpretation of ‘the negative-energy particle solutions propagating backward in time’ as ‘the positive-energy antiparticle solutions propagating forward in time’ provides a speedy solution to the scattering of this section from the result of the previous one: the current for the incident/outgoing antineutrino of Fig. \[fig:NuNuBar\](f) moving forward in time is identical to the one for an outgoing/incident neutrino moving backward in time. This implies that the result, obtained for the graph Fig. \[fig:NuNu\](c) (i.e., the result of Section \[sec:D1\]), may be used, along with the obvious transformations $p \to - p^\prime$ and $p^\prime \to -p$, and enable the nearly effortless extraction of the DCS of this section. Evidently, one simply needs to interchange the Mandelstam variables $s$ and $u$ in the $\lvert \mathscr{T} \rvert^2$ result of Section \[sec:D1\]. According to Eq. (\[eq:EQ120\]), only the Mandelstam variable $s$ enters $\lvert \mathscr{T} \rvert^2$, hence the result for the process of this section is $$\lvert \mathscr{T} \rvert^2 = 8 G_F^2 u^2 \, \, \, ,$$ leading to the DCS in the form $$\frac{d\sigma}{d\Omega} = 