= Solution
The <Charged current>[charged-current] process is
$$
\boxed{e^-+\mu^+\longrightarrow\nu_e+\bar\nu_\mu,}
$$
with a $t$-channel $W^-$ emitted at the electron-to-neutrino vertex and absorbed at the antimuon-to-antineutrino vertex. It is not an $s$-channel <Annihilation> of the unlike-flavour <charged leptons> into one $W$.
At high <momentum transfer> the incident <electron>'s <Chirality>[left-chiral] fraction tends to $4/5$. In the collision <Center-of-momentum frame>[centre-of-mass frame], the incoming <antimuon> moves along $-z$ and its <Charged current>[charged-current] allowed right <helicity> has <spin> along $-z$. A rest-frame <spin> at <Angle> $\theta$ to $+z$ projects onto that <spin> direction with <probability> $\sin^2(\theta/2)$. Therefore, if $\sigma_0$ denotes the <cross section> for a pure <Helicity>[left-helicity] <electron> and fully selected <antimuon> <spin>,
$$
\boxed{\sigma(\theta)\simeq\frac45\,\sigma_0\sin^2\frac\theta2=\frac25\sigma_0(1-\cos\theta).}
$$
It vanishes at $\theta=0$ in the ultrarelativistic selection limit and is maximal at $\theta=\pi$. Finite-mass corrections can fill the suppressed orientation. The absolute <normalization> depends on beam <energy> and the exchanged-boson <propagator>; the spin-angle dependence is what is fixed here.
\Image[/past-exam-of-the-particle-physics-course-of-the-university-of-oxford/2018-weak-scattering-channels.png]
{title=Leptonic and <Quark>[quark-level] exchange diagrams and polarized <Charged current>[charged-current] angular dependence}
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