= Solution
A leading valence-level process is $u\bar d\to W^+\to e^+\nu_e$, with the <up quark> supplied by the <proton> and the <down antiquark> by the <antiproton>. The same <Feynman diagram> works with $e^+\nu_e$ replaced by $\mu^+\nu_\mu$. Both vertices are <weak charged current> vertices; the production <scattering amplitude> contains the <CKM matrix> element $V_{ud}$ or its conjugate according to the chosen fermion-flow convention. The intermediate <W boson> is produced near its <mass shell>.
\Image[/past-exam-of-the-particle-physics-course-of-the-university-of-oxford/2019-w-production-and-jacobian.png]
{title=Original charged-current production diagram and the normalized ideal charged-lepton transverse-momentum density. The angular axis is measured from the proton-side incoming up quark.}
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