= Solution
Let $\theta^\star$ be the angle from the incoming <up quark>, along the <proton> direction, to the outgoing charged <antilepton> in the <W boson> rest frame. The initial and final <spin angular momentum>[spin projections] in <1/a/ii> are $-1$ and $+1$. The <Wigner small-d matrix> therefore gives
$$
\mathcal M(\theta^\star)\propto d^1_{-1,+1}(\theta^\star)
=\frac{1-\cos\theta^\star}{2}.
$$
After squaring and normalizing over $c=\cos\theta^\star$, the <charged lepton>[charged-lepton] <angular distribution> is
$$
\boxed{\frac1\Gamma\frac{d\Gamma}{dc}=\frac38(1-c)^2,\qquad -1<c<1.}
$$
Its integral is one since $\int_{-1}^{1}(1-c)^2dc=8/3$. The charged <antilepton> is preferentially emitted toward the <antiproton>. Measuring the angle from the incoming <down antiquark> instead replaces $c$ by $-c$ and produces $(1+c)^2$; the axis convention is part of the answer. Per unit solid angle the normalized density is $3(1-\cos\theta^\star)^2/(16\pi)$.
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