= Solution
For negligible <fermion> <masses>, the <Neutral current>[neutral-current] vertex connects a <fermion> and <antifermion> of opposite <helicity>. The two incoming choices are $e^-_Le^+_R$ and $e^-_Re^+_L$; the two outgoing choices are $f_L\bar f_R$ and $f_R\bar f_L$. Their combinations give four channels, denoted $LL,LR,RL,RR$, where each letter refers to the <fermion> rather than <antifermion>.
Take the incoming <electron> <momentum> along $+z$ and <Positron> <momentum> along $-z$. The initial $L$ pair has both <spins> along $-z$, giving total projection $-1$; the $R$ pair has both along $+z$, giving projection $+1$. If the outgoing <fermion> moves along $\hat{\mathbf n}$, an outgoing $L$ pair has both <spins> along $-\hat{\mathbf n}$ and an $R$ pair both along $+\hat{\mathbf n}$. The figure shows <momentum> and <spin> arrows separately for all four channels.
\Image[/past-exam-of-the-particle-physics-course-of-the-university-of-oxford/2018-z-helicity-channels.png]
{title=Four massless <Z boson> <helicity> channels with <momentum> and <spin> directions}
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