= Solution
The <Weak charged current>[charged weak current] contains $\bar\psi\gamma^\mu P_L\psi$. For an ultrarelativistic external <fermion>, it selects <Helicity>[negative helicity]; its <antifermion> partner has <Helicity>[positive helicity]. Wrong-helicity <Charged current>[charged-current] <Probability amplitudes>[amplitudes] vanish in the massless limit and are suppressed by powers of $m/E$ at finite <mass>. The active <neutrino> is consequently <Helicity>[left-helicity] and its <antineutrino> <Helicity>[right-helicity] in this limit.
This is a <Charged current>[charged-current] statement, not a prohibition on every <Helicity>[right-helicity] Standard Model <fermion>. Electromagnetic and strong vector interactions act on both <chiralities> and conserve <helicity> along a massless <fermion> line. A <Neutral current>[neutral-current] $Z$ vertex also couples to both charged-fermion <chiralities> with unequal strengths. For example, a <Helicity>[right-helicity] <electron> can scatter electromagnetically: it is not excluded from the Standard Model. In <Vector current>[vector-current] pair <annihilation> the massless <fermion> and <antifermion> have opposite <helicities>; a purely <Charged current>[charged-current] vertex keeps only the left-fermion/right-antifermion choice. <Yukawa interaction>[Yukawa interactions] connect left and right fields and must be distinguished from these <Vector current>[vector-current] selection rules.
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