Past exam of the particle and nuclear physics course of the University of Cambridge / 2021 / Section B / 5 / e

A spin-zero pion has no preferred emission direction. Choose the antineutrino momentum along $+z$. A massless antineutrino has positive helicity, so its spin points along $+z$. The muon moves along $-z$, and total $J_z=0$ requires its spin to point along $-z$:

The reversed configuration is obtained by rotating the entire event and is equally allowed. The two spins are opposite, while each is aligned with its own particle's momentum; both outgoing helicities are positive. A negative-helicity muon with this fixed antineutrino direction would give a nonzero total spin projection and is forbidden for this two-body decay of a spin-zero parent. The required positive-helicity charged lepton is the suppressed component of the weak charged current.