One-photon spin-zero transition prohibition
= One-photon spin-zero transition prohibition
A spin-zero particle cannot decay into a spin-zero particle and one on-shell real <photon>. In the parent rest frame the two final <helicities> must be equal for total <angular momentum> zero, whereas the <photon> has <helicity> $\pm1$ and the scalar has <helicity> zero. The prohibition is rotational and independent of charge-conjugation conservation. An off-shell <photon> producing additional particles is a different process.