G-parity selection rule for pion multiplicities
= G-parity selection rule for pion multiplicities
{c}
Each <pion> has <G parity> minus one, so an $n$-pion state has $G=(-1)^n$. In the isospin-symmetric <strong interaction>, a parent with positive G parity can decay only to an even pion multiplicity, and a negative parent only to an odd multiplicity. The rule does not forbid small isospin-breaking or electromagnetic amplitudes.