G-parity

In particle physics, G-parity is a multiplicative quantum number that results from the generalization of C-parity to multiplets of particles.

C-parity applies only to neutral systems; in the pion triplet, only π0 has C-parity. On the other hand, strong interaction does not see electrical charge, so it cannot distinguish amongst π+, π0 and π−. We can generalize the C-parity so it applies to all charge states of a given multiplet: \hat\mathcal G \begin{pmatrix} \pi^+ \\ \pi^0 \\ \pi^- \end{pmatrix} = \eta_G \begin{pmatrix} \pi^+ \\ \pi^0 \\ \pi^- \end{pmatrix} where ηG = ±1 are the eigenvalues of G-parity. The G-parity operator is defined as \hat\mathcal G = \hat\mathcal C \, e^{(i \pi \hat I_2)}