In theoretical 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:
where ηG = ±1 are the eigenvalues of G-parity. The G-parity operator is defined as
where
Since G-parity is applied on a whole multiplet, charge conjugation has to see the multiplet as a neutral entity. Thus, only multiplets with an average charge of 0 will be eigenstates of G, that is
(see Q, B, Y).
In general
where ηC is a C-parity eigenvalue, and I is the isospin. For fermion-antifermion systems, we have
where S is the total spin, L the total orbital angular momentum quantum number. For boson–antiboson systems we have