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:
where ηG = ±1 are the eigenvalues of G-parity. The G-parity operator is defined as
where is the C-parity operator, and is the operator associated with the 2nd component of the isospin "vector", which in case of isospin takes the form , where is the second Pauli matrix. G-parity is a combination of charge conjugation and a π rad (180°) rotation around the 2nd axis of isospin space. Given that charge and isospin are preserved by strong interactions, so is G. Weak and electromagnetic interactions, though, does not conserve G-parity.
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
In general
where ηC is a C-parity eigenvalue, and I is the isospin.
Since no matter whether the system is fermion–antifermion or boson–antiboson, always equals to , we have