Parity of a State

the parity of an energy state of a physical system, that is, the parity of a wave function. Such a characteristic of energy states is possible for a system of particles in which electromagnetic or nuclear forces that conserve parity act between the particles.

When weak interactions are taken into account, a state with a given parity acquires a small admixture of the opposite parity. In atomic nuclei, the relative magnitude of the admixture is of the order of 10^-6–10^-7.

If an energy state is degenerate, so that wave functions with different parities are associated with the state (as is the case with, for example, excited states of the hydrogen atom), states described by the superposition of such wave functions are possible; that is, the degenerate state may not have a definite parity, even if the forces acting in the system conserve parity.