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In quantum mechanics, the intrinsic parity is a phase factor that arises as an eigenvalue of the parity operation (a reflection about the origin). To see that the parity's eigenvalues are phase factors, we assume an eigenstate of the parity operation (this is realized because the intrinsic parity is a property of a particle species) and use the fact that two parity transformations leave the particle in the same state, thus the new wave function can differ by only a phase factor, i.e.: $$P^{2} \psi = e^{i \phi} \psi$$ thus $$P \psi = \pm e^{i \phi /2} \psi$$ , since these are the only eigenstates satisfying the above equation.

The intrinsic parity's phase is conserved for non-weak interactions (the product of the intrinsic parities is the same before and after the reaction), since [P,H]=0, i.e.: the Hamiltonian is invariant under a parity transformation. The intrinsic parity of a system is the product of the intrinsic parities of the particles, for instance for noninteracting particles we have $$P(|1\rangle|2\rangle)=(P|1\rangle)(P|2\rangle)$$ . Since the parity commutes with the Hamiltonian, its eigenvalue does not change with time, therefore the intrinsic parity's phase is a conserved quantity.

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