Landau–Yang theorem
In quantum mechanics, the Landau–Yang theorem is a selection rule for particles that decay into two on-shell photons. The theorem states that a massive particle with spin 1 cannot decay into two photons. Assumptions. A photon here is any particle with spin 1, without mass and without internal degrees of freedom. The photon is the only known particle with these properties. Consequences. The theorem has several consequences in particle physics. For example: