It is found that the invariance with respect to charge conjugation and charge symmetry combined leads to selection rules, additional to those following from covariance and gauge invariance arguments. Besides rigorous selection rules which hold for certain processes involving neutral particles (mesons and photons) only, there exist weak selection rules according to which certain boson-boson transitions are less probable by a factor $\frac{e^2}{\hslash c}$ than the corresponding transitions in which an additional photon is emitted. The rules are stated in Sec. III, and some examples are given.

• Received 8 April 1952

DOI:https://doi.org/10.1103/PhysRev.87.871