Possible three-photon couplings

One of the simplest modifications to quantum electrodynamics, if charge-conjugation invariance is abandoned, is the occurrence of direct coupling of three photons. This paper generalizes and extends Dolgov's phenomenological analysis of the consequences of such interactions and confirms that gauge invariance and Bose symmetry permit such interactions only if at least two of the photons are virtual. The effects of such couplings are considered for processes in which the third photon is real. The best present limit on the possible strength of such couplings is set by experiments on bremsstrahlung from $e^{+}{e}^{-}$ collisions.