Three-Photon Decay of PositroniumS1State as a Test of Charge Conjugation Invariance

In order to make comparisons with a possible experiment to test charge conjugation invariance, the angular distribution of decay photons is estimated for the forbidden $3\gamma$ decay of the ${}_{}{}^1{}_{}{}^{}S$ state of positronium using the simplest matrix element satisfying Lorentz, gauge, and $\mathrm{TCP}$ invariance.