The three-photon-annihilation contribution to the positronium hyperfine interval is obtained in analytic form. This contribution is of order m${\alpha }^6$, and has the value -0.969 MHz. The result of a previous calculation of this contribution is shown to be incomplete because of a subtlety in the regularization of infrared divergences.

• Received 24 December 1987

DOI:https://doi.org/10.1103/PhysRevA.37.4071