Calculation of the positronium hyperfine interval using the Bethe-Salpeter formalism

Abstract
We use a variation of the Bethe-Salpeter equation to complete the calculation of the one-photon annihilation contribution to the hyperfine interval of positronium at order mα6. Our results are in accord with a quite different calculation independently done using an effective field theory approach. This completes the evaluation of all the mα6 terms. We give the total theoretical value for this interval and compare with experiment.