One More Hard Three-Loop Correction to Parapositronium Energy Levels

Abstract

A hard three-loop correction to parapositronium energy levels of order $m{\alpha }^7$ is calculated. This nonlogarithmic contribution is due to the insertions of one-loop photon propagator in the fermion lines in the diagrams with virtual two-photon annihilation. We obtained $\Delta E=0.03297(2)(m{\alpha }^7/{\pi }^3)$ for this energy shift.

Comments: Version to be published in Phys. Rev.D, results unchanged