A hard three-loop correction to parapositronium energy levels of order mα7 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 ΔE = 0.03297(2)(mα7 / π3) for this energy shift.
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This work was supported by the NSF Grant No. PHY-1402593. The work of V. S. was also supported in part by the Russian Foundation for Basic Research Grant No. 16-02-00042.
Eides, Michael I. and Shelyuto, Valery A., "One More Hard Three-Loop Correction to Parapositronium Energy Levels" (2017). Physics and Astronomy Faculty Publications. 526.