Hard Three-Loop Corrections to Hyperfine Splitting in Positronium and Muonium

TL;DR

This paper calculates precise three-loop quantum corrections to hyperfine splitting in muonium and positronium, providing formulas valid for any electron-muon mass ratio and confirming previous small-mass ratio expansions.

Contribution

It offers the first comprehensive calculation of three-loop corrections without assuming a small mass ratio, applicable to both muonium and positronium.

Findings

Derived formulas for hyperfine splitting corrections valid for arbitrary mass ratios.

Confirmed consistency with previous small-mass ratio expansions for muonium.

Provided new nonlogarithmic corrections for positronium at order mα^7.

Abstract

We consider hard three-loop corrections to hyperfine splitting in muonium and positronium generated by the diagrams with closed electron loops. There are six gauge-invariant sets of such diagrams that generate corrections of order $m\alpha^7$. The contributions of these diagrams are calculated for an arbitrary electron-muon mass ratio without expansion in the small mass ratio. We obtain the formulae for contributions to hyperfine splitting that in the case of small mass ratio describe corrections for muonium and in the case of equal masses describe corrections for positronium. First few terms of the expansion of hard corrections in the small mass ratio were earlier calculated for muonium analytically. We check numerically that the new results coincide with the sum of the known terms of the expansion in the case of small mass ratio. In the case of equal masses we obtain hard nonlogarithmic corrections of order $m\alpha^7$ to hyperfine splitting in positronium.