Muon Loop Light-by-Light Contribution to Hyperfine Splitting in Muonium
  Three-loop corrections to hyperfine splitting in muonium generated by the gauge invariant sets of diagrams with muon and tauon loop light-by-light scattering blocks are calculated. These results complete calculations of all light-by-light scattering contributions to hyperfine splitting in muonium. 
Michael I. Eides and Valery A. Shelyuto
hep-ph physics.atom-ph