Positronium and Muonium 1S-2S Laser Spectroscopy as a Probe for the   Standard-Model Extension
  We present the status and the prospects of the ongoing efforts to improve the measurements of the 1S-2S transition in positronium and muonium. These purely leptonic systems represent ideal objects to test bound-state QED to high precision and the Standard-Model Extension. 
P. Crivelli and G. Wichmann
hep-ph physics.atom-ph