Measurement of the charged pion mass using X-ray spectroscopy of exotic   atoms
  The $5g-4f$ transitions in pionic nitrogen and muonic oxygen were measured simultaneously by using a gaseous nitrogen-oxygen mixture at 1.4\,bar. Due to the precise knowledge of the muon mass the muonic line provides the energy calibration for the pionic transition. A value of (139.57077\,$\pm$\,0.00018)\,MeV/c$^{2}$ ($\pm$\,1.3ppm) is derived for the mass of the negatively charged pion, which is 4.2ppm larger than the present world average. 
M Trassinelli (INSP), D.F. Anagnostopoulos (DMSE), G. Borchert (KFA),   A. Dax (PSI), J.P Egger, D. Gotta (KFA), M. Hennebac...
physics.atom-ph physics.ins-det