Measurement of the Muonium 1S-2S Transition Frequency
  A new measurement of the 1S-2S energy splitting of muonium by Doppler-free two-photon spectroscopy has been performed at the Rutherford Appleton Laboratory in Chilton, Didcot, UK. Increased accuracy is expected compared to a previous experiment. Spectroscopy of this transistion promises an improvement of the muon mass value. 
M1S2S-Collaboration ((1) U.Heidelberg,(2) U.Oxford,(3) RAL,(4) U.   Sussex,(5) ILP Novosibirsk, (6) Yale)