A new experiment has been set up at the Paul Scherrer Institut to search for muonium to antimuonium conversion. No event was found to fulfil the requested signature which consists of the coincident detection of both constituents of the antiatom in its decay. Assuming an effective (V - A) x (V - A) type interaction an improved upper limit is established for the conversion probability of P() less than or equal to 8 x 10(-9) (90% C.L.), which is almost 2 orders of magnitude lower compared to previous results and provides a sensitive test for theoretical extensions of the standard model.

Doppler-free two-photon laser spectroscopy has been employed to measure the 12S12−22S12 transition in the muonium atom (μ+e−). A value of 2 455 529 002(33) (46) MHz has been obtained, which agrees with QED calculations within two standard deviations. The Lamb shift contributions are tested to the level 8×10−3. The corresponding measurements in hydrogen and deuterium using the same apparatus and laser system provide a test of the applied systematic corrections and have verified the systematic error of 46 MHz quoted. The mass of the positive muon has been derived from the isotope shift in this transition and yields a value of 105.65880(29)(43) MeVc2.

A measurement of the 1S-2S energy splitting by Doppler-free two-photon spectroscopy yields a value of 2 455 529 002 (33)(46) MHz, which agrees with QED theory within two standard deviations. The mass of the positive muon is derived as 105.658 80 (29) (43) MeV/c/sup 2/ from the isotope shifts in this transition to hydrogen and deuterium.<>

In the standard model lepton number violating processes are forbidden, but are allowed in many extensions to it. Muonium to antimuonium conversion has been explicitly allowed in the framework of left‐right‐symmetric models or in supersymmetric models. A new experiment at PSI has demonstrated its ability to improve the sensitivity to such a rare process significantly.

The method of Doppler-free two-photon laser spectroscopy is an ideal method for studying transitions between the 1S and 2S states in one-electron systems. An experiment at the Rutherford Appleton Laboratory has observed an unambiguous signal in the exotic muonium (mu+e-) atom. The transition frequency is DELTAnu1S-2S = 2455529002(33)(46) MHz, where the first error is of a statistical nature and the second one arises from systematic corrections, which mainly are related to the properties of the high-power pulsed laser system. There is agreement with a prior, less accurate, independent experiment at KEK and with QED theory within two standard deviations. The Lamb shift contributions are tested at the 8 x 10(-3) level. The muonium-hydrogen and muonium-deuterium isotope shifts of the 1S-2S transition have a high potential for a precise mass determination of the positive muon.