Steve Kettell

The $\rm(2\sp2 S\sb{1/2}$-$\rm 2\sp2 P\sb{3/2})$ fine structure transition in muonium has been observed for the first time. The measured value is 9895$\sbsp{-30}{+35}$ MHz. This measurement, when included with the theoretical value for the $\rm 2\sp2 P\sb{1/2}$-$\rm 2\sp2 P\sb{3/2}$ fine structure interval, gives a value for the Lamb shift $\rm(2\sp2 S\sb{1/2}$-$\rm 2\sp2 P\sb{1/2})$ independent of previous direct measurements. From the theoretical value for the fine structure interval, 10921.833(3) MHz (Lep84), the value for the Lamb shift determined from this experiment is 1027$\sbsp{-35}{+30}$ MHz and is in agreement with the prediction of quantum electrodynamics (QED) of 1047.5(3) MHz (Eri88). Previous experimental values for the Lamb shift $\rm (2\sp2 S\sb{1/2}$-$\rm 2\sp2 P\sb{1/2})$ in muonium are 1042$\sbsp{-23}{+21}$ MHz (Woo89) and 1070$\sbsp{-15}{+12}$ MHz (Ora84a). Combining this result with these previous results gives a new experimental value of 1058$\sbsp{-12}{+10}$ for the Lamb shift in muonium. Muonium, the bound state of two structureless leptons $\rm (\mu\sp+ e\sp-),$ is an ideal system for testing bound state quantum electrodynamics (QED) because of the lack of hadronic structure as exists in the hydrogen system.

The measurement makes use of the technique of atomic beam microwave spectroscopy. Muonium atoms $\rm (\mu\sp+ e\sp-)$ in the 2S state are produced by the beam-foil technique at the Clinton P. Auderson Meson Physics Facility (LAMPF) with a low momentum, sub-surface muon beam. A variable frequency microwave field is applied to drive the atoms from the 2S to the 2P states, with the subsequent observation of the Lyman alpha photon from the decay of the 2P state to the 1S ground state. The frequency is varied from 9.0-11.0 GHz, driving the F = 0 $\to$ F = 1, F = 1 $\to$ F = 1 and F = 1 $\to$ F = 2 transitions.