Muonium vacuum-state

Here the muonium hyperfme constant, relative to the vacuum-state value, is plotted versus electron affinity for a variety of materials, from insulators... 

The hyperfine constant for the free-atom or vacuum-state muonium is, 4) = 4.46 GHz. This is approximately equal to the value for hydrogen, when scaled by the ratio of magnetic moments of the muon and the proton, i.e. 3.18. This implies essentially the same electron spin density at the nucleus for muonium and hydrogen, providing further justification for considering muonium as an isotope of hydrogen. 

The time window can be extended to even shorter times if there is a muonium precursor state. Evolution of spin polarization in Mu occurs partly at frequencies near the Mu hyperfine frequency (4463 MHz in vacuum, broken arrows in Figure 2), which sets the timescale for loss of phase coherence during formation of the observed muonated species. The formation process can be studied indirectly in transverse fields by interpretation of shifts in the initial phase and concomitant loss of amplitude. Thus, processes occurring on a timescale down to 10 ps can be analysed, but the results rely on the validity of the underlying model. 

The dominant interaction within the muonium atom is electromagnetic. This can be treated most accurately within the framework of bound state Quantum Electrodynamics (QED). There are also contributions from weak interaction which arise from Z°-boson exchange and from strong interaction due to vacuum polarization loops with hadronic content. Standard theory, which encompasses all these forces, allows to calculate the level energies of muonium to the required level of accuracy for all modern precision experiments1. 

The 1S-2S transition in muonium has also been measured by laser spectroscopy. The transition is induced by a two-photon Doppler-free process and detected through the subsequent photoionization of the 2S state in the laser field. The key to success in this experiment was the production of muonium into vacuum from the surface of heated W or of Si02 powder. The discovery experiment(33) was done at the KEK facility in Japan with a pulsed muon beam and an intense pulsed laser system. A subsequent experiment(34) done with the pulsed beam at RAL and a similar pulsed laser has improved the signal substantially and has achieved a a precision of about lO" in the 1S- 2S interval, thus determining the Lamb shift in the IS state to about 1% accuracy (Fig. 22). The precision of this experiment should be greatly improved in a new experiment now underway at RAL. This experiment will provide a precise... 

The very precise measurement of the ground state hyperfine structure (hfs) is described. A new successful technique for producing muonium in vacuum has been developed and possible future experiments using this technique are presented in the second part. 

The fact that y e can now be studied in vacuum is very important for a number of fundamental experiments. One is the measurement of the Lamb shift in the first excited state of muonium (Fig.5). The beam-foil method produces not only the n=l state but excited states as well l with a probability roughly as l/n. We expect that 15% of the muonium formed is in the 2S state. 

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