Some problems of the physics of exotic atoms

The current status of the physics of exotic atoms is reviewed. The problem of the anomalous width of the kaonic helium 2P-state is analyzed, experimental CPT invariance tests for the antihydrogen atom are discussed, and results on the metastability of the antiproton (as well as of pi (-), mu (-), K-, Sigma (-) etc.) in helium are considered. A comparison is made of the well-known Kondo model and the ionic model. In the latter, the ion ((p) over bar alpha ee) - the so-called 'ioncule' - forms after (p) over bar is stopped in its motion in helium. One of its electrons is weakly bound, occupies one of the Efimov levels, and orbits at similar to 20 a.u. in the dipole field of the neutral 'atomcule' ((p) over bar alphae). The lifetime of the ioncule is long (>10(-5) s) and the antiproton's vibrational-rotational transition frequencies omega are practically equal to the atomcule value (Delta omega/omega similar to 10(-5)-10(-6)) - in agreement with laser-spectroscopy data. While ionic model results on the cross-sections of metastability quenching by impurity atoms and molecules in helium agree well with experiment, the atomcule-based Kondo model leads to a 3-4 orders of magnitude underestimate. The ionic model is also in agreement with some other available data. Direct experimental tests of this model are discussed. A new interpretation of E Zavattini group's laser spectroscopy results on the 2s-2p-splitting in muon helium is presented.