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 $2\mathrm p$-state is analyzed, experimental CPT invariance tests for the antihydrogen are discussed, and results of investigations 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 Condo model and the ionic model. In the latter, the ion $(\bar{\mathrm p}\alpha\mathrm e\mathrm e)$ – a so-called 'ioncule' – forms after $\bar{\mathrm p}$ is stopped in its motion in helium. One of the ioncule's electrons is weakly bound, resides at one of the Efimov levels, and orbits at $\sim20$ a.u. in the dipole field of the neutral 'atomcule' $(\bar{\mathrm p}\alpha\mathrm e)$. The lifetime of the ioncule is long ($\ge 10^{-5}$ s) and the antiproton's vibrational-rotational transition frequencies $\omega$ are practically equal to the atomcule value ($\Delta\omega/\omega\sim10^{-5}$–$10^{-6}$) – in agreement with laser-spectroscopy data. While the ionic model results on the quenching cross sections of metastable states by impurity atoms and molecules in helium agree well with experiment, the atomcule-based Condo model underrates the cross sections by $3$–$4$ orders of magnitude. The ionic model is also consistent with other experiments. Direct experimental tests of this model are discussed. A new interpretation of E Zavattini's group's laser-spectroscopy results on the $2\mathrm s$–$2\mathrm p$-splitting in muonic helium is presented.