Magnetic dipole transitions in atomic and particle physics: ions and psions

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Abstract
Reviews the theory of magnetic dipole transitions (M1 transitions) in H and H-like ions, the history of the development of interest in the analogous transition 23S1 to 11S0+ gamma in He and He-like ions is sketched and the corresponding theory is developed. Experimental values for the lifetime of the 23S1 state are now available in the helium isoelectronic sequence. A systematic comparison of the theoretical and experimental values provides a new and stringent test for quantum electrodynamics. The spectacular discovery, in 1974, of the J/ psi and psi ' particles in high-energy proton-nucleus and electron-positron collisions is reviewed and the theoretical background is given for the far-reaching hypothesis that these particles can be regarded as 3S1 bound states of a 'charmed quark' and its antiparticle. The qualitative success of this assumption, resulting from the subsequent discovery of predicted other narrow resonances in the 3-4 GeV region coupled to the J/ psi and psi ' by one-photon emission, is described and the ideas of the so-called 'naive charmonium model' are outlined. The theory of the radiative transitions of bound states of two spin-1/2 particles is then developed in some detail, with special emphasis on magnetic dipole transitions. The theory is applied to the charmonium model and the great sensitivity of the relativistic M1 decays to the form of the relativistic interaction between the quark and the antiquark is illustrated. A brief survey is given of recent attempts to overcome the deficiencies of the model.