It has been well established1 that the muon possesses characteristic Bohr orbits of its own around the nucleus and that the trapping of the muon into these orbits via ordinary atomic interaction is the precursor of any specific reaction with the nucleus. It is also known from the early experiments of Conversi, Pancini, and Piccioni2 that in light elements a negative μ-meson in the K-orbit generally decays before it is captured by the nucleus. Only for Z > 10 does capture become more probable than decay. Thus, the experiments become not only difficult but also unreliable for low Z(Z < 10), because the free μ-decay rate is then greater than the μ-capture rate, and, in fact, one often measures the sum of the two rates. So the heavier nuclei are favored for the study of muon capture by the experimentalists, but the theoretical study is rendered difficult by the complexity of the structure of the heavier nuclei. The interaction Hamiltonian of the basic elementary processes μ - + p → n + ν is not yet well known. Although the universal Fermi interaction is assumed to be operative in the weak processes such as β-decay, μ-decay, and μ-capture, only the μ-decay is free from the effects of strong interactions; μ-capture is affected by the presence of strong interaction currents3 which considerably alter the structure of the interaction Hamiltonian. The fundamental universal “bare” four-fermion interaction, which is assumed to be a V-A interaction, is modified by the effects of strong interaction to yield other types of interactions, namely, pseudoscalar, scalar, tensor, and weak magnetism.
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