Abstract
We discuss the relevance of the on-shell pion-nucleus coupling constants to modern theories of pion and isobar degrees of freedom in nuclei. The excitation of specific transitions is first considered and the on-shell coupling constant is shown to carry other information than the familiar Gamow-Teller constant measured in β-decay; we present a critical discussion of the scarce existing determinations which are at considerable variance with our theoretical expectations. Next, the sum rule for the “effective” pion-nucleus coupling constant is calculated and predicted to be drastically reduced by δ-hole renormalization of the pion propagator besides the Lorentz-Lorenz effect. Dispersion relations strongly favour the validity of the model.
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