S-wave pion-nucleus dynamics in the sigma + ω model

The sigma + ω model of nuclear forces has many desirable properties, including approximate chiral symmetry (partial conservation of axial vector current) and renormalizability, and it gives a good account of nuclear structure. It is therefore a useful framework within which to explore the modification of pion-nucleon dynamics in a nuclear medium, a problem which resists the usual soft-pion current-algebra cum analyticity techniques owing to the presence of anomalous thresholds (nuclear structure effects). It is found that through order G4 (for pion elastic scattering) and G3 (for pion absorption or emission) the chiral invariance ensures sufficient cancellations between otherwise large terms so as to produce little renormalization of free-particle dynamics. Moreover, the pseudoscalar-coupled sigma + ω model is equivalent in pion emission or absorption, through O (G3), to a pseudovector-coupled phenomenological model (with no π-sigma interaction, but with a form of partially conserved axial vector current). Thus, the question of whether one can distinguish experimentally between pseudoscalar and pseudovector πNN coupling is probably meaningless