Meson-exchange model for πN scattering and γN→πN reaction

Phys. Rev. C 54, 2660 – Published 1 November 1996
T. Sato and T.-S. H. Lee


An effective Hamiltonian consisting of bare Δ⇆πN, γN vertex interactions and energy-independent meson-exchange πN⇆πNN transition operators is derived by applying a unitary transformation to a model Lagrangian with N,Δ,π, ρ, ω, and γ fields. With appropriate phenomenological form factors and coupling constants for ρ and Δ, the model can give a good description of πN scattering phase shifts up to the Δ excitation energy region. It is shown that the best reproduction of the recent LEGS data of the photon-asymmetry ratios in γpπ0p reactions provides rather restricted constraints on the coupling strengths GE of the electric E2 and GM of the magnetic M1 transitions of the bare Δ⇆γN vertex and the less well-determined coupling constant gωNN of ω meson. Within the ranges that GM=1.9±0.05, GE=0.0±0.025, and 7≤gωNN≤10.5, the predicted differential cross sections and photon-asymmetry ratios are in an overall good agreement with the data of γpπ0p, γpπ+n, and γnπ-p reactions from 180 MeV to the Δ excitation region. The predicted M1+ and E1+ multipole amplitudes are also in good agreement with the empirical values determined by the amplitude analyses. The constructed effective Hamiltonian is free of the nucleon renormalization problem and hence is suitable for nuclear many-body calculations. We have also shown that the assumptions made in the K-matrix method, commonly used in extracting empirically the γN→Δ transition amplitudes from the data, are consistent with our meson-exchange dynamical model. It is found that the helicity amplitudes calculated from our bare γN→Δ vertex are in good agreement with the predictions of the constituent quark model. The differences between these bare amplitudes and the dressed amplitudes, which are closer to the empirical values listed by the Particle Data Group, are shown to be due to the nonresonant meson exchange mechanisms. Within the range 7≤gωNN≤10.5 of the ω meson coupling favored by the data of the photon-asymmetry ratios in γpπ0p reactions, our values of the E2/M1 ratio for the γN→Δ transition are (0.0±1.3)% for the bare vertex and (-1.8±0.9)% for the dressed vertex. © 1996 The American Physical Society.


  • Received 7 June 1996
  • Published in the issue dated November 1996

© 1996 The American Physical Society

Authors & Affiliations

T. Sato and T.-S. H. Lee

  • Physics Division, Argonne National Laboratory, Argonne, Illinois 60439-4843 Department of Physics, Osaka University, Toyonaka, Osaka 560, Japan

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