Chiral unitary meson-baryon dynamics in the presence of resonances: elastic pion-nucleon scattering

We develop a novel approach to chiral meson-baryon dynamics incorporating u nitarity constraints and explicit resonance fields. It is based on the most general structure of any pion-nucleon partial wave amplitude neglecting th e unphysical cuts as derived from the N/D method. This amplitude is then ma tched to the one-loop heavy baryon chiral perturbation theory result at thi rd order and to tree level exchanges of baryon- and meson-states in the s, t and u channels. This generates the left-hand cuts. The unitarization proc edure does not involve form factors or regulator functions. The resonance p arameters are determined from fits to the S- and P-wave pion-nucleon partia l wave amplitudes for energies up to 1.3 GeV. In particular, the Delta(1232 ) is accurately reproduced whereas scalar and vector meson couplings are le ss precisely pinned down. We also obtain a satisfactory description of the pi N threshold parameters. Further extensions of this method to coupled cha nnels and the three-flavor case are briefly discussed. (C) 2000 Elsevier Sc ience B.V. All rights reserved.