ASPECTS OF CHIRAL PION-NUCLEON PHYSICS

The next-to-leading order chiral pion-nucleon Lagrangian contains seve n finite low-energy constants. Two can be fixed from the nucleon anoma lous magnetic moments and another one from the quark mass contribution to the neutron-proton mass splitting. We find a set of nine observabl es, which to one-loop order only depend on the remaining four dimensio n-two couplings. These are then determined from a best fit. We also sh ow that their values can be understood in terms of resonance exchange related to Delta excitation as well as vector and scalar meson exchang e, Ln particular, we discuss the role of the fictitious scalar-isoscal ar meson. We also investigate the chiral expansion of the two P-wave s cattering volumes P-1(-) and P-2(+) as well as the isovector S-wave ef fective range parameter b(-). The one-loop calculation is in good agre ement with the data. The difference P-1(-) - P-2(+) signals chiral loo p effects in the pi N P-waves. The calculated D- and F-wave threshold parameters compare well with the empirical values. (C) 1997 Elsevier S cience B.V.