ON-SHELL APPROACH TO PION-NUCLEON PHYSICS

We discuss an on-shell approach to pion-nucleon physics that is consis tent order by order in a 1/f(pi) expansion with the chiral reduction f ormula, crossing, and relativistic unitarity. A number of constraints between the on-shelf low-energy parameters are derived at tree level i n the presence of the pion-nucleon sigma term, and found to be in fair agreement with experiment. We analyze the nucleon form factors, and t he pi N --> pi N scattering amplitude to one-loop, as well as pi N --> pi pi N to tree level. We use the latter to derive a new constraint f or the pion-nucleon sigma term at threshold. We compare our results to both relativistic and nonrelativistic chiral perturbation theory, and discuss the convergence character of the expansion in light of experi ment.