PION-NUCLEON INTERACTION AND 2-PION-EXCHANGE 3-NUCLEON FORCES

We compare the pi N scattering amplitudes that underlie 2 pi-exchange three-nucleon forces (TBFs) with the experimental pi N amplitudes in t he form of partial-wave phase shifts and subthreshold invariant amplit udes. The amplitudes of the Tucson-Melbourne and Brazil TBFs when take n on-pion-mass-shell predict scattering lengths at threshold and phase shifts (slightly) above threshold which are in good agreement with th e experimental amplitudes, except for the S-waves. Partial wave amplit udes from separable potentials, recently employed in a 2 pi-TBF calcul ation, were continued below threshold, summed into invariant amplitude s, and compared with the experimental amplitudes in this kinematic reg ion, which is most relevant to the kinematics of TBFs. The separable-p otential invariant amplitudes, in contrast to those of TM and Brazil T BFs, do not compare well quantitatively with the experimental amplitud es in this region but have a similar qualitative behaviour. The very s mall TBF effect in the triton of the separable-potential amplitude app ears to be due to the pi NN vertex function rather than the pi N ampli tude itself.