We have analyzed low-energy pion-nucleon data for isospin invariance by comparing chargeexchange amplitudes derived from charge-exchange data with those predicted from recent~p elastic data through the application of isospin invariance. A discrepancy of the order of 7% is observed beyond the contributions of the~p Coulomb interaction and the hadronic mass differences. PACS numbers: 13.75.Gx, 24.80.Dc, 25.80.Gn fcEx = Of the eight reactions of the form~N~' N' the only three directly accessible experimentally are those with charged pion beams and proton targets: 7Tp elasticscattering, with amplitudes f, and 7r p~~" n chargeexchange (CEX) scattering, with amplitude fcEx. Isospin conservation gives a relationship among these three amplitudes, the triangle identity

An analysis of low-energy charged pion-nucleon data from recent π ± p experiments is presented. From the scattering lengths and the GMO sum rule we find a value of the pion-nucleon coupling constant of f 2 = 0.0764 ± 0.0007. We also find, contrary to most previous analyses, that the scattering volumes for the P31 and P13 partial waves are equal, within errors, corresponding to a symmetry found in the hamiltonian of many theories. For the potential models used, the amplitudes are extrapolated into the subthreshold region to estimate the value of the Σ-term. Off-shell amplitudes are also provided.

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