Determination of the real part of the isospin-even forward-scattering amplitude of pion-nucleon scattering at 55 MeV as a test of low-energy quantum chromodynamics

The real part of the isospin-even forward-scattering amplitude of pion-nucleon scattering has been determined at a pion energy of $T_{\pi }$=55 MeV by measurement of the elastic scattering of positive and negative pions on protons within the Coulomb-nuclear interference region. The value confirms the prediction of the Karlsruhe-Helsinki phase-shift analysis for that energy. These phases have been used to determine the σ term of pion-nucleon scattering by means of dispersion relations, resulting in a value for σ which is in contradiction with chiral perturbation theory of QCD.