An inconsistency in low-energy pion physics

Abstract

Results are reported of measurements of charge-exchange scattering of negative pions by protons, and of elastic scattering of negative pions by protons, at a mean energy of 39 MeV. These results, combined with those obtained from an earlier experiment at 35 MeV, reveal an inconsistency between the hypothesis of charge independence and measured values of the s-wave scattering lengths obtained from elastic scattering and from charge-exchange scattering.