Isospin mixing of the 1 + doublet in 12C determined by low-energy pion scattering

Abstract

With the Low Energy Pion Spectrometer (LEPS) at PSI the inelastic scattering of 50 MeV positive and negative pions to the 1 + excitations at 12.71 MeV ( T = 0) and 15.11 MeV ( T = 1) in 12C has been measured in the angular range 35°−105°. The angular distributions are in good agreement with impulse approximation calcculations. From the double ratio of the 1 + cross sections for π + and π − scattering an isospin mixing matrix element of H 01 = 157 ± 35 keV is derived. In contrast to a previous study of this reaction these results do not question the validity of the impulse approximation at low energies.