Investigation of nuclear charge symmetry by pion elastic scattering from H3 and He3

We have measured differential cross sections for pion elastic scattering from H-3 and He-3 in the angular region near the minimum in the non-spin-flip amplitude. Data were acquired for incident pion energies of 180, 220, 256, and 295 MeV. Nuclear charge symmetry is investigated with the aid of several charge-symmetric ratios formed from combinations of measured cross sections. A particularly intriguing result is obtained from the superratio R, which is defined as R=d sigma(pi(+3)H)d sigma(pi(-3)H)/d sigma(pi(+3)He)d sigma(pi(-3)He). R is found to be greater than unity at 180 MeV and significantly smaller than unity at 256 MeV, with the transition occurring at around 210 MeV. The charge-symmetry prediction for this ratio (after allowance for the Coulomb force) is one, and is independent of energy and angle.