Test of isospin symmetry via low energy ${}^1$H(${\pi }^{-}$,${\pi }^o$)$n$ charge exchange

We report measurements of the ${\pi }^{-}p\to {\pi }^{o}n$ differential cross sections at six momenta (104-143 MeV/c) and four angles (0-40 deg) by detection of $\gamma$-ray pairs from ${\pi }^o\to \gamma \gamma$ decays using the TRIUMF RMC spectrometer. This region exhibits a vanishing zero-degree cross section from destructive interference between s-- and p–waves, thus yielding special sensitivity to pion-nucleon dynamics and isospin symmetry breaking. Our data and previous data do not agree, with important implications for earlier claims of large isospin violating effects in low energy pion-nucleon interactions.