Measurement of the weak pion nucleon coupling constant, h1(pi), from backward pion photo-production near threshold on the proton

The longest range weak pion-nucleon coupling constant, h1/n is important for nuclear parity violation. However, after considerable effort in the past two decades, its value is still poorly known largely due to many-body theoretical uncertainties. Prospects of a new measurement of h#sup 1#/#sub n# in a theoretically clean process are presented. A measurement of the parity-violating asymmetry in pion photoproduction off the proton is related to h1/n in a low-energy theorem for the photon polarization asymmetry at threshold in the chiral limit. At present two completed experiments - photon circular polarization for 18F and the anapole moment of 133Cs - have been interpreted to give very different values of h1/n. This experiment will be the first attempt to measure h1/n in the single nucleon system. A reliable measurement of h1/n provides a crucial test of the meson-exchange picture of the weak NN interaction. Such a test of the meson-exchange picture will shed light on low energy QCD.