The weak parity-violating pion-nucleon coupling

Abstract We use QCD sum rules to obtain the weak parity-violating pion-nucleon coupling constant f πNN . We find that f πNN ≈ 2 × 10 −8 , about an order of magnitude smaller than the “best estimates” based on quark models. This result follows from the cancellation between pertubative and nonperturbative QCD processes not found in quark models, but explicit in the QCD sum rule method. Our result is consistent with the experimental upper limit found from 18 F parity-violating measurements.