The weak parity-violating pion-nucleon coupling

We use QCD sum rules to obtain the weak parity-violating pion-nucleon coupling constant fπNN. We find that fπNN ≈ 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 18F parity-violating measurements.