Weak pion-nucleon coupling strength: New constraint from parity mixing in /sup 18/F

The circular polarization of the 1.081 MeV γ rays from /sup 18/F has been measured to be (1.7 +- 5.8) x 10/sup -4/. /sup 18/F was produced by a 4.05 MeV 3He+ beam impinging on a recirculating water target with a thin entrance window. The circular polarization was measured with four magnetic transmission polarimeters, each backed by a 150 cm3 n-type Ge detector. A fast data acquisition system limited overall peak losses to 30% at a count rate of 60 kHz per detector. Supplementary experiments were carried out to investigate the sensitivity of the circular polarization measurement to systematic variations of beam position and intensity. These effects were found to be negligible relative to the statistical uncertainty. The experimental result is interpreted in terms of an isovector parity-nonconserving matrix element between the chemical bond0+,T = 1,1.042 MeV> and chemical bond0-,T = 0,1.081 MeV> states in /sup 18/F that is dominated by the weak pion exchange contribution. The present result for the weak pion-nucleon coupling strength is chemical bondf/sub π/1chemical bond = (0.4/sub -0.4//sup +1.4/) x 10/sup -7/, which is significantly smaller than recent theoretical predictions based on the Weinberg-Salam model