Weak pion-nucleon coupling strength: New constraint from parity mixing inF18

The circular polarization of the 1.081 MeV γ rays from ${}_{}{}^{18}{}_{}{}^{}\mathrm{F}$ has been measured to be (1.7±5.8)×${10}^{\mathrm{-}4}$. ${}_{}{}^{18}{}_{}{}^{}\mathrm{F}$ was produced by a 4.05 MeV ${}_{}{}^3{}_{}{}^{}$ ${\mathrm{He}}^{+}$ 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 ${\mathrm{cm}}^3$ 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 ‖$0^{+}$,T=1,1.042 MeV〉 and ‖$0^{\mathrm{-}}$,T=0,1.081 MeV〉 states in ${}_{}{}^{18}{}_{}{}^{}\mathrm{F}$ that is dominated by the weak pion exchange contribution. The present result for the weak pion-nucleon coupling strength is ‖$f_{\pi }^1$‖=(0.$4_{-0.4}^{+1.4\mathrm{}}$)×${10}^{-7}$, which is significantly smaller than recent theoretical predictions based on the Weinberg-Salam model.