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

Page, S. A. and Evans, H. C. and Ewan, G. T. and Kwan, S.-P. and Leslie, J. R. and MacArthur, J. D. and McLatchie, W. and Skensved, P. and Wang, S.-S. and Mak, H.-B. and McDonald, A. B. and Barnes, C. A. and Alexander, T. K. and Clifford, E. T. H. (1987) Weak pion-nucleon coupling strength: New constraint from parity mixing in 18F. Physical Review C, 35 (3). pp. 1119-1131. ISSN 0556-2813. doi:10.1103/PhysRevC.35.1119.

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The circular polarization of the 1.081 MeV γ rays from 18F has been measured to be (1.7±5.8)×10-4. 18F 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 ‖0+,T=1,1.042 MeV〉 and ‖0-,T=0,1.081 MeV〉 states in 18F that is dominated by the weak pion exchange contribution. The present result for the weak pion-nucleon coupling strength is ‖fπ1‖=(0.4-0.4+1.4)×10-7, which is significantly smaller than recent theoretical predictions based on the Weinberg-Salam model.

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Additional Information:©1987 The American Physical Society. Received 28 October 1986. The authors wish to thank A.L. Bell, H. Janzen, and R.L. Stevenson of Queen's University for their many contributions to the experiment. The assistance of J.L. Gallant of Chalk River Nuclear Laboratories in the preparation of the target foils is greatly appreciated. The work was carried out with the help of grants from the Natural Sciences and Engineering Research Council of Canada. Further assistance was provided by Queen's University. Partial support of one of us (C.A.B.) wsa provided by the National Science Foundation under Grant Nos. PHY-82-15500 and PHY-85-08562 and to another of us (A.B.M.) by Grant No. ?PHY-86-045101.
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Deposited On:01 May 2008
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