The parity-nonconservation asymmetry A y in the decay of polarized 19 F* (110-keV) nuclei has been measured to high precision and is found to be A y = -(6.8 ± 1.8) x 10" 5 . Absolute uncertainties have been largely removed. This new value for A y together with recent nuclear structure calculations provides severe constraints on the weak meson-nucleon coupling constants, in particular on f" and hp.Parity nonconservation in the purely hadronic weak interaction is predicted by modern gauge theories. In the last few years, increasing experimental evidence for the validity of the GlashowWeinberg-Salam theory has been found. Weak lepton-lepton and lepton-quark interactions are well described by this model. Flavor-changing weak interactions between quarks have been extensively studied in strangeness-changing weak nonleptonic decays (AS=1), while the strangeness-conserving weak quark-quark interaction, which is found to be particularly sensitive to Z° exchange contributions, 1 is accessible to nuclear physics experiments only and is not well known at present.The parity-nonconserving weak interaction between nucleons is commonly parametrized in terms of boson exchange contributions and described by weak meson-nucleon coupling constants. Since isospin is not conserved in weak interaction processes, all possible isospin configurations yield different couplings for the same meson (e.g., hp, hp l > h* for p-nucleon weak coupling). The weak meson-nucleon coupling constants have to be calculated from basic weak interactions in conjunction with the quark model and QCD. Furthermore, for a nuclear process, nuclear structure models are needed to connect the parity-nonconserving observables to these coupling constants. Considerable progress was recently achieved in the calculation of the coupling constants by Desplanques, Donoghue, and Holstein (DDH). 1 From extensive studies of both the weak and the strong interaction in the calculation, these authors give "best values" and "reasonable ranges" for all the weak mesonnucleon coupling constants.Experimental evidence for parity nonconservation in the strong interaction first came from the circular polarization of y rays in the decay of unpolarized heavy nuclei. 2 Extremely large effects have been found recently in neutron resonance scattering. 3 Although this gives qualitative confirmation of parity mixing in the energy levels of nuclei, the complexity of the nuclear structure tends to obscure the basic features of the parity-nonconserving N-N interaction. Clearly, one should be able to determine the weak meson-nucleon coupling constants from simple systems, i.e., from two-nucleon experiments. (In fact, parity, nonconservation was found in elastic p -p scattering, whereas in n-p experiments no finite effects have been measured. 4 ) It is, however, not possible to have constraints on all weak meson-nucleon coupling constants from simple systems, and information from more complex nuclei is urgently needed.Decays of light nuclei (A = 20) are found to be a highly valuable compromise betwee...

No abstract

No abstract

No abstract

No abstract