Measurement of the weak neutral current via nuclear parity violation

Present low-energy experiments which detect parity violation in nuclei are analyzed, and a consistent set of weak couplings is extracted. Using symmetries and the quark model, it is shown that a parity-violating weak-neutral-current contribution is required by the data, and we extract values ${\sin }^2$ ${\mathrm{\theta }}_{\mathrm{W}}$=0.0±0.4 or ρ=1.1±0.2, consistent with other determinations. The error quoted is statistical only, and does not include a possibly sizable theoretical uncertainty. On the other hand, if we assume the world-average values for ρ and ${\theta }_W$, we can obtain from the data (with minimal quark-model assumptions) the matrix element of an operator which is closely related to operators involved in the so-called ‘‘penguin’’ diagrams of ΔS=1 hyperon decay. The matrix element so obtained is consistent with that calculated in a modified form of the vacuum-saturation technique, and has a magnitude such that the penguin contributions to hyperon decays are moderate but not dominant.