A theoretical and experimental review of the weak neutral current: A determination of its structure and limits on deviations from the minimal SU(2)L×U(1) electroweak theory

A detailed analysis of existing neutral-current data has been performed in order (a) to determine as fully as possible the structure of the hadronic and leptonic neutral currents without recourse to a specific weak-interaction model; (b) to search for the effects of small deviations from the Weinberg-Salam (WS-GIM) model; and (c) to determine the value of sin2θw as accurately as possible. The authors attempt to incorporate the best possible theoretical expressions in the treatment of each of the reactions. For deep-inelastic scattering, for example, the effects of quantum chromodynamics, including the contributions of the s and c quarks, have been included. The sensitivity of the results both to systematic uncertainties in the data and to theoretical uncertainties in the treatment of deep-inelastic scattering, semi-inclusive pion production, ν elastic scattering from protons, and the asymmetry in polarized eD scattering have been considered; the systematic errors are generally found to be smaller than the statistical uncertainties. In the model-independent analyses the authors find that the hadronic neutral-current parameters are uniquely determined to lie within a small domain consistent with the WS-GIM model. The leptonic couplings are determined to within a twofold ambiguity; one solution, the axial-vector-dominant, is consistent with the WS-GIM model. If factorization is assumed then the axial-dominant solution is uniquely determined and null atomic parity violation experiments are inconsistent with other neutral-current experiments. Within generalized SU(2)×U(1) models we find the following limits on mixing between right-handed singlets and doublets: sin2αu≤0.103, sin2αd≤0.348, and sin2αe≤0.064. Assuming these mixing angles to be zero, a fit to the most accurate data (deep-inelastic and the polarized eD asymmetry) yields ρ=0.992±0.017 (±0.011) and sin2θw=0.224±0.015 (±0.012), where ρ=MW2MZ2cos2θw and the numbers in parentheses are the theoretical uncertainties. The value of ρ is remarkably close to 1.0 and strongly suggests that the Higgs mesons occur only as doublets and singlets. If one makes this assumption, then the limit on ρ implies mL≤500 GeV, where mL is the mass of any heavy lepton with a massless partner. In addition, for ρ=1.0, the authors determine sin2θw=0.229±0.009 (±0.005). Fits which also include the semi-inclusive, elastic, and leptonic data yield very similar results. A two-parameter fit gives ρ=1.002±0.015 (±0.011) and sin2θw=0.234±0.013 (±0.009), while a one-parameter fit to sin2θw gives sin2θw=0.233±0.009 (±0.005). Finally, the authors have found no evidence for a violation of factorization or for the existence of additional Z bosons. Fits to two explicit two-boson models yield the lower limits MZ2MZ1>1.61 and 3.44 for the mass of the second Z boson. The desirability of a complete analysis of radiative and higher-order weak corrections, which have not been included in the authors' theoretical uncertainties, is emphasized.