Neutral currents within beyond the standard model

Abstract

We present new, model-independent analyses of the observables in low-energy neutral-current experiments in the neutrino-hadron, neutrino-electron, electron/muon-hadron and electron- muon/tau sectors. We combine them with the latest experimental measurements of the W and Z masses to make global fits in the context of the standard model and of superstring-inspired models. We find in the standard model that sin 2θ w = 0.228 ± 0.004 if ϱ is fixed to be 1, whilst a two parameter fit yields sin 2θ w = 0.229 ± 0.006 and ϱ = 1.001 ± 0.007. This new value of sin 2θ w for ϱ = 1 lies uncomfortably outside the predicted region of the minimal SU(5) model, but is consistent with supersymmetric SU(5) models. In the minimal standard model with ϱ = 1 and equal Higgs and Z masses we find that mt<168 GeV at the 90% confidence level. We establish lower bounds on the masses of additional neutral gauge bosons Z′ in three superstring-inspired models and upper limits on their mixing with the standard model Z 0. In particular, we find that mZ′129 GeV at the 90% confidence level in a minimal rank-5 superstring-inspired model with canonical Higgs structure.