Are there anomalous Z fermion couplings ?

Abstract

The couplings of the fermions to the Z boson are of great importance in establishing the validity of the Standard Model and in looking for physics beyond it. The couplings of the b-quark to the Z boson have been the subject of much experimental study and theoretical interpretation. The apparent excess in the value of $\Rbbz$, the ratio of the partial width of the Z boson to $\bbbar$ to its total hadronic width, above the Standard Model expectation reported a few years ago has now become much less significant. However, the measurements of the pole forward-backward asymmetry $\Afbzb$ for b-quarks at the Z pole and of the {\it polarisation parameter} $\Ab$, obtained using a polarised electron beam, have improved considerably in accuracy. The latest data are examined and values of the vector and axial-vector b-quark and c-quark couplings to the Z are extracted. The left and right handed couplings are also extracted. It is found that whereas the c-quark couplings are compatible with the Standard Model, those of the b-quark data are only compatible with the Standard Model at about the 1% level. In addition, the individual lepton couplings are extracted and the hypothesis of {\it lepton universality} is examined. The sensitivity of the limits from electroweak fits to the Higgs boson mass to these data is examined.