Theory of Z boson decays

The precision data on Z boson decays from LEP-I and SLC colliders are compa red with the predictions based on the minimal standard theory. The Born app roximation of the theory is based on three most accurately known observable s: G(mu)-the four fermion coupling constant of muon decay, m(Z)-the mass of the Z boson, and alpha(m(Z))-the value of the 'running fine structure cons tant' at the scale of m(Z). The electroweak loop corrections are expressed, in addition, in terms of the masses of higgs, m(H), of the top and bottom quarks, m(t) and m(b), and of the strong interaction constant alpha(s)(m(Z) ). The main emphasis of the review is focused on the one-electroweak-loop a pproximation. Two electroweak loops have been calculated in the literature only partly. Possible manifestations of new physics are briefly discussed.