Electroweak fits at LEP

Abstract

High precision electroweak measurements performed over ten years at LEP and SLC have allowed to constrain the Standard Model of electroweak interactions. The model have been used to predict the mass of the top quark and to set limits on the mass of the Higgs boson. Why electroweak fits?Electroweak fits have been extensively used at LEP and SLC for testing the Standard Model at the level of its quantum corrections, searching for deviations that may signal the presence of new physics. They have also successively predicted the top quak mass prior to its discovery and recently, allowed to set limit on the Higgs boson mass. Sensitivity to top quark and Higgs boson masses can be infered from the following exemple relation :where ∆r contains several terms: ∆α (due to light fermion masses), a negative term (several %) proportional to m 2 t and a relatively small (below 1%) term proportional to ln(m H ). Therefore, the inferred constraints on m H are much weaker than those on m t .Before LEP startup, the W and Z boson masses were measured to be 80.000 ± 0.360 GeV and 91.120 ± 0.160 GeV respectively. These measurements were equivalent to a precision on the weak mixing angle sin 2 θ W of 3.7% worse than the direct measurement itself: 0.227 ± 0.006 corresponding to 2.6%. That is the reason why inputs to the Standard Model fits were α, G F and sin 2 θ W .