Tests of the Electroweak Theory at LEP

Abstract

independent search for new physics.the Standard Model multiplet structure and an analysis in a framework which provides a model constraints on Standard Model parameters and on new particles, the sensitivity to deviations from so far. \Ve present several analyses which reveal specific aspects of the results, such as their of t.hese measurements a.nd their theoretical parametrization and summarize the results available charge asymmetry and of final state radiation in hadronic events. \Ye discuss experimental aspects partial widths and forward-backward asymmetries of heavy quark flavours, of the inclusive q § and leptonic cross sections, leptonic forward-backward asymmetries, T polarization asymmetries, LEP offers a rich choice of tests of the electroweak t.heorv such as the measurement of hadronic provide a window to new physics in an energy domain which is beyond the reach of present or even the with masses far beyond the range of direct production. Their precise measurement might therefore contrast to pure QED. radiative corrections in the electroweak theory are also sensitive to particles at the Z0 resonance can be compared with tests of QED like those of the g -2 experiments. In probe tiny modifications to the tree level processes. The electroweak precision tests in e+e" collisions to extend these tests of the theory to a variety of aspects inherent to its quantum structure and to colliders LEP and SLC The analysis of e+e" collisions at LEP and SLC offers a unique opportunity to establish the theory in considerable detail. well before the start of Z0-pole physics at the e+e" ofthe neutral current in neutrino scattering experiments Since then many experiments contributedThe first experimental success of the Standard Model of electroweak interactions [1] was the discovery

Two Higgs Doublet Model and model independent interpretation of neutral Higgs boson searches

“…In the region m S < 30 GeV the high energy data (LEP2) have little exclusion power while for m S > 50 GeV the Z 0 data (LEP1) contribute little to the determination of the experimental limit. The s 2 limit is calculated only for m S ≥ 5 GeV, since below this mass value the direct search rapidly loses sensitivity and the limit is extracted by a different method [40], which makes use of the electroweak precision measurements of the Z 0 width and provides an s 2 limit of about 0.5 × 10 −2 . The limit on m S for s 2 = 1 assuming SM branching ratios is 91 GeV in complete agreement with the result obtained by the SM search [30] at √ s≈ 189 GeV.…”

“…This limit translates directly into an upper limit on the 2HDM(II) production cross-section for m h below 9.5 GeV. Another powerful experimental constraint on extensions of the SM is the determination of the total width of the Z 0 boson at LEP [40]. Any possible excess width obtained when subtracting the predicted SM width from the measured Γ Z value can be used to place upper limits on the cross-section of Z 0 decays into final states with h 0 and A 0 bosons [43].…”

The epsilon variables for electroweak precision tests: a reappraisal

“…The minimum amount of model dependence is introduced by including other purely leptonic quantities at the Z pole such as A τ pol , A e (measured [4] from the angular dependence of the τ polarisation) and A LR (measured by SLD [18]). At this stage, one is simply relying on lepton universality.…”

“…Note that this is true for whatever For a global fit of all high energy data we consider m W /m Z , Γ T , R h , σ h , R bh and x=g V /g A given in table 1.The relations between these quantities and the epsilons, valid in any model of the assumed type, are given in eqs.1a,3a,4,5. For LEP data, we have taken the correlation matrix for Γ T , R h and σ h given by the LEP experiments [4], while we have considered the additional information on R bh and x as independent. We obtain (SLD is also included): At this stage, the epsilons have acquired also a dependence on α s (m Z ).…”

“…These important goals of the electroweak precision tests are indeed possible in a near future. [28] 0.8798 + _ 0.0020 α s (m Z ) [19] 0.118 + _ 0.007 Table 1 Summary of the data [4] used in the present paper Table 2 Values of the epsilons in the Standard Model as functions of m t and m H as obtained from recent versions of ZFITTER [13] and TOPAZ0 [14]. These values are obtained for for α s (m Z )=0.118, α(m Z )=1/128.87 but are essentially independent of these input parameters.…”