Lepton Forward-backward Asymmetry Research Articles

Tests of the Electroweak Theory at LEP

LEP offers a rich choice of tests of the electroweak theory such as the measurement of hadronic and leptonic cross sections, leptonic forward-backward asymmetries, τ polarization asymmetries, partial widths and forward-backward asymmetries of heavy quark flavours, of the inclusive qq charge asymmetry and of final state radiation in hadronic events. We discuss experimental aspects of these measurements and their theoretical parametrization and summarize the results available so far. We present several analyses which reveal specific aspects of the results, such as their constraints on Standard Model parameters and on new particles, the sensitivity to deviations from the Standard Model multiplet structure and an analysis in a framework which provides a model independent search for new physics.

Improved measurements of the neutral current from hadron and lepton production at LEP

We present an update with increased statistics to our published analysis of hadronic and leptonic cross sections and of the leptonic forward-backward asymmetries ine + e − collisions. The published results were based on a total 454 000 hadronic and 58 000 leptonic events. This analysis adds 733 000 hadronic and 88 000 leptonic events recorded at theZ 0 peak in 1992 by the OPAL experiment at LEP. A model independent analysis ofZ 0 parameters based on an extension of the improved Born approximation leads to tests of lepton universality and gives an interpretation of the results within the Standard Model framework. We also present a model independent test for new physics.

Precision measurements of the neutral current from hadron and lepton production at LEP

New measurements of the hadronic and leptonic cross sections and of the leptonic forward-backward asymmetries ine+e− collisions are presented. The analysis includes data recorded up to the end of 1991 by the OPAL experiment at LEP, with centre-of-mass energies within ±3 GeV of the Z0 mass. The results are based on a recorded total of 454 000 hadronic and 58 000 leptonic events. A model independent analysis of Z0 parameters based on an extension of the improved Born approximation is presented leading to test of lepton universality and an interpretation of the results within the Standard Model framework. The determination of the mass and width of the Z0 benefit from an improved understanding of the LEP energy calibration.

Constraints on physics in and beyond the Minimal Standard Model from LEP experiments

The results of fits to the LEP data, taken up to the end of 1990, on cross-sections for hadron and lepton final states and lepton forward-backward charge asymmetries are presented. The fits are first of all performed to extract the parameters of the improved Born approximation. Within the context of the Minimal Standard Model (MSM), the constraints on αs,m t andm H from these LEP data, and with the inclusion of other LEP and electroweak data, are examined. The results favour a top-quark mass in the region of about 140 GeV, and a relatively light Higgs mass. However, the upper limit onm H from such fits is sensitive to the value of the strong coupling constant $$\alpha _s (M_Z )_{\overline {MS} } $$ . Some differences between the data and the MSM are observed, although these are not yet at a level of significance to indicate physics beyond the MSM. Lower limits on the mass of a possible secondZ 0 are examined. Results are then presented in terms of the variablesS, T andU. These variables provide a general framework for the description of electroweak radiative effects, and are thus suitable for the investigation of physics beyond the MSM. The variables are defined such thatS=T=U=0 corresponds to the MSM prediction form t=140 GeV andm H =300 GeV. A fit to the LEP data on cross-sections for hadron and lepton final states and lepton forward-back-ward charge asymmetries givesS=−0.44±0.91 andT=−0.15±0.52. With the addition to the fit of other LEP and electroweak data, the values obtained areS=−1.03±0.66,T=−0.46±0.41 andU=−0.13±0.94. These results are only just compatible with the assumed central MSM valuesS=T=U=0. Some constraints from these results on models beyond the MSM are discussed.

Identifying the origin of new gauge bosons at the Superconducting Super Collider.

We examine the ability of the detector of the Solenoidal Detector Collaboration (SDC) to identify the origin of a new neutral gauge boson ${Z}_{2}$ in the TeV mass range at the Superconducting Super Collider. Specifically, given the measurements of the ${Z}_{2}$ production cross section, width, and leptonic forward-backward asymmetry, together with their associated statistical and systematic errors, we address two related questions: (i) For two differnet extended electroweak models, up to what mass can their corresponding ${Z}_{2}$ bosons be distinguished, and (ii) how well can the ${Z}_{2}$ couplings be determined? Our calculations include $O({\ensuremath{\alpha}}_{s}^{2})$ QCD, as well as oblique electroweak radiative corrections to the above quantities, and allow for uncertainties due to structure functions, detector efficiencies, lepton identification, luminosity measurement errors, and finite lepton-pair mass resolution, using the specifications of the SDC detector. Nine distinct classes of extended electroweak models are investigated and results are obtained for integrated luminosities of ${10}^{4}$ and ${10}^{5}$ ${\mathrm{pb}}^{\ensuremath{-}1}$.