Spin Physics at e^+e^- Colliders

A large number of measurements with polarized beams and/or spin analysis of final state particles has been performed at the e^+e^- colliders LEP and SLC, providing important information on the dynamics of high energy interactions. In this paper three subjects, for which the role of spin studies was particularly relevant, will be covered: the measurements of the electroweak couplings, the study of fragmentation dynamics and the search for physics beyond the Standard Model.Comment: 11 pages, Invited talk given at the International Workshop on Symmetry and Spin - Prague, Czech Republic, August 30 - September 5, 199

Complete LEP Data: Status of Higgs Boson Searches

The LEP experiments completed data-taking in November 2000. New preliminary combined results of the four LEP experiments ALEPH, DELPHI, L3 and OPAL are presented for various Higgs boson searches.Comment: 9 pages, 12 figures. Proc. Third Int.Conference on Non-Accelerator New Physics, NANP-01, Moscow, 200

Unanswered Questions in the Electroweak Theory

This article is devoted to the status of the electroweak theory on the eve of experimentation at CERN's Large Hadron Collider. A compact summary of the logic and structure of the electroweak theory precedes an examination of what experimental tests have established so far. The outstanding unconfirmed prediction of the electroweak theory is the existence of the Higgs boson, a weakly interacting spin-zero particle that is the agent of electroweak symmetry breaking, the giver of mass to the weak gauge bosons, the quarks, and the leptons. General arguments imply that the Higgs boson or other new physics is required on the TeV energy scale. Indirect constraints from global analyses of electroweak measurements suggest that the mass of the standard-model Higgs boson is less than 200 GeV. Once its mass is assumed, the properties of the Higgs boson follow from the electroweak theory, and these inform the search for the Higgs boson. Alternative mechanisms for electroweak symmetry breaking are reviewed, and the importance of electroweak symmetry breaking is illuminated by considering a world without a specific mechanism to hide the electroweak symmetry. For all its triumphs, the electroweak theory has many shortcomings. . . .Comment: 31 pages, 20 figures; prepared for Annual Review of Nuclear and Particle Science (minor changes

Search for R-parity violating chargino and neutralino decays in e⁺e⁻ collisions up to s=183 GeV

A search for chargino and neutralino pair production in e + e − collisions at center-of-mass energies between 161 GeV and 183 GeV is performed under the assumptions that R-parity is not conserved and that only purely leptonic or hadronic R-parity violating decays are allowed. No signal is found in the data. Limits on the production cross sections, on the Minimal Supersymmetric Standard Model parameters and on the masses of the supersymmetric particles are derived

Measurement of <i>η </i>production in two and three jet events from hadronic Z decays at LEP

The inclusive production of η mesons has been studied using 1.6 million hadronic Z decays collected with the L3 detector. The η multiplicity per event, the multiplicity for two-jet and three-jet events separately, and the multiplicity in each jet have been measured and compared with the predictions of different Monte Carlo programs. The momentum spectra of η in each jet have also been measured. We observe that the measured η momentum spectrum in quark-enriched jets agrees well with the Monte Carlo prediction while in gluon-enriched jets it is harder than that predicted by the Monte Carlo models

Searches for scalar top and scalar bottom quarks in e⁺e⁻ interactions at 161 GeV ≤ √s ≤ 183 GeV

Searches for scalar top and scalar bottom quarks have been performed at center-of-mass energies between 161 GeV and 183 GeV using the L3 detector at LEP. No signal is observed. Model-independent limits on production cross sections are determined for the two decay channels t ̃ 1 →c χ ̃ 1 0 and b ̃ 1 →b χ ̃ 1 0 . Within the framework of the Minimal Supersymmetric extension of the Standard Model mass limits are derived. For mass differences between t ̃ 1 and χ ̃ 1 0 greater than 10 GeV a 95% C.L. limit of 81.5 GeV is set on the mass of the Supersymmetric partner of the left-handed top. A supersymmetric partner of the left-handed bottom with a mass below 80 GeV is excluded at 95% C.L. if the mass difference between b ̃ 1 and χ ̃ 1 0 is greater than 20 GeV