[en] The electroweak process e⁺e^- → μ⁺μ^- and e⁺e^- → τ⁺τ^- are studied in the energy range √s = 52.0 ∼ 61.4 GeV, using the TOPAZ detector at the TRISTAN e⁺e^- collider. From an integrated luminosity of L = 74 pb^-1, the lowest order cross sections and forward-backward asymmetries measured for these reactions are [σ_μμ] = 25.4 ± 0.9 ± 1.2 pb, [A_μμ] = 32.2 ± 3.1 ± 1.1% [σ_ττ] = 27.1 ± 1.1 ± 1.2 pb, [A_ττ] = -33.9 ± 4.9 ± 1.0%, at an average energy of (√s) = 57.87 GeV. At ∼10 standard deviations from QED, this represents the most significant measurement of the asymmetry for lepton pairs to the present date. Weak axial vector couplings of a_ea_μ = 0.96 ± 0.09 ± 0.01, and a_e a_τ = 1.01 ± 0.14 ± 0.01 are determined from the asymmetry. These results agree with the electroweak standard model prediction for γ-Z⁰ interference below the Z⁰ pole. Lepton compositeness is also tested. Lower limits in the range 2 ∼ 5 TeV at the 95% confidence level are placed on the compositeness scale parameters for μ and τ. No evidence for compositeness is observed

[en] Two new methods are proposed to extract the flavour contents of the events produced at LEP/SLC, together with the classification matrix of a tagging by hemispheres. By utilising the tagging obtained in both hemispheres, the efficiencies, backgrounds and flavour compositions are directly obtained by fitting the data. A minimal dependence on modelling and a consistent treatment of systematic errors are achieved by applying these methods. The choice of the tagging algorithm is irrelevant in the methods, provided that similar efficiencies are reached. As an example, a multivariate analysis technique combining the tracking information given by a microvertex detector has been applied to extract the Z →b b branching ratio using a standard simulation of a LEP/SLC experiment. (orig.)

[en] It has been proposed that the mass of the Z⁰ boson should be defined as the real part, and the width of the Z⁰ as the imaginary part of the complex pole M_c, for the reason that M_c is gauge invariant (g.i.). Clearly, this definition is only useful if we can calculate the S-matrix up to some accuracy in a g.i. way. This is not at all obvious, since the use of the Z⁰ dressed propagator mixes up the order of the diagrams, and thus order by order gauge invariance of the S-matrix, in this formulation, becomes an ill-defined concept. By means of an infinitesimal change of the gauge breaking term, we investigate what happens to perturbative gauge invariance of the S-matrix in the non-perturbative energy region of the Z⁰ resonance. Within the language of perturbation theory, we find that in fact there exists a Ward identity valid in the complex plane, i.e. for complex values of the momenta of the external legs, from which we can derive the gauge invariance of M_c. (orig.)

[en] Short communication. 6 refs

[en] We investigate the prospects of measuring anomalous couplings between gauge bosons at electron-positron-colliders with optimal observables. Such observables are shown to contain all information on the coupling parameters that can be extracted in a given reaction. Their sensitivity to the form factors in the general expressions of the triple gauge vertices WWZ and WWγ, including CP violating terms and absorptive parts, is calculated in view of LEP2 and the NLC. (orig.)

[en] Within the strongly-interacting electroweak symmetry breaking scenario (SIH) we investigate a few observables which are sensitive to SIH effects in e⁺e^-→W⁺W^- at TeV collision energies. (orig.)

[en] Considering the physics potential of an e^-e^- collider in the TeV energy range, we indicate a few interesting examples for exotic processes and discuss the standard model backgrounds. Focussing on pair production of weak gauge bosons, we report some illustrative predictions. ((orig.))

[en] The prospects to search for scalar and vector leptoquarks at high energy e⁺e^- colliders are reviewed. We compare production cross sections in the energy range between √(s)=O(200GeV) and 1TeV for e⁺e^- annihilation. QED and QCD corrections and the effect of beamstrahlung in these processes are discussed. For the case of linear colliders the search potential in the different possible collider modes as e⁺e^- annihilation, e^±γ scattering, and γγ fusion is compared. ((orig.))

[en] We study the accuracy to be obtained in measuring trilinear Z⁰W⁺W^- and γW⁺W^- couplings in the reaction e⁺e^- → W⁺W^- at a ''new linear collider'' energies of 500GeV to 1000GeV. We derive a simple approximate formula which says that the sentitivity for the determination of these couplings increases as √s.L, where √s denotes the center-of mass energy and L the integrated luminosity. Detailed investigations based on various fits confirm this simple estimate and show that an accuracy of the order of standard radiative corrections can be reached at NLC for designed values of luminosity. (orig.)

[en] We analyze the production in e⁺e^- collisions of new heavy fermions stemming from extensions of the Standard Model. We write down the most general expression for the production of two heavy fermions and their subsequent decays, allowing for the polarization of the e⁺e^- initial state and taking into account the final polarization of the fermions. We then discuss the various decay modes including cascade and three body decays, and the production mechanisms, both pair production and single production in association with ordinary fermions. (orig.)