[en] A monitoring system was built for tracking the behaviour of each BGO crystal and associated electronics in the L3 electromagnetic calorimeter and for maintaining its calibration. The construction of the system is described in detail. This monitoring system was first used at the CERN SPS, where each crystal was calibrated in an electron beam, and then on the LEP collider. A procedure combining the information of the xenon monitor and of the Bhabba events allows to maintain the absolute calibration of each crystal to within 0.9%. (orig.)

[en] During 1991 two cosmic rays runs took place for the calibration of the L3 electromagnetic calorimeter. In this paper we present the results of the first high statistics cosmic ray calibration of the calorimeter in situ, including the end caps. Results show that the accuracy on the measurement of the calibration constants that can be achieved in one month of data taking is of 1.3%. (orig.)

[en] We have measured the cross-section of the reaction e⁺e^-→γγ at center of mass energies around the Z⁰ mass. The results are in good agreement with QED predictions. For the QED cutoff parameters the limit of Λ₊>103 GeV and Λ_->118 GeV are found. For the decays Z⁰→γγ, Z⁰→π⁰γ, Z⁰→ηγ and Z⁰→γγγ we find upper limits of 2.9x10^-4, 2.9x10^-4, 4.1x10^-4 and 1.2x10^-4, respectively. All limits are at 95% CL. (orig.)

[en] We have searched for the pair produced charged Higgs particles in the decays of Z⁰ for the decay channels of H⁺H^-→τ⁺ντ^-anti ν, H⁺H^-→τνcs and H⁺H^-→canti santi cs. The data sample analyzed corresponds to approximately 50 000 hadronic decays of Z⁰. A lower limit of 36.5 GeV is obtained at the 95% confidence level for the mass of charged Higgs particle, independent of its decay branching ratio. (orig.)

[en] We have searched for lepton flavour violation in Z⁰ boson decays into lepton pairs, Z⁰→μτ, Z⁰→eτ, and Z^-→eμ. The data sample is based on an integrated luminosity of 10.4 pb^-1 corresponding to 370 000 Z⁰'s produced. We obtain upper limits on the branching ratios of 4.8x10^-5 for the μτ, 3.4x10^-5 for the eτ and 2.4x10^-5 for the eμ decay modes at the 95% confidence level. (orig.)

[en] We have measured the partial widths for the three reactions e⁺e^-→Z⁰→e⁺e^-, μ⁺μ^-, τ⁺τ^-. The results are Γ_ee=84.3±1.3 MeV, √Γ_eeΓ_μμ=83.3±1.3 MeV, and √Γ_eeΓ_ττ=83.9±1.4 MeV, where the errors are statistical. The systematic errors are estimated to be 1.0 MeV, 0.9 MeV, and 1.4 MeV, respectively. We perform a simultaneous fit to the cross sections for the e⁺e^-→e⁺e^-, μ⁺μ^-, and τ⁺τ^- data, the differential cross section as a function of polar angle for the electron data, and the forward-backward asymmetry for the muon data. We obtain the leptonic partial with Γ_ll=84.0±0.9(stat). MeV. The systematic error is estimated to be 0.8 MeV. Also, we obtain the axial-vector and vector weak coupling constants of charged leptons, g_A=-0.500±0.003 and g_V=-0.064_-0.013^+0.017. (orig.)

[en] We have searched for the neutral Higgs boson produced in the decays of the Z⁰ through the process Z⁰→H⁰l⁺l^- in the mass range Msub(H⁰)<2 GeV. From the analysis of a data sample corresponding to about 70 000 Z⁰→hadrons we exclude a minimal standard model Higgs boson in the mass range 0< Msub(H⁰)<2 GeV at the 99% confidence level independent of the Higgs decay channels. (orig.)

[en] We have searched for excited taus from Z⁰ decay in the channels e⁺e^-→τ*⁺τ*^-→τ⁺τ^-γγ and e⁺e^-→ττ*→τ⁺τ^-γ, using the L3 detector at LEP. Using the τ* pair production channel, we can exclude a τ* up to a mass of 45.5 GeV at 95% confidence level. We have also determined upper limits on the ττ*Z⁰ and ττ*γ couplings as a function of m_τ* up to 89.3 GeV. (orig.)