[en] This work is based on data taken with the L3 detector at LEP (electron-positron collider) built at the CERN laboratories in Geneva. The first results on the measurement of the e⁺e^- →νantiνγ cross section are presented. In the first part of the thesis we give a general overview on the recently obtained values for N_ν through various methods. We also point out the importance of measuring the single photon cross section in order to determine this parameter in the framework of the Standard Model. This measurement can also give valuable insights as to what happens beyond the S.M. specially concerning supersymmetric and composite models. We end with a review of the generators used for simulating the signal and the various background processes present in this measurement. The second chapter describes the L3 detector with a particular emphasis given to the electromagnetic calorimeter, built using BGO crystals. We also describe the simulation software used by L3. In the third chapter we give a detailed presentation of the routines and criteria used for our single photon signal data acquisition. We show how, by also selecting single electrons, we can study this sample in order to gain a better understanding of the background generator. The last chapter describes the analysis work done on the various data samples, the comparison between Monte Carlo and data, and the procedure used to extract the number of families N_ν. With our sample of single photons, corresponding to an integrated luminosity of 2.98 pb^-1, with a center of mass energy from M_z^o - 3GeV to M_z^o + 3 GeV, we obtain N_ν=3.3±0.6(stat.)±0.3(syst.)

[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 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 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 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.)

[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.)