[en] This thesis is devoted to the search for neutral Higgs bosons predicted by various theoretical models. The Higgs boson arises as a result of spontaneous breaking of SU(2) symmetry leading to the generation of masses of fermions and weak bosons. The search is done in all experimentally related channels using the data collected at center-of-mass energies √(s) up to 209 GeV in the years 1998-2000 with the L3 detector at the large electron positron collider, LEP. Here the study of the final states characterised by four jets is described in detail. For other final states the analyses are briefly reviewed and the results are reported. The data are compared with the expectation from the Standard Model background processes and with various signal hypotheses. A model independent search for neutral Higgs bosons is also performed. No evidence for the production of Higgs bosons is found. New mass limits are determined superseding previous mass limits established by L3 and other experiments. First I describe the analysis searching for the Standard Model Higgs Boson. Its production at LEP is expected mainly via the Higgs-strahlung process, e⁺e^- → Z^* → HZ. In the mass range accessible at LEP the Standard Model Higgs Boson is predicted to decay dominantly into a pair of b and anti-b quarks, hence the dedicated analyses are optimised for the H → b anti b decay mode. The HZ → q anti qq anti q channel is investigated and then combined with the HZ → q anti qν anti ν, HZ → q anti ql⁺l^-(l=e,μ,τ) and HZ → τ⁺τ^-q anti q channels leading to a lower mass limit of m_H > 112.0 GeV at 95% C.L. The results of the search are also interpreted in terms of limits on the HZZ coupling. In the minimal supersymmetric standard model (MSSM) the Higgs sector is extended to five physical states. The Higgs-strahlung process e⁺e^- → hZ is complemented by the mechanism of the Higgs boson pair production e⁺e^- → hA. Therefore, for the interpretation of the results in the framework of the MSSM the Standard Model analyses are combined with the hA → b anti bτ⁺τ^- (hA → τ⁺τ^-b anti b), hA → b anti bb anti b and hZ → AAq anti q channels. Three benchmark MSSM scenarios denoted ''m_h - max'', ''no mixing'' and ''large-μ'' are considered. Using L3 data the lower bounds on the Higgs boson masses m_h > 84.5 GeV, m_A > 86.3 GeV are derived at 95% C.L. for tanβ >0.7. For the ''m_b - max'', ''no mixing'' and ''large-μ'' scenarios, ranges 0.55 ≤ tan β ≤ 2.2, 0.4 ≤ tan β ≤ 5.4 and 0.7 ≤ tan β ≤ 6.7, respectively, are ruled out. (orig.)

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[en] The potential of a linear e⁺e^- collider operated at a centre-of-mass energy of 350 GeV is studied for the measurement of the Higgs boson mass. An integrated luminosity of 500 fb^-1 is assumed. For Higgs boson masses of 120, 150 and 180 GeV the uncertainty on the Higgs boson mass measurement is estimated to be 40, 65 and 70 MeV, respectively. The effects of beam related systematics, namely a bias in the beam energy measurement, the beam energy spread and the luminosity spectrum due to beamstrahlung, on the precision of the Higgs boson mass measurement are investigated. In order to keep the systematic uncertainty on the Higgs boson mass well below the level of the statistical error, the beam energy measurement must be controlled with a relative precision better than 10^-4. (orig.)

[en] The potential of a linear e⁺e^- collider operated at a centre-of-mass energy of 350 GeV is studied for the measurement of the Higgs boson mass. An integrated luminosity of 500 fb^-1 is assumed. For Higgs boson masses of 120, 150 and 180 GeV the uncertainty on the Higgs boson mass measurement is estimated to be 40, 65 and 70 MeV, respectively. The effects of beam related systematics, namely a bias in the beam energy measurement, the beam energy spread and the luminosity spectrum due to beamstrahlung, on the precision of the Higgs boson mass measurement are investigated. In order to keep the systematic uncertainty on the Higgs boson mass well below the level of the statistical error, the beam energy measurement must be controlled with a relative precision better than 10^-4. (orig.)

[en] We report on the accuracy of the measurement of the Higgs boson mass that would be achieved in a linear collider operating at a center-of-mass energy of 350 GeV, assuming an integrated luminosity of 500 fb-1. For that we have exploited the exclusive Higgs decays into b quarks and W bosons. The Higgs mass is determined with an accuracy of about 40 MeV for m_H=120 GeV and 80 MeV for m_H=180 GeV

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[en] The process e⁺e^- → ZH allows to measure the Higgs boson in the recoil mass spectrum against the Z boson without any assumptions on the Higgs boson decay. We performed a full simulation and reconstruction of e⁺e^- → ZH using the Mokka and Marlin packages describing the LDC detector. The Z is reconstructed from its decays into electrons and muons. The mass of the Higgs boson is set to 120GeV. Assuming a centre-of-mass energy of 250GeV and an integrated luminosity of 50 fb^-1 the Higgs boson mass and the Higgs-strahlung cross section can be measured with a precision of 120MeV and 9%, respectively. (orig.)

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