Recherche de bosons de Higgs neutres h et A dans les états finals à quatre jets, avec le détecteur DELPHI à LEP200

Searches for Higgs bosons are presented in the framework of the standard model, of the minimal supersymmetric model, and of its first extension. The data sample used has been collected by Delphi in 1997 and 1998, and represents an integrated luminosity of 212 $pb^{-1}$ . The final states considered are the $b\overline{b}b\overline{b}$ decay of the $e^+e^- \to hA$ process, and the $b\overline{b}q\overline{q}$ decay of the $e^+e^- \to hZ$ process. The methods used for the analyses and for the interpretation of their results are described, as well as the implementation of b-quark identification from leptonic decays. After a first selection, the discrimination between background and signal events is studied, and the most powerful quantities are combined into a single variable. The best signal sensitivity is obtained by a weighted counting of all events, where the weight of the events is defined on the basis of the single variable. An excess of data, encountered in the search for hZ, justifies a separate discussion. The main instrumental and theoretical uncertainties, from which this excess could originate, are discussed. It is shown that the majority of the events in excess do not correspond to a Higgs signal, and that it is difficult to understand the excess as a consequence of imperfect standard process simulation. The results of the searches allow to give limits on the masses of the Higgs bosons in the models under consideration. The data imply $m_h$>93 GeV in the standard model. In the MSSM, one finds $m_h$>82.4 GeV and $m_A$ >83.3 GeV, for any tan $\beta$. On the simplest extension of the MSSM, the pseudoscalar mass parameter $m^{'}_{A}$ must be greater than 69 GeV for tan $\beta$>1. No constraint is found for tan $\beta$<1. All limits are given at 95% C.L.