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Neutral Higgs bosons search in four jet events with the dectector DELPHI

Recherche des bosons de Higgs neutres dans les etats finals a quatre jets avec le detecteur DELPHI

Abstract : The search for the Higgs bosons is one of the priorities of LEP200 at CERN. Indeed the Higgs mechanism which predicts the existence of scalar bosons whose mass is unknown, allows to generate masses to all standard and supersymmetric particles. We have searched for the Higgs bosons produced in the two channels at LEP200 $\mathrm(e^+e^-\rightarrow Z^* \rightarrow hZ)$, and $\mathrm(e^+e^-\rightarrow Z^*\rightarrow hA)$. We have focussed our studies on final states containing four jets. They represent the majority of the final states. We have developed analysis tools to improve the reconstruction of the mass of the Higgs boson produced in the hZ, channel, as well as the efficiency of the event selection. We have analysed all events recorded by DELPHI from 1995 to 1997 (ecm=161, 172 et 183 GeV). Taking into account results from the other decay modes of the Z, DELPHI has obtained the following 95% confidence level exclusion limit on the lightest Higgs boson mass: \mathrm(m_\Phz \ge 84.4~ GeV)/c^2~~~~~~95\%~\mathrm( C.L. ~~(observed~ limit))\nonumber \mathrm(m_\Phz \ge 86.4~ GeV)/c^2~~~~~~95\%~\mathrm( C.L. ~~(expected~ limit))\nonumber An original analysis, explained in this thesis, allows to reach an expected limit of 86.5 GeV/c2, using the four jets channel alone. The interpretation, in the minimal supersymmetric model (MSSM), of the results obtained in the hZ, and hA, channels has lead to the following 95\% confidence level exclusion limit on both neutral Higgs bosons h, et A: \begin(eqnarray) \mathrm(m_\Phz \ge 74.4~ GeV)/c^2~~~~~~95\%~\mathrm( C.L. ~~~~\forall~ tan\beta) \nonumber \mathrm(m_\PAz \ge 75.2~ GeV)/c^2~~~~~~95\%~\mathrm( C.L. ~~~~\forall~ tan\beta) \nonumber \end(eqnarray) Finally, a special analysis of the four jets events has been performed in the framework of an analysis proposed by the ALEPH Collaboration which had seen an excess of four jets events in the data recorded in 1995. Our results have not confirmed this excess.