Search for neutral Higgs bosons in ${\mathrm{e}}^+ {\mathrm{e}}^-$ collisions at $\sqrt s \approx$ 189 GeV

Abstract.

Searches for the neutral Higgs bosons predicted by the Standard Model (SM) and the Minimal Supersymmetric extension of the Standard Model (MSSM) have been performed with the OPAL detector at LEP. Approximately 170 pb$^{-1}$ of $\mathrm{e}^+ \mathrm{e}^-$ collision data collected at $\sqrt{s} \approx 189$ GeV were used to search for Higgs boson production in the SM process ${\mathrm{e}}^+{\mathrm{e}}^- \rightarrow \mathrm{H}^{0}\mathrm{Z}^{0}$ and the MSSM processes ${\mathrm{e}}^+{\mathrm{e}}^- \rightarrow \mathrm{h}^{0} \mathrm{Z}^{0}$ and ${\mathrm{e}}^+ {\mathrm{e}}^- \rightarrow \mathrm{A}^{0} \mathrm{h}^{0}$. The searches are sensitive to the $\mathrm{b}\bar{\mathrm{b}}$ and $\tau^+\tau^-$ decay modes of the Higgs bosons, and also to the MSSM decay mode $\mathrm{h}^{0} \rightarrow \mathrm{A}^{0} \mathrm{A}^{0}$. OPAL search results at lower centre-of-mass energies have been incorporated in the limits, which are valid at the 95% confidence level. For the SM Higgs boson, a lower mass bound of 91.0 GeV is obtained. In the MSSM, the limits are $m_{\mathrm{H}} >74.8$ GeV and $m_{\mathrm{A}} >76.5$ GeV, assuming $\tan\beta > 1$, that the mixing of the scalar top quarks is either zero or maximal, and that the soft SUSY-breaking masses are 1 TeV. For the case of zero scalar top mixing, the values of $\tan\beta$ between 0.72 and 2.19 are excluded.