Search for Yukawa Production of a light neutral Higgs Boson at LEP

Within a Two-Higgs-Doublet Model (2HDM) a search for a light Higgs boson in the mass range of 4-12 GeV has been performed in the Yukawa process e+e ! b¯ bA/h ! bbτ +τ , using the data collected by the OPAL detector at LEP between 1992 and 1995 in e+e collisions at about 91 GeV centre-of-mass energy. A likelihood selection is applied to separate background and signal. The number of observed events is in good agreement with the expected background. Within a CP-conserving 2HDM type II model the cross-section for Yukawa production depends on ξA d = |tan β| and ξh d = |sin α/cos β| for the production of the CP-odd A and the CP-even h, respectively, where tan β is the ratio of the vacuum expectation values of the Higgs doublets and α is the mixing angle between the neutral CP-even Higgs bosons. From our data 95% C.L. upper limits are derived for ξA d within the range of 8.5 to 13.6 and for ξh d between 8.2 to 13.7, depending on the mass of the Higgs boson, assuming a branching fraction into τ +τ of 100%. An interpretation of the limits within a 2HDM type II model with Standard Model particle content is given. These results impose constraints on several models that have been proposed to explain the recent BNL measurement of the muon anomalous magnetic moment.