Searches for the neutral Higgs bosons of the MSSM in e +e − collisions at centre-of-mass energies of 181–184 GeV

The data collected by ALEPH at LEP at centre-of-mass energies ranging from 181 to 184 GeV, corresponding to an integrated luminosity of 57 pb −1, are analysed to search for pair-produced neutral Higgs bosons h and A, in the b b ̄ b b ̄ and τ + τ − b b ̄ final states. Two events are found in the data with 2.5 expected from standard model processes. When combined with the lower energy data collected by ALEPH and with earlier reported searches for associated hZ production, these analyses are interpreted in the context of the minimal supersymmetric extension of the standard model (MSSM). For standard choices of MSSM parameter sets, this combination results in 95% C.L. exclusion lower limits of 72.2 and 76.1 GeV/ c 2 for m h and m A, irrespective of tan β. A scan of the MSSM parameter space is performed in which the model parameters are varied over wide ranges. For low values of tan β, i.e., for 1< tanβ≲ 2 , the limit on m h of ∼88 GeV/ c 2 is shown to be robust, being satisfied in essentially all of the physically allowed domain.