Flavour-independent h0A0 search and two Higgs doublet model interpretation of neutral Higgs boson searches at LEP

Upper limits on the cross-section of the pair-production process e +e- →h0 A0, assuming 100 % branching fraction to hadrons, are derived from a new search for the →h0 A0→ hadrons final state, independently of the hadronic flavour of the decay products. This study, combined with previously published searches for the neutral Higgs bosons h0 and A 0, is used to constrain the Type II Two Higgs Doublet Model (2HDM(II)) with no CP violation in the Higgs sector and no additional non-Standard Model particles besides the five Higgs bosons. The analysis combines LEP1 and LEP2 data collected with the OPAL detector up to the highest available centre-of-mass energies. The searches are sensitive to the h 0, A0→qq̄, gg, τ+ τ- h0A0A0 decay modes of the Higgs bosons. A benchmark scan of the 2HDM(II) parameter space is performed. Large regions of the 2HDM(II) parameter space explored are excluded at the 95% CL in the mh, mA, (mh, \tanβ planes, using both direct neutral Higgs boson searches and indirect limits derived from Standard Model high precision measurements. The region 1 ≲ mh ≲ 55 GeV and 3 ≲ mA ≲ 63 GeV is excluded at 95 % CL, independently of mh, mA, tanβ and for selected values of α which are representative of a complete α -scan. © Springer-Verlag /Societa Italiana di Fisica 2005.