Search for a low mass CP-odd higgs boson in e+e- collisions with the OPAL detector at LEP2

Abstract

We have analysed the data collected by OPAL at centre-of-mass energies between 189 and 209 GeV searching for Higgs boson candidates from the process e+e- → h0Z0 followed by the decay of h0 → A0A0 where A0 is the CP-odd Higgs boson. The search is done in the region where the A0 mass, mA, is below the production threshold for bb̄, and the CP-even Higgs boson mass mh is within the range 45-86 GeV/c2. In this kinematic range, the decay of h0 → A0A0 may be dominant and previous Higgs boson searches have very small sensitivities. This search can be interpreted within any model that predicts the existence of at least one scalar and one pseudoscalar Higgs boson. No excess of events is observed above the expected Standard Model backgrounds. Model-independent limits on the cross-section for the process e+e- → h0Z0 are derived assuming 100% decays of the h0 into A0A0 and 100% decays of the A0A0 into each of the following final 3states: cc̄cc̄, gggg, τ+τ-τ+τ-, cc̄gg, ggτ+τ- and cc̄τ+τ-. The results are also interpreted in the CP-conserving no-mixing MSSM scenario, where the region 45 ≤ mh ≤ 85 GeV/c2 and 2 ≤ mA ≤ 9.5 GeV/c2 is excluded.