Search for a low mass CP-odd Higgs boson in $\mathrm{e^+e^-}$ 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 $\mathrm {e^+e^-} \to \mathrm {h^0}\mathrm {Z}^0$ followed by the decay of $\mathrm {h^0} \to \mathrm {A^0} \mathrm {A^0}$ where $\mathrm {A^0}$ is the CP-odd Higgs boson. The search is done in the region where the $\mathrm {A^0}$ mass, $m_{\mathrm{A}}$, is below the production threshold for $\mathrm {b \bar b}$, and the CP-even Higgs boson mass $m_{\mathrm{h}}$ is within the range 45-86 GeV/c ². In this kinematic range, the decay of $\mathrm {h^0} \to \mathrm {A^0} \mathrm {A^0}$ 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 $\mathrm {e^+e^-}\rightarrow \mathrm {h^0} \mathrm {Z}^0$ are derived assuming 100% decays of the $\mathrm {h^0}$ into $\mathrm {A^0} \mathrm {A^0}$ and 100% decays of the $\mathrm {A^0} \mathrm {A^0}$ into each of the following final states: $\mathrm {c \bar c} \mathrm {c \bar c}$, $\mathrm {gggg}$, $\tau^+ \tau^- \tau^+ \tau^-$, $\mathrm {c \bar c} \mathrm {gg}$, $\mathrm {gg} \tau^+ \tau^-$ and $\mathrm {c \bar c} \tau^+ \tau^-$. The results are also interpreted in the CP-conserving no-mixing MSSM scenario, where the region $45 \le m_{\mathrm{h}} \le 85 {\mathrm{GeV}}/c^2$ and $2\le m_{\mathrm{A}} \le 9.5 {\mathrm{GeV}}/c^2$ is excluded.