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

TL;DR

The OPAL collaboration conducted a targeted search for a low-mass CP-odd Higgs A0 produced via e+e−→h0Z0 with h0→A0A0 in LEP2 data (188–209 GeV). By analyzing Z0 decays to νν, μ+μ−, and e+e− and employing likelihood discriminants, they derived model-independent cross-section limits for h0Z0 production and interpreted the results within the MSSM no-mixing benchmark. No excess above Standard Model backgrounds was observed; the study excludes, at 95% CL, the region 2 ≤ mA ≤ 9.5 GeV and 45 ≤ mh ≤ 85 GeV in the MSSM no-mixing scenario, representing the strongest constraints in this parameter space to date. The work provides comprehensive limit maps across multiple decay channels (cc̄, gg, ττ) and final states, contributing valuable constraints for extended Higgs sectors in e+e− collisions.

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 bbbar, and the CP-even Higgs boson mass mh is within the range 45-86 GeV/c^2. 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 states: ccbarccbar, gggg, tau+tau-tau+tau-, ccbargg ggtau+tau- and ccbartau+tau-. The results are also interpreted in the CP-conserving no-mixing MSSM scenario, where the region 45 <= mh <= 85 GeV/c^2 and 2<= mA <= 9.5 GeV/c^2 is excluded.

Figures (8)

• Figure 1: The Feynman diagram for the processes considered in this analysis.
• Figure 2: The four input variables used for the likelihood function of the $\hbox{$\mathrm{ Z}^0$} \to \hbox{$\mathrm {\nu \bar{\nu}}$}$ channel after all preselection cuts are applied. The contributions from the Standard Model backgrounds are added and normalised to the data integrated luminosity. The results are shown here for $E_{\mathrm CM}$=189--209 GeV combined. The contribution from the reference signal is scaled up by a factor of five.
• Figure 3: The four input variables used for the likelihood function in the muon channel after all preselection cuts for $E_{\mathrm CM}$=189--209 ${\mathrm GeV}/c^2$ combined, where the labels $1$ and $2$ refer to the more and less energetic muon and jet, respectively. The only contribution from Standard Model backgrounds surviving the preselection, namely the 4-fermion sample, is compared to the data. The contribution from the reference signal is also shown.
• Figure 4: The five input variables used for the likelihood function after all preselection cuts in the electron channel for $E_{\mathrm CM}$=189--209 GeV combined, where the labels $1$ and $2$ refer to the more and less energetic electron and jet, respectively. The contributions of Standard Model backgrounds surviving the preselection, namely the 4-fermion, 2-fermion and 2-photon samples, are compared to the data. The contribution from the reference signal is also shown.
• Figure 5: The likelihood distribution functions in the (a) $\hbox{$\mathrm{ Z}^0$} \to \hbox{$\mathrm {\nu \bar{\nu}}$}$ (b) $\hbox{$\mathrm{ Z}^0$} \to \hbox{$\mathrm {\mu^+\mu^-}$}$ and (c) $\hbox{$\mathrm{ Z}^0$} \to \hbox{$\mathrm {e^+e^-}$}$ channels are shown for the data, the Standard Model backgrounds and the reference signal (the mixture of all signal hypotheses) for $E_{\mathrm CM}$=189--209 GeV combined. The backgrounds are added and normalised to the data integrated luminosity.