Abstract: | This paper describes a topological search for an invisibly decaying Higgs boson, H, produced via the Bjorken process (e+e-→HZ). The analysis is based on data recorded using the OPAL detector at LEP at centre-of-mass energies from 183 to 209 GeV
corresponding to a total integrated luminosity of 629 pb-1. In the analysis only hadronic decays of the Z boson are considered. A scan over Higgs boson masses from 1 to 120 GeV and
decay widths from 1 to 3000 GeV revealed no indication for a signal in the data. From a likelihood ratio of expected signal
and standard model background we determine upper limits on cross-section times branching ratio to an invisible final state.
For moderate Higgs boson decay widths, these range from about 0.07 pb (MH=60 GeV) to 0.57 pb (MH=114 GeV). For decay widths above 200 GeV the upper limits are of the order of 0.15 pb. The results can be interpreted in
general scenarios predicting a large invisible decay width of the Higgs boson. As an example we interpret the results in the
so-called stealthy Higgs scenario. The limits from this analysis exclude a large part of the parameter range of this scenario
experimentally accessible at LEP 2. |