Search for Invisibly Decaying Higgs Bosons with Large Decay Width Using the OPAL Detector at LEP

TL;DR

This study conducted a search for invisibly decaying Higgs bosons with large decay widths using OPAL detector data from LEP, setting upper limits on production cross-sections and excluding significant parameter space of certain theoretical models.

Contribution

First search to set experimental limits on invisibly decaying Higgs bosons with large decay widths at LEP energies.

Findings

No signal observed in the data.

Upper limits on cross-section times branching ratio were established.

Large parts of the stealthy Higgs scenario parameter space were excluded.

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 629pb-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 0.07pb Mh = 60GeV) to 0.57pb (Mh = 114GeV). For decay widths above 200GeV the upper limits are of the order of 0.15pb. 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 LEP2.