Decay-mode independent searches for new scalar bosons with the OPAL detector at LEP

This paper describes topological searches for neutral scalar bosons S0 produced in association with a Z0 boson via the Bjorken process \(e^+e^- \to{\rm S}^0{}{\rm Z}^0\) at centre-of-mass energies of 91 GeV and 183-209 GeV. These searches are based on studies of the recoil mass spectrum of \({\rm Z}^0 \to e^+e^-\) and \(\mu^+ \mu^-\) events and on a search for \({\rm S}^0{\rm Z}^0\) with \({\rm Z}^0 \to \nu\bar{\nu}\) and S\(^0 \to e^+ e^-\) or photons. They cover the decays of the S0 into an arbitrary combination of hadrons, leptons, photons and invisible particles as well as the possibility that it might be stable. No indication for a signal is found in the data and upper limits on the cross section of the Bjorken process are calculated. Cross-section limits are given in terms of a scale factor k with respect to the Standart Model cross section for the Higgs-strahlung process \(e^+ e^-\to{\rm H}_{\rm SM}{\rm Z}^0\). These results can be interpreted in general scenarios independently of the decay modes of the S0. The examples considered here are the production of a single new scalar particle with a decay width smaller than the detector mass resolution, and for the first time, two scenarios with continuous mass distributions, due to a single very broad state or several states close in mass.