Search for Higgs bosons and other massive states decaying into two photons in e+e− collisions at 189 GeV

A search is described for the generic process e+e−→XY, where X is a neutral heavy scalar boson decaying into a pair of photons, and Y is a neutral heavy boson (scalar or vector) decaying into a fermion pair. The search is motivated mainly by the cases where either X, or both X and Y, are Higgs bosons. In particular, we investigate the case where X is the Standard Model Higgs boson and Y the Z0 boson. Other models with enhanced Higgs boson decay couplings to photon pairs are also considered. The present search combines the data set collected by the OPAL collaboration at 189 GeV collider energy, having an integrated luminosity of 182.6 pb−1, with data samples collected at lower energies. The search results have been used to put 95% confidence level bounds, as functions of the mass MX, on the product of the cross-section and the relevant branching ratios, both in a model independent manner and for the particular models considered.