Search for neutral higgs bosons in Z(0) decays using the OPAL detector at LEP

A search for neutral Higgs bosons has been performed using the full sample of Z(0) decays collected by the OPAL detector at LEP up to 1995. The data were taken at centre-of-mass energies between 88 GeV and 95 GeV and correspond to an integrated luminosity of approximately 160 pb(-1). The present search addresses the processes Z(0) --> H(0)Z* and h(0)Z*, where H-0 is the Higgs boson predicted by the Standard Model and h(0) the lightest neutral scalar Higgs boson predicted in the framework of the Minimal Supersymmetric Standard Model. For the virtual Z(0) boson, Z*, the following decay channels are considered: Z* --> nu<(nu)over bar>, e(+)e(-) and mu(+)mu(-). Two candidate events have been found in the nu<(nu)over bar>H-0 channel and one in the mu(+)mu(-) H-0 channel. Combined with earlier searches, the present search excludes the SM Higgs boson, at the 95% confidence level (CL), from the mass range below 59.6 GeV. In the framework of the Minimal Supersymmetric Standard Model, allowing a wide range of variation for most relevant model parameters, a 95% CL lower limit of 44.3 GeV is obtained for the mass of the h(0) boson. Combined with earlier direct searches for the Higgs boson pair production process Z(0) --> h(0)A(0) and with measurements of the Z(0) line shape, a 95% CL lower limit of 23.5 GeV is obtained for the mass of the pseudoscalar Higgs boson A(0), assuming tan beta greater than or equal to 1.