Search for a Neutral Higgs Boson with DELPHI

Abstract: The mass of the neutral Higgs boson cannot be predicted by the Standard Model. Therefore, the particle is scanned for at different assumed masses. The search described here was done using data taken with the DELPHI detector during 1993-1994 (Ecm=91 GeV), 1996 (Ecm=161 and 172 GeV) and during 1997 (Ecm=183 GeV).The Higgs boson was searched for through the Bjorken process where the decay of the Z' into two neutrinos was assumed. In order to reduce the background to a level where a discovery would be possible, new analysis methods using discriminant analysis were developed. This led to a very good background rejection and high signal efficiency. One event, with a mass of 27.5±3.6 GeV/c2, was selected from the 1993 real data at Ecm=91 GeV; one event, mH=64.6+2.4-1.2 GeV/c2, from 1996 at Ecm=161 GeV; and one event, mH=88 GeV/c2, from 1997.In the absence of a clear signal, these results were translated into a limit on the Higgs boson mass mH>58.3 GeV/c2 with 1993 data, verified with 1994 data, mH>64.6 GeV/c2 with 1996 data, and mH>85.7 GeV/c2 with 1997 data; all at 95% confidence level.