Search for the Standard Model Higgs Boson at LEP

The four LEP collaborations, ALEPH, DELPHI, L3 and OPAL, have collected 2461 pb−1 of e+e− collision data at centre-of-mass energies between 189 and 209 GeV, of which 536 pb−1 have energies above 206 GeV. The data have been used to search for the Standard Model Higgs boson. The search results of the four collaborations are combined and examined for their consistency with two hypotheses: (a) the Standard Model background hypothesis and (b) the signal plus background hypothesis. The confidence levels for the two hypotheses have been computed in a likelihood analysis as a function of the hypothetical Higgs boson mass. The evolution of the likelihood as a function of the test-mass shows a slight preference for the signal plus background hypothesis for a Higgs boson mass in the vicinity of 116 GeV, where the confidence level for the signal plus background hypothesis is 37% while that for the background hypothesis is about 8%. A lower bound of 114.4 GeV has been established, at the 95% confidence level, for the mass of the Standard Model Higgs boson.