Search for the Standard Model Higgs Boson at LEP
ALEPH Collaboration, DELPHI Collaboration, L3 Collaboration, OPAL Collaboration, the LEP Higgs Working Group
TL;DR
The paper combines LEP data from four experiments (ALEPH, DELPHI, L3, OPAL) collected at 189–209 GeV to search for the Standard Model Higgs boson via the Higgsstrahlung process. Using a likelihood framework based on reconstructed mass and discriminant variables, it derives a 95% CL lower bound of 114.1 GeV for the Higgs mass and finds a preferred mass near 115.6 GeV with a background fluctuation probability of 3.4%, driven mainly by ALEPH's four-jet final state. The analysis rigorously handles input histograms, systematic uncertainties, and cross-checks, presenting the result as a robust, though not final, LEP Higgs constraint. This work informs the viable Higgs mass range and demonstrates the power and limitations of combining multi-experiment searches at high-energy colliders.
Abstract
The four LEP collaborations, ALEPH, DELPHI, L3 and OPAL, have collected 2465 pb-1 of e+e- collision data at energies between 189 and 209 GeV, of which 542 pb-1 were collected above 206 GeV. Searches for the Standard Model Higgs boson have been performed by each of the LEP collaborations. Their data have been combined and examined for their consistency with the Standard Model background and various Standard Model Higgs boson mass hypotheses. A lower bound of 114.1 GeV has been obtained at the 95% confidence level for the mass of the Higgs boson. The likelihood analysis shows a preference for a Higgs boson with a mass of 115.6 GeV. At this mass, the probability for the background to generate the observed effect is 3.4%.