Search for the Standard Model Higgs Boson at LEP

Search for the Standard Model Higgs Boson at LEP

October 27, 2018 | ALEPH, DELPHI, L3 and OPAL Collaborations
The four LEP collaborations—ALEPH, DELPHI, L3, and OPAL—collected 2465 pb⁻¹ of e⁺e⁻ collision data at energies between 189 and 209 GeV, with 542 pb⁻¹ above 206 GeV. They searched for the Standard Model (SM) Higgs boson, combining their data to examine consistency with SM background and Higgs mass hypotheses. A lower bound of 114.1 GeV was obtained at 95% confidence level for the Higgs mass. The likelihood analysis favored a Higgs mass of 115.6 GeV, with a 3.4% probability for background to generate the observed effect. The results are not final. The Higgs boson is expected to be produced mainly via Higgsstrahlung (e⁺e⁻ → HZ) at LEP energies. Searches focused on decay channels like b̄b, τ⁺τ⁻, and WW. Preselection cuts reduced backgrounds from two-photon processes and radiative returns. Remaining backgrounds were reduced using kinematic differences and b-flavour tagging. The combination procedure used likelihood ratios and test statistics to evaluate Higgs mass hypotheses. The observed test statistic -2lnQ had a minimum at 115.6 GeV, indicating a deviation from the background hypothesis. The probability of background producing the observed effect was 3.4%. The results were consistent across different decay channels and experiments, with the four-jet channel showing the strongest signal. Systematic errors were estimated at 5% for signal and 10% for background. Tests showed that ignoring these errors reduced the background probability from 3.4% to 3.2%. The results were consistent with the SM, with a lower bound of 114.1 GeV at 95% confidence level. The Higgs mass of 115.6 GeV was preferred, with the ALEPH data and four-jet final state contributing significantly. The results are not final as they combine preliminary results from three experiments with final results from one. The findings highlight the importance of further data to confirm the Higgs boson signal.The four LEP collaborations—ALEPH, DELPHI, L3, and OPAL—collected 2465 pb⁻¹ of e⁺e⁻ collision data at energies between 189 and 209 GeV, with 542 pb⁻¹ above 206 GeV. They searched for the Standard Model (SM) Higgs boson, combining their data to examine consistency with SM background and Higgs mass hypotheses. A lower bound of 114.1 GeV was obtained at 95% confidence level for the Higgs mass. The likelihood analysis favored a Higgs mass of 115.6 GeV, with a 3.4% probability for background to generate the observed effect. The results are not final. The Higgs boson is expected to be produced mainly via Higgsstrahlung (e⁺e⁻ → HZ) at LEP energies. Searches focused on decay channels like b̄b, τ⁺τ⁻, and WW. Preselection cuts reduced backgrounds from two-photon processes and radiative returns. Remaining backgrounds were reduced using kinematic differences and b-flavour tagging. The combination procedure used likelihood ratios and test statistics to evaluate Higgs mass hypotheses. The observed test statistic -2lnQ had a minimum at 115.6 GeV, indicating a deviation from the background hypothesis. The probability of background producing the observed effect was 3.4%. The results were consistent across different decay channels and experiments, with the four-jet channel showing the strongest signal. Systematic errors were estimated at 5% for signal and 10% for background. Tests showed that ignoring these errors reduced the background probability from 3.4% to 3.2%. The results were consistent with the SM, with a lower bound of 114.1 GeV at 95% confidence level. The Higgs mass of 115.6 GeV was preferred, with the ALEPH data and four-jet final state contributing significantly. The results are not final as they combine preliminary results from three experiments with final results from one. The findings highlight the importance of further data to confirm the Higgs boson signal.
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