Abstract
A search for a Higgs boson produced in association with a pair of top quarks (t¯) and decaying into a pair of bottom quarks (H→ b¯ b) is presented. The analysis uses 4.7 fb −1 of pp collision data at √ s = 7 TeV collected in 2011 with the ATLAS detector at the CERN Large Hadron Collider. The search is focused on the semileptonic decay mode of the t¯ t system which, together with the H → bdecay, typically results in a final state signature characterised by one high transverse momentum isolated electron or muon, high transverse missing momentum and six jets, of which four jets originate from b quarks. Events are classified in nine different topologies depending on their jet and b-tagged jet multiplicities in order to improve the sensitivity of the search. No significant excess of events above the background expectation is observed and 95% confidence- level upper limits on the production cross section times branching ratio, �(t¯ tH)× BR(H→ b¯), are derived for a Higgs boson with a mass between 110 and 140 GeV. For a Higgs boson with a mass of 125 GeV, an observed (expected) 95% confidence-level upper limit of 13.1 (10.5) times the Standard Model cross section is obtained.
Highlights
The observation of a Higgs-like particle was reported by the ATLAS [1] and CMS [2] collaborations
Smaller background contributions arise from W+jets, single top, diboson (WW, WZ, ZZ) and Z+jets production. These physics backgrounds are predicted from the Monte Carlo (MC) simulation and are normalised to higher-order theoretical cross sections
Hypothesis testing is performed using a modified frequentist method as implemented in RooStats [12] and based on a profile likelihood which takes into account the systematic uncertainties as nuisance parameters which are fitted to the data. This procedure allows the degrading impact of systematic uncertainties on the search sensitivity to be minimized, by taking advantage of the high-statistics backgrounddominated control channels included in the likelihood fit
Summary
The observation of a Higgs-like particle was reported by the ATLAS [1] and CMS [2] collaborations. √A search for ttH (H → bb) production in pp collisions at s = 7 TeV using data corresponding to an integrated luminosity of 4.7 fb−1 collected by the ATLAS [3] experiment is presented [4]. This search is simultaneously sensitive to the Yukawa coupling between the top quark and the Higgs boson and the H → bbbranching ratio. The only assumption made is that the Higgs boson is a narrow scalar particle
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