Abstract
A search for the associated production of the Higgs boson with a top quark pair is performed in multilepton final states using 20.3 fb−1 of proton–proton collision data recorded by the ATLAS experiment at s=8 TeV at the Large Hadron Collider. Five final states, targeting the decays H→WW⁎, ττ, and ZZ⁎, are examined for the presence of the Standard Model (SM) Higgs boson: two same-charge light leptons (e or μ) without a hadronically decaying τ lepton; three light leptons; two same-charge light leptons with a hadronically decaying τ lepton; four light leptons; and one light lepton and two hadronically decaying τ leptons. No significant excess of events is observed above the background expectation. The best fit for the tt¯H production cross section, assuming a Higgs boson mass of 125 GeV, is 2.1−1.2+1.4 times the SM expectation, and the observed (expected) upper limit at the 95% confidence level is 4.7 (2.4) times the SM rate. The p-value for compatibility with the background-only hypothesis is 1.8σ; the expectation in the presence of a Standard Model signal is 0.9σ.
Highlights
The discovery of a new particle H with a mass of about 125 GeV in searches for the Standard Model (SM) [1,2,3] Higgs boson [4,5,6,7] at the LHC was reported by the ATLAS [8] and CMS [9] Collaborations in July 2012
The particle has been observed in the decays H → γ γ [10,11], H → Z Z ∗ → 4 [12,13], and H → W W ∗ → ν ν [14,15], and evidence has been reported for H → τ τ [16,17], consistent with the rates expected for the SM
The best-fit value of the signal strength μ = σttH,obs/σttH,SM is determined using a maximum likelihood fit to the data yields of the categories listed in Table 3, which are treated as independent Poisson terms in the likelihood
Summary
The discovery of a new particle H with a mass of about 125 GeV in searches for the Standard Model (SM) [1,2,3] Higgs boson [4,5,6,7] at the LHC was reported by the ATLAS [8] and CMS [9] Collaborations in July 2012. The observation of the process in which the Higgs boson is produced in association with a pair of top quarks (ttH ) would permit a direct measurement of the top quark–Higgs boson Yukawa coupling in a process that is tree-level at the lowest order, which is otherwise accessible primarily through loop effects. Having both the tree- and loop-level measurements would allow disambiguation of new physics effects that could affect the two differently, such as dimension-six operators contributing to the g g H vertex. The main backgrounds to the signal arise from ttproduction with additional jets and non-prompt leptons, associated production of a top quark pair and a vector boson W or Z (collectively denoted ttV ), and other processes where the electron charge is incorrectly measured or where quark or gluon jets are incorrectly identified as τ candidates
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