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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 \(\sqrt {s}\)=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 t\(\overline{t}\)H production cross section, assuming a Higgs boson mass of 125 GeV, is 2.1\(^{+1.4}_{-1.2}\) 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.
A search is presented for the direct pair production of a chargino and a neutralino pp → \(\tilde{χ}\)\(^{±}_{1}\)\(\tilde{χ}\)\(^{0}_{2}\), where the chargino decays to the lightest neutralino and the W boson, \(\tilde{χ}\)\(^{±}_{1}\)→\(\tilde{χ}\)\(^{0}_{1}\)(W\(^{±}\)→ℓ\(^{±}\)ν), while the neutralino decays to the lightest neutralino and the 125 GeV Higgs boson, \(\tilde{χ}\)\(^{0}_{2}\)→\(\tilde{χ}\)\(^{0}_{1}\)(h→bb/γγ/ℓ\(^{±}\)νqq). The final states considered for the search have large missing transverse momentum, an isolated electron or muon, and one of the following: either two jets identified as originating from bottom quarks, or two photons, or a second electron or muon with the same electric charge. The analysis is based on 20.3 fb\(^{-1}\) of \(\sqrt {s}\) = 8 TeV proton–proton collision data delivered by the Large Hadron Collider and recorded with the ATLAS detector. Observations are consistent with the Standard Model expectations, and limits are set in the context of a simplified supersymmetric model.
A search for the Standard Model Higgs boson produced in association with a top-quark pair, t\(\overline{t}\)H, is presented. The analysis uses 20.3 fb\(^{−1}\) of pp collision data at \(\sqrt {s}\) = 8 TeV, collected with the ATLAS detector at the Large Hadron Collider during 2012. The search is designed for the H→b\(\overline{b}\) decay mode and uses events containing one or two electrons or muons. In order to improve the sensitivity of the search, events are categorised according to their jet and b-tagged jet multiplicities. A neural network is used to discriminate between signal and background events, the latter being dominated by t\(\overline{t}\)+jets production. In the single-lepton channel, variables calculated using a matrix element method are included as inputs to the neural network to improve discrimination of the irreducible t\(\overline{t}\)+b\(\overline{b}\) background. No significant excess of events above the background expectation is found and an observed (expected) limit of 3.4 (2.2) times the Standard Model cross section is obtained at 95 % confidence level. The ratio of the measured t\(\overline{t}\)H signal cross section to the Standard Model expectation is found to be μ = 1.5 ± 1.1 assuming a Higgs boson mass of 125 GeV.
A search is performed for WH production with a light Higgs boson decaying to hidden-sector particles resulting in clusters of collimated electrons, known as electron-jets. The search is performed with \(2.04 fb^{−1}\) of data collected in 2011 with the ATLAS detector at the Large Hadron Collider in proton–proton collisions at \(\sqrt {s}=7 TeV\) . One event satisfying the signal selection criteria is observed, which is consistent with the expected background rate. Limits on the product of the WH production cross section and the branching ratio of a Higgs boson decaying to prompt electron-jets are calculated as a function of a Higgs boson mass in the range from 100 to 140 GeV.