Searches for the Standard Model Higgs boson produced in association with a pair of top quarks, ttH, are presented. The ATLAS analysis uses 20.3 fb−1 of pp collision data at s√=8 TeV, collected with the ATLAS detector at the Large Hadron Collider during 2012. The search is designed for the H→bbˉ decay mode and uses events containing one or two electrons or muons. The Higgs boson is assumed to have a mass of 125 GeV. In order to improve the sensitivity of the search, events are categorised according to their jet and b-tagged jet multiplicities. A multivariate technique is used to discriminate between signal and background events, which are dominated by ttˉ+jets production. No significant excess of events above the background expectation is found and an observed (expected) 95 % confidence-level limit of 4.1 (2.6) times the Standard Model cross section is obtained. By performing a fit allowing for an arbitrary Higgs boson signal strength, the ratio of the measured signal strength to the Standard Model expectation is found to be μ=1.7±1.4. Results and analysis methods are compared to the existing CMS analysis in this channel, which is based on a multivariate analysis (MVA) techniques are used to discriminate between background and signal events. A simultaneous fit for signal and background is performed using the MVA output distributions. Assuming SM Higgs Boson branching fractions, a 95% C.L. upper limit on the ttH production cross section is presented.
