Search for the Standard Model Higgs boson produced in association with $t\bar{t}$ and decaying into $b\bar{b}$ at $\sqrt{s} = 8$ TeV with the ATLAS detector using the Matrix Element Method

A search for the Standard Model Higgs boson produced in association with a pair of top quarks ($t\bar{t}H$) 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 $H \to b\bar{b}$ decay mode and is performed in the single lepton (electrons or muons) decay channel of the top quark pair. In order to improve the sensitivity of the search, events are categorised according to their jet and $b$-tagged jet multiplicities into nine different analysis regions. A matrix element method is developed and applied to regions with six jets to obtain discriminants separating $t\bar{t}H$ events from the irreducible $t\bar{t} + b\bar{b}$ background. In signal-enriched regions, a neural network is employed combining kinematic variables and variables obtained from the matrix element method to maximise the separation between signal and background events. The nine analysis regions are statistically combined using a profile likelihood fit to improve the background predictions and reduce the systematic uncertainties. The ratio of the measured $t\bar{t}H$ signal cross section to the Standard Model expectation is found to be $\mu = 1.2 \pm 1.3$ assuming a Higgs boson mass of 125 GeV. No significant excess of events above the background expectation is found and an observed (expected) limit of $\mu < 3.6 (2.6)$ is set at 95% confidence level. The single lepton channel is combined with the dilepton channel of the $t\bar{t}$ decay resulting in $\mu = 1.5 \pm 1.1$ and an observed (expected) upper limit of 3.4 (2.2) times the Standard Model cross section at 95% confidence level.