Search for the decay $H\to b\overline{b}$ of the standard model Higgs boson produced in association with hadronically decaying top quarks in $pp$ collisions 8 TeV with the ATLAS detector at the LHC

Abstract : In 2012 a new particle was discovered at the Large Hadron Collider (LHC) by the ATLAS and CMS experiments, that is likely to be the Brout-Englert-Higgs boson, hereafter called Higgs boson for simplicity, predicted by the Standard Model (SM) of particle physics. To asses if this newly discovered particle is indeed the one predicted by the SM, the measurement of its properties is necessary. One of the most important properties to be measured is its coupling to the top quark. This is because the coupling of the Higgs boson to fermions is proportional to fermions' masses. As the top-quark is the heaviest known elementary particle, the strength of this coupling is of the order of 1. A precise measurement of this parameter can give hints on the energy scale where physics beyond the SM can manifest itself. A direct test of this parameter is the measurement of the Higgs boson production cross section in association with a pair of top anti-top quarks ($t\overline{t}H$). In this doctoral thesis the search of the $t\overline{t}H$ ($H\to b\overline{b}$) process is presented in the topology where the two top quarks are decaying hadronically. The combination of all $t\overline{t}H$ searches performed by ATLAS in Run 1 is also presented. The fully hadronic analysis uses 20.3 fb$^{-1}$ at $\sqrt{s}= 8$~TeV, collected with the ATLAS detector during Run 1 of the LHC. The search uses events with at least five energetic jets and uses a boosted decision tree to discriminate the signal from the overwhelming multijet background which is estimated using a new data driven technique. Moreover, for this analysis, new techniques have been developed for the estimation of trigger efficiencies and scale factors. For a Higgs Boson mass of 125 GeV, a 95\% CL upper limit of 6.5 (resp. 5.4) times the SM cross section is observed (resp. expected). A $t\overline{t}H$ signal strength $\mu = 1.6 \pm 2.6$ times the SM value is obtained. After combination of all $t\overline{t}H$ searches carried out by ATLAS in Run 1, an 95\% CL upper limit of 3.12 (resp. 1.44) is observed (resp. expected) with a signal strength of $\mu = 1.7 \pm 0.8$. The fully hadronic $t\overline{t}H$ ($H\to b\overline{b}$) analysis presented in this doctoral thesis is the first one performed in this channel at the LHC.
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Daniele Madaffari. Search for the decay $H\to b\overline{b}$ of the standard model Higgs boson produced in association with hadronically decaying top quarks in $pp$ collisions 8 TeV with the ATLAS detector at the LHC. High Energy Physics - Experiment [hep-ex]. Aix Marseille Université, 2015. English. ⟨tel-01326964⟩

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