Abstract
A search for the Standard Model Higgs boson produced in association with a top-quark pair, $t\bar{t}H$, is presented. The analysis uses 36.1 fb$^{-1}$ of $pp$ collision data at $\sqrt{s}$ = 13 TeV, collected with the ATLAS detector at the Large Hadron Collider in 2015 and 2016. The search targets the $H \to b\bar{b}$ decay mode. The selected events contain either one or two electrons or muons from the top-quark decays, and are then categorized according to the number of jets and how likely these are to contain $b$-hadrons. Multivariate techniques are used to discriminate between signal and background events, the latter being dominated by $t\bar{t}$ + jets production. For a Higgs boson mass of 125 GeV, the ratio of the measured $t\bar{t}H$ signal cross-section to the Standard Model expectation is found to be $\mu = 0.84^{+0.64}_{-0.61}$. A value of $\mu$ greater than 2.0 is excluded at 95% confidence level while the expected upper limit is $\mu < 1.2$ in the absence of a $t\bar{t}H$ signal.
Highlights
After the discovery of the Higgs boson [1,2,3] in 2012 by the ATLAS [4] and CMS [5] Collaborations, attention has turned to more detailed measurements of its properties and couplings as a means of testing the predictions of the standard model (SM) [6,7,8]
The combination of the results presented in this article with the results from other analyses targeting ttH production with different final states is reported in Ref. [21]
The muon spectrometer measures the deflection of muons with jηj < 2.7 using multiple layers of high-precision tracking chambers located in a toroidal field
Summary
After the discovery of the Higgs boson [1,2,3] in 2012 by the ATLAS [4] and CMS [5] Collaborations, attention has turned to more detailed measurements of its properties and couplings as a means of testing the predictions of the standard model (SM) [6,7,8]. Assuming that no BSM particle couples to the Higgs boson, the ATLAS and CMS experiments measured a value of the top-quark’s Yukawa coupling equal to 0.87 Æ 0.15 times the SM prediction by combining [10] their respective Higgsboson measurements from the Run 1 dataset collected at center-of-mass energies of 7 and 8 TeV at the Large Hadron Collider (LHC) This measurement relies largely on the gluon–gluon fusion production mode and on the decay mode to photons, which both depend on loop contributions with a top quark. TeV and pffiffi s using tt decays with a single-lepton or dilepton in the final state, obtaining a signal strength of 0.7 Æ 1.9 [18] These results were combined with each other, and with results for Higgs boson decay to vector bosons, to τ-leptons or to photons [18,19,20], resulting in an observed (expected) significance of 4.4 (2.0) standard deviations for ttH production [10].
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