Search for the Standard Model Higgs boson produced in association with top quarks and decaying into $b\bar{b}$ in pp collisions at $\sqrt{s}$ = 8 TeV with the ATLAS detector

TL;DR

The ATLAS study searches for the Standard Model Higgs boson produced in association with top-quark pairs with $H\to b\bar b$ using $20.3~\mathrm{fb}^{-1}$ of data at $\sqrt{s}=8~\mathrm{TeV}$. It combines region-based event categorization by jet and $b$-tag multiplicities with neural networks and matrix-element method discriminants to separate signal from the dominant $t\bar t$+jets background. No significant excess is found; the observed 95% CL upper limit on $\sigma(t\bar t H)$ is $3.4\times\sigma_{\mathrm{SM}}$, and the measured signal strength is $\mu=1.5\pm1.1$ for $m_H=125~\mathrm{GeV}$. The analysis demonstrates the importance of heavy-flavour modelling and advanced multivariate techniques in probing the top-quark Yukawa coupling through $t\bar t H$ production.

Abstract

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 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 $t\bar{t}$+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 $t\bar{t}$+$b\bar{b}$ 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 $t\bar{t}H$ signal cross section to the Standard Model expectation is found to be $μ$ = 1.5 $\pm$ 1.1 assuming a Higgs boson mass of 125 GeV.

Figures (34)

• Figure 1: Representative tree-level Feynman diagrams for the production of the Higgs boson in association with a top-quark pair ($t\bar{t}H$) and the subsequent decay of the Higgs to , (a) and (b), and for the main background + (c).
• Figure 2: Single-lepton channel: (a) $S/\sqrt{B}$ ratio for each of the regions assuming SM cross sections and branching fractions, and $\mH=125\gev$. Each row shows the plots for a specific jet multiplicity (4, 5, $\geq$6), and the columns show the $b$-jet multiplicity (2, 3, $\geq$4). Signal-rich regions are shaded in dark red, while the rest are shown in light blue. The $S/B$ ratio for each region is also noted. (b) The fractional contributions of the various backgrounds to the total background prediction in each considered region. The ordering of the rows and columns is the same as in (a).
• Figure 3: Dilepton channel: (a) The $S/\sqrt{B}$ ratio for each of the regions assuming SM cross sections and branching fractions and $\mH=125\gev$. Each row shows the plots for a specific jet multiplicity (2, 3, $\geq$4), and the columns show the $b$-jet multiplicity (2, 3, $\geq$4). Signal-rich regions are shaded in dark red, while the rest are shown in light blue. The $S/B$ ratio for each region is also noted. (b) The fractional contributions of the various backgrounds to the total background prediction in each considered region. The ordering of the rows and columns is the same as in (a).
• Figure 4: Relative contributions of different categories of + events in Powheg+ Pythia, Madgraph+ Pythia and SherpaOL samples. Labels "+MPI" and "+FSR" refer to events where heavy flavour is produced via multiparton interaction (MPI) or final state radiation (FSR), respectively. These contributions are not included in the SherpaOL calculation. An arrow indicates that the point is off-scale. Uncertainties are from the limited MC sample sizes.
• Figure 5: The exclusive 2-$b$-tag region of the single-lepton channel before and after the reweighting of the of the system and the of the top quark of the Powheg+Pythia$t\bar{t}$ sample. The jet multiplicity distribution (a) before and (b) after the reweighting; $\HT^{\rm had}$ distributions (c) before and (d) after the reweighting.