Search for the standard model Higgs boson produced in association with a top-quark pair in pp collisions at the LHC

TL;DR

CMS performed a search for the SM Higgs boson produced in association with a top-quark pair (ttH) using 7 and 8 TeV pp collision data, focusing on H → bb decays. A multivariate approach based on artificial neural networks discriminates ttH from the dominant tt+jets background across lepton+jets and dilepton tt decays, with events categorized by jet and b-tag multiplicities to optimize sensitivity. No significant excess is observed; the combined data yield a 95% CL upper limit of 5.8 times the SM expectation for m_H = 125 GeV, illustrating the method’s viability for probing the ttH coupling at the LHC.

Abstract

A search for the standard model Higgs boson produced in association with a top-quark pair is presented using data samples corresponding to an integrated luminosity of 5.0 inverse femtobarns (5.1 inverse femtobarns) collected in pp collisions at the center-of-mass energy of 7 TeV (8 TeV). Events are considered where the top-quark pair decays to either one lepton+jets (t tbar to ell nu q q' b bbar) or dileptons (t tbar to ell(+) nu ell(-) nu b bbar), ell being an electron or a muon. The search is optimized for the decay mode H to b bbar. The largest background to the t tbar H signal is top-quark pair production with additional jets. Artificial neural networks are used to discriminate between signal and background events. Combining the results from the 7 TeV and 8 TeV samples, the observed (expected) limit on the cross section for Higgs boson production in association with top-quark pairs for a Higgs boson mass of 125 GeV is 5.8 (5.2) times the standard model expectation.

Figures (12)

• Figure 1: A leading-order Feynman diagram for ${t}\overline{{t}}\xspace {H}\xspace$ production, illustrating the two top-quark pair system decay channels considered here, and the ${H}\xspace \to {b}\overline{{b}}\xspace$ decay mode for which the analysis is optimized.
• Figure 2: Number of jets (left) and number of b-tagged jets (right) in data and simulation for events with $\geq$4 jets + $\geq$2 b-tags in the lepton+jets channel at $7\,\text{Te\spaceV}\xspace$ (top) and $8\,\text{Te\spaceV}\xspace$ (bottom). The background is normalized to the SM expectation; the uncertainty band (shown as a hatched band in the stack plot and a green band in the ratio plot) includes statistical and systematic uncertainties that affect both the rate and shape of the background distributions. The ${t}\overline{{t}}\xspace {H}\xspace$ signal ($m_{{H}\xspace} = 125\,\text{Ge\spaceV}\xspace\xspace$) is normalized to 30 $\times$ SM expectation.
• Figure 3: Distributions of the five ANN input variables with rankings 1 through 5, in terms of separation, for the 5 jets + 3 b-tags category of the lepton+jets channel at $8 \,\text{Te\spaceV}\xspace$. Definitions of the variables are given in the text. The background is normalized to the SM expectation; the uncertainty band (shown as a hatched band in the stack plot and a green band in the ratio plot) includes statistical and systematic uncertainties that affect both the rate and shape of the background distributions. The ${t}\overline{{t}}\xspace {H}\xspace$ signal ($m_{{H}\xspace} = 125\,\text{Ge\spaceV}\xspace\xspace$) is normalized to $\sim$150 $\times$ SM expectation, equal to the total background yield, for easier comparison of the shapes.
• Figure 4: Distributions of the five ANN input variables with rankings 6 through 10, in terms of separation, for the 5 jets + 3 b-tags category of the lepton+jets channel at $8 \,\text{Te\spaceV}\xspace$. Definitions of the variables are given in the text. The background is normalized to the SM expectation; the uncertainty band (shown as a hatched band in the stack plot and a green band in the ratio plot) includes statistical and systematic uncertainties that affect both the rate and shape of the background distributions. The ${t}\overline{{t}}\xspace {H}\xspace$ signal ($m_{{H}\xspace} = 125\,\text{Ge\spaceV}\xspace\xspace$) is normalized to $\sim$150 $\times$ SM expectation, equal to the total background yield, for easier comparison of the shapes.
• Figure 5: Distributions of ANN input variables for the 2 jets + 2 b-tags category of the dilepton channel at $8\,\text{Te\spaceV}\xspace$. Definitions of the variables are given in the text. The background is normalized to the SM expectation; the uncertainty band (shown as a hatched band in the stack plot and a green band in the ratio plot) includes statistical and systematic uncertainties that affect both the rate and shape of the background distributions. The ${t}\overline{{t}}\xspace {H}\xspace$ signal ($m_{{H}\xspace} =125\,\text{Ge\spaceV}\xspace\xspace$) is normalized to $\sim$7000 $\times$ SM expectation, equal to the total background yield, for easier comparison of the shapes.