Search for the Standard Model Higgs boson produced in association with top quarks and decaying into [Formula: see text] in [Formula: see text] collisions at [Formula: see text] with the ATLAS detector

Abstract

A search for the Standard Model Higgs boson produced in association with a top-quark pair, [Formula: see text], is presented. The analysis uses 20.3 fb^-1 of pp collision data at [Formula: see text], collected with the ATLAS detector at the Large Hadron Collider during 2012. The search is designed for the [Formula: see text] 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 [Formula: see text]+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 [Formula: see text] 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 [Formula: see text] signal cross section to the Standard Model expectation is found to be [Formula: see text] assuming a Higgs boson mass of 125[Formula: see text].

Fig. 1

Representative tree-level Feynman diagrams for the production of the Higgs boson in association with a top-quark pair ($t\overline{t}H$) and the subsequent decay of the Higgs to $b\overline{b}$, (a, b) for the main background $t\overline{t}+b\overline{b}$ (c)

Fig. 2

Single-lepton channel: a $S/\sqrt{B}$ ratio for each of the regions assuming SM cross sections and branching fractions, and $m_H=125\mathrm{G}eV$. Each row shows the plots for a specific jet multiplicity (4, 5, $\ge$6), and the columns show the b-jet multiplicity (2, 3, $\ge$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

Fig. 3

Dilepton channel: a The $S/\sqrt{B}$ ratio for each of the regions assuming SM cross sections and branching fractions and $m_H=125\mathrm{G}eV$. Each row shows the plots for a specific jet multiplicity (2, 3, $\ge$4), and the columns show the b-jet multiplicity (2, 3, $\ge$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

Fig. 4

Relative contributions of different categories of $t\overline{t}+b\overline{b}$ events in Powheg+Pythia, Madgraph+Pythia and SherpaOL samples. Labels “$t\overline{t}$+MPI” and “$t\overline{t}$+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

Fig. 5

The exclusive 2-b-tag region of the single-lepton channel before and after the reweighting of the $p_{\mathrm{T}}$ of the $t\overline{t}$ system and the $p_{\mathrm{T}}$ of the top quark of the Powheg+Pythia $t\overline{t}$ sample. The jet multiplicity distribution (a) before and (b) after the reweighting; ${H_{\mathrm{T}}}^{\mathrm{had}}$ distribution c before and d after the reweighting

Fig. 6

Comparison of prediction to data in all analysis regions before the fit to data in a the single-lepton channel and b the dilepton channel. The signal, normalised to the SM prediction, is shown both as a filled red area stacked on the backgrounds and separately as a dashed red line. The hashed area corresponds to the total uncertainty on the yields

Fig. 7

Single-lepton channel: NN output for the different regions. In the $(5\mathrm{j},3\mathrm{b})$ region (a), the $t\overline{t}$+HF production is considered as signal and $t\overline{t}$+light as background whereas in the $(5\mathrm{j},\ge 4\mathrm{b})$ (b), $(\ge 6\mathrm{j},3\mathrm{b})$ (c), and $(\ge 6\mathrm{j},\ge 4\mathrm{b})$ (d) regions the NN output is for the $t\overline{t}H$ signal and total background. The distributions are normalised to unit area

Fig. 8

Dilepton channel: NN output for the $t\overline{t}H$ signal and total background in the a $(3\mathrm{j},3\mathrm{b})$, b $(\ge 4\mathrm{j},3\mathrm{b})$, and c $(\ge 4\mathrm{j},\ge 4\mathrm{b})$ regions. The distributions are normalised to unit area

Fig. 9

Systematic uncertainties on the $t\overline{t}+b\overline{b}$ contribution based on a scale variations and b PDF choice and shower recoil model of the SherpaOL simulation. The effect of a given systematic uncertainty is shown across the different $t\overline{t}+b\overline{b}$ categories. The effect of migration between categories is covered by variations of these systematic uncertainties

Fig. 10

Event yields in all analysis regions in a the single-lepton channel and b the dilepton channel after the combined fit to data under the signal-plus-background hypothesis. The signal, normalised to the fitted $\mathit{\mu }$, is shown both as a filled area stacked on the other backgrounds and separately as a dashed line. The hashed area represents the total uncertainty on the yields

Fig. 11

Single-lepton channel: comparison between data and prediction for the discriminant variable used in the $(4\mathrm{j},2\mathrm{b})$ region a before the fit and b after the fit, in the $(4\mathrm{j},2\mathrm{b})$ region c before the fit and d after the fit, in the $(4\mathrm{j},4\mathrm{b})$ region e before the fit and f after the fit. The fit is performed on data under the signal-plus-background hypothesis. The last bin in all figures contains the overflow. The bottom panel displays the ratio of data to the total prediction. An arrow indicates that the point is off-scale. The hashed area represents the uncertainty on the background. The $t\overline{t}H$ signal yield (solid) is normalised to the SM cross section before the fit and to the fitted $\mathit{\mu }$ after the fit. In several regions, predominantly the control regions, the $t\overline{t}H$ signal yield is not visible on top of the large background

Fig. 12

Single-lepton channel: comparison of data and prediction for the discriminant variable used in the $(5\mathrm{j},2\mathrm{b})$ region a before the fit and b after the fit, in the $(5\mathrm{j},3\mathrm{b})$ region c before the fit and d after the fit, in the $(5\mathrm{j},\ge 4\mathrm{b})$ region e before the fit and f after the fit. The fit is peformed on data under the signal-plus-background hypothesis. The last bin in all figures contains the overflow. The bottom panel displays the ratio of data to the total prediction. An arrow indicates that the point is off-scale. The hashed area represents the uncertainty on the background. The dashed line shows $t\overline{t}H$ signal distribution normalised to background yield. The $t\overline{t}H$ signal yield (solid) is normalised to the SM cross section before the fit and to the fitted $\mathit{\mu }$ after the fit. In several regions, predominantly the control regions, the $t\overline{t}H$ signal yield is not visible on top of the large background

Fig. 13

Single-lepton channel: comparison of data and prediction for the discriminant variable used in the $(\ge 6\mathrm{j},2\mathrm{b})$ region a before the fit and b after the fit, in the $(\ge 6\mathrm{j},3\mathrm{b})$ region c before the fit and d after the fit, in the $(\ge 6\mathrm{j},\ge 4\mathrm{b})$ region e before the fit and f after the fit. The fit is performed on data under the signal-plus-background hypothesis. The last bin in all figures contains the overflow. The bottom panel displays the ratio of data to the total prediction. An arrow indicates that the point is off-scale. The hashed area represents the uncertainty on the background. The dashed line shows $t\overline{t}H$ signal distribution normalised to background yield. The $t\overline{t}H$ signal yield (solid) is normalised to the SM cross section before the fit and to the fitted $\mathit{\mu }$ after the fit. In several regions, predominantly the control regions, the $t\overline{t}H$ signal yield is not visible on top of the large background

Fig. 14

Dilepton channel: comparison of data and prediction for the discriminant variable used in the $(2\mathrm{j},2\mathrm{b})$ region a before the fit and b after the fit, in the $(3\mathrm{j},2\mathrm{b})$ region c before the fit and d after the fit, in the $(3\mathrm{j},3\mathrm{b})$ region e before the fit and f after the fit. The fit is performed on data under the signal-plus-background hypothesis. The last bin in all figures contains the overflow. The bottom panel displays the ratio of data to the total prediction. An arrow indicates that the point is off-scale. The hashed area represents the uncertainty on the background. The dashed line shows $t\overline{t}H$ signal distribution normalised to background yield. The $t\overline{t}H$ signal yield (solid) is normalised to the SM cross section before the fit and to the fitted $\mathit{\mu }$ after the fit. In several regions, predominantly the control regions, the $t\overline{t}H$ signal yield is not visible on top of the large background

Fig. 15

Dilepton channel: comparison of data and prediction for the discriminant variable used in the $(\ge 4\mathrm{j},2\mathrm{b})$ region a before the fit and b after the fit, in the $(\ge 4\mathrm{j},3\mathrm{b})$ region c before the fit and d after the fit, in the $(\ge 4\mathrm{j},\ge 4\mathrm{b})$ region e before the fit and f after the fit. The fit is performed on data under the signal-plus-background hypothesis. The last bin in all figures contains the overflow. The bottom panel displays the ratio of data to the total prediction. An arrow indicates that the point is off-scale. The hashed area represents the uncertainty on the background. The dashed line shows $t\overline{t}H$ signal distribution normalised to background yield. The $t\overline{t}H$ signal yield (solid) is normalised to the SM cross section before the fit and to the fitted $\mathit{\mu }$ after the fit. In several regions, predominantly the control regions, the $t\overline{t}H$ signal yield is not visible on top of the large background

Fig. 16

The fitted values of the signal strength and their uncertainties for the individual channels and their combination. The green line shows the statistical uncertainty on the signal strength

Fig. 17

95 % CL upper limits on $\mathit{\sigma }(t\overline{t}H)$ relative to the SM prediction, $\mathit{\sigma }/{\mathit{\sigma }}_{\mathrm{SM}}$, for the individual channels as well as their combination. The observed limits (solid lines) are compared to the expected (median) limits under the background-only hypothesis and under the signal-plus-background hypothesis assuming the SM prediction for $\mathit{\sigma }(t\overline{t}H)$ and pre-fit prediction for the background. The surrounding shaded bands correspond to the 68 and 95 % confidence intervals around the expected limits under the background-only hypothesis, denoted by $\pm 1\mathit{\sigma }$ and $\pm 2\mathit{\sigma }$, respectively

Fig. 18

Event yields as a function of ${log}_{10}(S/B)$, where S (signal yield) and B (background yield) are taken from the ${H_{\mathrm{T}}}^{\mathrm{had}}$, $H_{\mathrm{T}}$, and NN output bin of each event. Events in all fitted regions are included. The predicted background is obtained from the global signal-plus-background fit. The $t\overline{t}H$ signal is shown both for the best fit value ($\mathit{\mu }=1.5$) and for the upper limit at 95 % CL ($\mathit{\mu }=3.4$)

Fig. 19

The fitted values of the nuisance parameters with the largest impact on the measured signal strength. The points, which are drawn conforming to the scale of the bottom axis, show the deviation of each of the fitted nuisance parameters, $\widehat{\mathit{\theta }}$, from ${\mathit{\theta }}_0$, which is the nominal value of that nuisance parameter, in units of the pre-fit standard deviation $\mathrm{\Delta }\mathit{\theta }$. The error bars show the post-fit uncertainties, ${\mathit{\sigma }}_{\mathit{\theta }}$, which are close to 1 if the data do not provide any further constraint on that uncertainty. Conversely, a value of ${\mathit{\sigma }}_{\mathit{\theta }}$ much smaller than 1 indicates a significant reduction with respect to the original uncertainty. The nuisance parameters are sorted according to the post-fit effect of each on $\mathit{\mu }$ (hashed blue area) conforming to the scale of the top axis, with those with the largest impact at the top

Fig. 20

Contribution of various Higgs boson decay modes to the analysis regions in a the single-lepton channel and b the dilepton channel

Fig. 21

Single-lepton channel: comparison of $t\overline{t}$+HF (dashed) and $t\overline{t}$+light (solid) background for the four top-ranked input variables in the $(5\mathrm{j},3\mathrm{b})$ region where the NN is designed to separate these two backgrounds. The plots include a $m_{\mathrm{bb}}^{\max {\mathrm{p}}_{\mathrm{T}}}$, b $m_{\mathrm{uu}}^{\min \mathrm{\Delta }\mathrm{R}}$, c $p_{\mathrm{T},\mathrm{uu}}^{\min \mathrm{\Delta }\mathrm{R}}$ and d $m_{\mathrm{bb}}^{\min \mathrm{\Delta }\mathrm{R}}$

Fig. 22

Single-lepton channel: comparison of $t\overline{t}H$ signal (dashed) and background (solid) for the four top-ranked input variables in the $(5\mathrm{j},\ge 4\mathrm{b})$ region. The plots include a $\mathrm{Centrality}$, b H1, c $N_{40}^{\mathrm{jet}}$ and d $m_{\mathrm{bb}}^{\min \mathrm{\Delta }\mathrm{R}}$

Fig. 23

Single-lepton channel: comparison of $t\overline{t}H$ signal (dashed) and background (solid) for the four top-ranked input variables in the $(\ge 6\mathrm{j},3\mathrm{b})$ region. The plots include a $N_{40}^{\mathrm{jet}}$, b $\mathrm{Centrality}$, c H1, and d SSLL

Fig. 24

Single-lepton channel: comparison of $t\overline{t}H$ signal (dashed) and background (solid) for the four top-ranked input variables in the $(\ge 6\mathrm{j},\ge 4\mathrm{b})$ region. The plots include a D1, b $\mathrm{Centrality}$, c ${p_{\mathrm{T}}}^{\mathrm{jet}5}$, and d H1

Fig. 25

Dilepton channel: comparison of $t\overline{t}H$ signal (dashed) and background (solid) for the four top-ranked input variables in the $(3\mathrm{j},3\mathrm{b})$ region. The plots include a $\mathrm{\Delta }{\mathit{\eta }}_{\mathrm{jj}}^{\max \mathrm{\Delta }\mathit{\eta }}$, b $m_{\mathrm{jj}}^{\min \mathrm{m}}$, c $H_{\mathrm{T}}$, and d H4

Fig. 26

Dilepton channel: comparison of $t\overline{t}H$ signal (dashed) and background (solid) for the four top-ranked input variables in the $(\ge 4\mathrm{j},3\mathrm{b})$ region. The plots include a $\mathrm{\Delta }{\mathit{\eta }}_{\mathrm{jj}}^{\max \mathrm{\Delta }\mathit{\eta }}$, b ${\mathrm{N}}_{30}^{\mathrm{Higgs}}$, c $m_{\mathrm{jj}}^{\min \mathrm{m}}$, and d $\mathrm{\Delta }{R}_{\mathrm{bb}}^{\max {\mathrm{p}}_{\mathrm{T}}}$

Fig. 27

Dilepton channel: comparison of $t\overline{t}H$ signal (dashed) and background (solid) for the four top-ranked input variables in the $(\ge 4\mathrm{j},\ge 4\mathrm{b})$ region. The plots include a $\mathrm{\Delta }{\mathit{\eta }}_{\mathrm{jj}}^{\max \mathrm{\Delta }\mathit{\eta }}$, b $m_{\mathrm{bb}}^{\min \mathrm{\Delta }\mathrm{R}}$, c $m_{b\overline{b}}$, and d $\mathrm{\Delta }{R}_{\mathrm{hl}}^{\min \mathrm{\Delta }\mathrm{R}}$

Fig. 28

Single-lepton channel: post-fit comparison of data and prediction for the four top-ranked input variables in the $(5\mathrm{j},3\mathrm{b})$ region. The plots include a $m_{\mathrm{bb}}^{\max {\mathrm{p}}_{\mathrm{T}}}$, b $m_{\mathrm{uu}}^{\min \mathrm{\Delta }\mathrm{R}}$, c $p_{\mathrm{T},\mathrm{uu}}^{\min \mathrm{\Delta }\mathrm{R}}$ and d $m_{\mathrm{bb}}^{\min \mathrm{\Delta }\mathrm{R}}$. The first and last bins in all figures contain the underflow and overflow, respectively. The bottom panel displays the ratio of data to the total prediction. An arrow indicates that the point is off-scale. The hashed area represents the uncertainty on the background

Fig. 29

Single-lepton channel: post-fit comparison of data and prediction for the four top-ranked input variables in the $(5\mathrm{j},\ge 4\mathrm{b})$ region. The plots include a $\mathrm{Centrality}$, b H1, c $N_{40}^{\mathrm{jet}}$ and d $m_{\mathrm{bb}}^{\min \mathrm{\Delta }\mathrm{R}}$. The first and last bins in all figures contain the underflow and overflow, respectively. The bottom panel displays the ratio of data to the total prediction. An arrow indicates that the point is off-scale. The hashed area represents the uncertainty on the background. The dashed line shows $t\overline{t}H$ signal distribution normalised to background yield. The $t\overline{t}H$ signal yield (solid) is normalised to the fitted $\mathit{\mu }$

Fig. 30

Single-lepton channel: post-fit comparison of data and prediction for the four top-ranked input variables in $(\ge 6\mathrm{j},3\mathrm{b})$ region. The plots include a $N_{40}^{\mathrm{jet}}$, b $\mathrm{Centrality}$, c H1, and d SSLL. The first and last bins in all figures contain the underflow and overflow, respectively. The bottom panel displays the ratio of data to the total prediction. An arrow indicates that the point is off-scale. The hashed area represents the uncertainty on the background. The dashed line shows $t\overline{t}H$ signal distribution normalised to background yield. The $t\overline{t}H$ signal yield (solid) is normalised to the fitted $\mathit{\mu }$

Fig. 31

Single-lepton channel: post-fit comparison of data and prediction for the four top-ranked input variables in $(\ge 6\mathrm{j},\ge 4\mathrm{b})$ region. The plots include a D1, b $\mathrm{Centrality}$, c ${p_{\mathrm{T}}}^{\mathrm{jet}5}$, and d H1. The first and last bins in all figures contain the underflow and overflow, respectively. The bottom panel displays the ratio of data to the total prediction. An arrow indicates that the point is off-scale. The hashed area represents the uncertainty on the background. The dashed line shows $t\overline{t}H$ signal distribution normalised to background yield. The $t\overline{t}H$ signal yield (solid) is normalised to the fitted $\mathit{\mu }$

Fig. 32

Dilepton channel: post-fit comparison of data and prediction for the four top-ranked input variables in the $(3\mathrm{j},3\mathrm{b})$ region. The plots include a $\mathrm{\Delta }{\mathit{\eta }}_{\mathrm{jj}}^{\max \mathrm{\Delta }\mathit{\eta }}$, b $m_{\mathrm{jj}}^{\min \mathrm{m}}$, c $H_{\mathrm{T}}$, and d H4. The first and last bins in all figures contain the underflow and overflow, respectively. The bottom panel displays the ratio of data to the total prediction. An arrow indicates that the point is off-scale. The hashed area represents the uncertainty on the background. The dashed line shows $t\overline{t}H$ signal distribution normalised to background yield. The $t\overline{t}H$ signal yield (solid) is normalised to the fitted $\mathit{\mu }$

Fig. 33

Dilepton channel: post-fit comparison of data and prediction for the four top-ranked input variables in the $(\ge 4\mathrm{j},3\mathrm{b})$ region. The plots include a $\mathrm{\Delta }{\mathit{\eta }}_{\mathrm{jj}}^{\max \mathrm{\Delta }\mathit{\eta }}$, b ${\mathrm{N}}_{30}^{\mathrm{Higgs}}$, c $m_{\mathrm{jj}}^{\min \mathrm{m}}$, and d $\mathrm{\Delta }{R}_{\mathrm{bb}}^{\max {\mathrm{p}}_{\mathrm{T}}}$. The first and last bins in all figures contain the underflow and overflow, respectively. The bottom panel displays the ratio of data to the total prediction. An arrow indicates that the point is off-scale. The hashed area represents the uncertainty on the background. The dashed line shows $t\overline{t}H$ signal distribution normalised to background yield. The $t\overline{t}H$ signal yield (solid) is normalised to the fitted $\mathit{\mu }$

Fig. 34

Dilepton channel: post-fit comparison of data and prediction for the four top-ranked input variables in the $(\ge 4\mathrm{j},\ge 4\mathrm{b})$ region. The plots include a $\mathrm{\Delta }{\mathit{\eta }}_{\mathrm{jj}}^{\max \mathrm{\Delta }\mathit{\eta }}$, b $m_{\mathrm{bb}}^{\min \mathrm{\Delta }\mathrm{R}}$, c $m_{b\overline{b}}$, and d $\mathrm{\Delta }{R}_{\mathrm{hl}}^{\min \mathrm{\Delta }\mathrm{R}}$. The first and last bins in all figures contain the underflow and overflow, respectively. The bottom panel displays the ratio of data to the total prediction. An arrow indicates that the point is off-scale. The hashed area represents the uncertainty on the background. The dashed line shows $t\overline{t}H$ signal distribution normalised to background yield. The $t\overline{t}H$ signal yield (solid) is normalised to the fitted $\mathit{\mu }$