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Evidence for the associated production of the Higgs boson and a top quark pair with the ATLAS detector ATLAS-CONF-2017-077 23 October 2017
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| Main document (CDS record), Physics Briefing - internal | pdf from CDS | |
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| Abstract | ||
| A search for the associated production of the Higgs boson with a top quark pair ($t\bar t H$) in multilepton final states using a dataset corresponding to an integrated luminosity of 36.1 fb$^{-1}$ of proton--proton collision data recorded by the ATLAS experiment at a centre-of-mass energy $\sqrt{s} = 13$ TeV at the Large Hadron Collider is reported. Higgs boson decays to $WW^*$, $\tau\tau$, and $ZZ^*$ are targeted. Seven final states, categorised by the number and flavour of charged-lepton candidates, are examined for the presence of the Standard Model Higgs boson with a mass of 125 GeV and a pair of top quarks. An excess of events over the expected background from other Standard Model processes is found with an observed significance of 4.1 standard deviations, compared to an expectation of 2.8 standard deviations. The best fit for the $t\bar t H$ production cross section is $\sigma(t\bar t H) = 790^{+230}_{-210}$ fb, in agreement with the Standard Model prediction of $507^{+35}_{-50}$ fb. The combination of this result with other $t\bar t H$ searches from the ATLAS experiment using the Higgs boson decay modes to $b\bar b$, $\gamma\gamma$ and $ZZ^* \to 4\ell$, has an observed significance of 4.2 standard deviations, compared to an expectation of 3.8 standard deviations. This provides evidence for the $t\bar t H$ production mode. | ||
| Figures | ||
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Figure 01a: Example of tree-level Feynman diagrams for the production of the Higgs boson in association with a pair of top quarks. Higgs boson decays to (left) WW/ZZ or (right) ττ are shown. png (35kB) eps (166kB) pdf (9kB) |
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Figure 01b: Example of tree-level Feynman diagrams for the production of the Higgs boson in association with a pair of top quarks. Higgs boson decays to (left) WW/ZZ or (right) ττ are shown. png (31kB) eps (166kB) pdf (9kB) |
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Figure 02a: The efficiency to select well-identified prompt muons (left) and electrons (right) at the chosen non-prompt lepton BDT working point, as a function of the lepton pT. The muons are required to pass the loose identification requirements, while the electrons are required to pass the tight identification requirements. The measurements in data (simulation) are shown as full black (open red) circles. The bottom panel displays the ratio of data to simulation results, with the blue (yellow) band representing the statistical (total) uncertainty. This ratio is the scale factors that are applied to correct the simulation. png (85kB) pdf (14kB) |
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Figure 02b: The efficiency to select well-identified prompt muons (left) and electrons (right) at the chosen non-prompt lepton BDT working point, as a function of the lepton pT. The muons are required to pass the loose identification requirements, while the electrons are required to pass the tight identification requirements. The measurements in data (simulation) are shown as full black (open red) circles. The bottom panel displays the ratio of data to simulation results, with the blue (yellow) band representing the statistical (total) uncertainty. This ratio is the scale factors that are applied to correct the simulation. png (86kB) pdf (15kB) |
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Figure 03: The channels used in the analysis organised according to the number of selected light leptons and τhad candidates. The selection requirements for each channel are in Table 3. png (29kB) pdf (28kB) |
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Figure 04a: Left: The fraction of the expected ttH signal arising from different Higgs boson decay modes in each signal region. The decays labelled as "other" are dominantly H→μμ and H→ bb. Right: Pre-fit S/B (black line) and S/√ B (red dashed line) ratios for each of the twelve analysis categories including the four 3ℓ control regions. The background prediction methods are described in Section 6. png (36kB) pdf (15kB) |
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Figure 04b: Left: The fraction of the expected ttH signal arising from different Higgs boson decay modes in each signal region. The decays labelled as "other" are dominantly H→μμ and H→ bb. Right: Pre-fit S/B (black line) and S/√ B (red dashed line) ratios for each of the twelve analysis categories including the four 3ℓ control regions. The background prediction methods are described in Section 6. png (37kB) pdf (14kB) |
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Figure 05: The fractional contributions of the various backgrounds to the total predicted background in each of the twelve analysis categories. The background prediction methods are described in Section 6 : "Non-prompt", "Fake τhad" and "q mis-id" refer to the data-driven background estimates (largely tt but also include other electroweak processes), and rare processes (tZ, tW, tWZ, ttWW, triboson production, ttt, ttttH, tH, rare top decay) are labelled as "Other". png (163kB) pdf (118kB) |
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Figure 06a: Comparison of data and prediction of the jet multiplicity in (left) the 3ℓ ttZ and (right) the ttW control regions. The last bin in all figures contains the overflow. The bottom panel displays the ratio of data to the total prediction. The hashed area represents the total uncertainty in the background. The background prediction for non-prompt leptons is described in Section 6.2 and the other backgrounds are normalised according to the predictions from simulation. png (137kB) pdf (109kB) |
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Figure 06b: Comparison of data and prediction of the jet multiplicity in (left) the 3ℓ ttZ and (right) the ttW control regions. The last bin in all figures contains the overflow. The bottom panel displays the ratio of data to the total prediction. The hashed area represents the total uncertainty in the background. The background prediction for non-prompt leptons is described in Section 6.2 and the other backgrounds are normalised according to the predictions from simulation. png (133kB) pdf (109kB) |
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Figure 07a: The composition from simulation of (left) the fake and non-prompt light leptons and (right) the fake τhad in selected analysis regions. The light-lepton composition is shown separately depending on the lepton flavour in the regions used in the estimate of the non-prompt contribution. The control regions labelled '2lSSxx' are used for the 2ℓSS and 3ℓ channels; those labelled '3lx' are used for the 4ℓ channel where x denotes the flavour of the lowest pT lepton and those labelled '2lSSx+1τ' are used for the 2ℓSS+1τhad channel. The non-prompt lepton background has been separated into the components from b-jets, c-jets, other jets, J/ψ, photon conversions and other contributions. The τhad composition is shown both in the control regions used in the estimates and in the signal regions of each channel. The τhad background has been separated into the components from b-jets, c-jets, light quark jets, gluon jets, electrons and other contributions. The latter includes muons, hadrons and cases where associating reconstructed leptons to a particular source can't be done without ambiguity. png (35kB) pdf (15kB) |
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Figure 07b: The composition from simulation of (left) the fake and non-prompt light leptons and (right) the fake τhad in selected analysis regions. The light-lepton composition is shown separately depending on the lepton flavour in the regions used in the estimate of the non-prompt contribution. The control regions labelled '2lSSxx' are used for the 2ℓSS and 3ℓ channels; those labelled '3lx' are used for the 4ℓ channel where x denotes the flavour of the lowest pT lepton and those labelled '2lSSx+1τ' are used for the 2ℓSS+1τhad channel. The non-prompt lepton background has been separated into the components from b-jets, c-jets, other jets, J/ψ, photon conversions and other contributions. The τhad composition is shown both in the control regions used in the estimates and in the signal regions of each channel. The τhad background has been separated into the components from b-jets, c-jets, light quark jets, gluon jets, electrons and other contributions. The latter includes muons, hadrons and cases where associating reconstructed leptons to a particular source can't be done without ambiguity. png (39kB) pdf (15kB) |
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Figure 08a: Comparison of data and prediction of (a) the angular distance between the subleading lepton and the closest jet in the μμ channel and (b) the subleading lepton pT in the opposite-flavour channel, in a 2ℓSS low-Njets validation region (VR); (c) the b-tagged jet multiplicity in a validation region similar to the control region used in the 4ℓ channel but at higher Njets multiplicity (called 3ℓ VR), with the leptons categorised according to their origin: prompt, heavy-flavour (HF) and light-flavour (LF), see text; (d) the jet multiplicity in the 2ℓOS+1τhad category. The last bin in all figures contains the overflow. The bottom panel displays the ratio of data to the total prediction. The hashed area represents the total uncertainty in the background. png (136kB) pdf (82kB) |
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Figure 08b: Comparison of data and prediction of (a) the angular distance between the subleading lepton and the closest jet in the μμ channel and (b) the subleading lepton pT in the opposite-flavour channel, in a 2ℓSS low-Njets validation region (VR); (c) the b-tagged jet multiplicity in a validation region similar to the control region used in the 4ℓ channel but at higher Njets multiplicity (called 3ℓ VR), with the leptons categorised according to their origin: prompt, heavy-flavour (HF) and light-flavour (LF), see text; (d) the jet multiplicity in the 2ℓOS+1τhad category. The last bin in all figures contains the overflow. The bottom panel displays the ratio of data to the total prediction. The hashed area represents the total uncertainty in the background. png (140kB) pdf (83kB) |
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Figure 08c: Comparison of data and prediction of (a) the angular distance between the subleading lepton and the closest jet in the μμ channel and (b) the subleading lepton pT in the opposite-flavour channel, in a 2ℓSS low-Njets validation region (VR); (c) the b-tagged jet multiplicity in a validation region similar to the control region used in the 4ℓ channel but at higher Njets multiplicity (called 3ℓ VR), with the leptons categorised according to their origin: prompt, heavy-flavour (HF) and light-flavour (LF), see text; (d) the jet multiplicity in the 2ℓOS+1τhad category. The last bin in all figures contains the overflow. The bottom panel displays the ratio of data to the total prediction. The hashed area represents the total uncertainty in the background. png (105kB) pdf (89kB) |
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Figure 08d: Comparison of data and prediction of (a) the angular distance between the subleading lepton and the closest jet in the μμ channel and (b) the subleading lepton pT in the opposite-flavour channel, in a 2ℓSS low-Njets validation region (VR); (c) the b-tagged jet multiplicity in a validation region similar to the control region used in the 4ℓ channel but at higher Njets multiplicity (called 3ℓ VR), with the leptons categorised according to their origin: prompt, heavy-flavour (HF) and light-flavour (LF), see text; (d) the jet multiplicity in the 2ℓOS+1τhad category. The last bin in all figures contains the overflow. The bottom panel displays the ratio of data to the total prediction. The hashed area represents the total uncertainty in the background. png (116kB) pdf (93kB) |
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Figure 09: The impact of systematic uncertainties on the fitted signal-strength parameter μ̂ for the combined fit of all channels. The systematic uncertainties are listed in decreasing order of their impact on μ̂ on the y-axis, and only the fifteen most important ones are displayed. The filled blue boxes show the variations of μ̂ with respect to the central value, Δμ, referring to the upper x-axis, when fixing the corresponding individual nuisance parameter, θ, to its post-fit value θ̂ modified upwards or downwards by its post-fit uncertainty, and repeating the fit. The empty blue boxes represent the corresponding pre-fit impact. The black points, which refer to the lower x-axis, show the fitted values and uncertainties of the nuisance parameters, with respect to their pre-fit values, θ0, and uncertainties, Δθ. The black lines show the post-fit uncertainties of the nuisance parameters, relative to their nominal uncertainties, which are indicated by the dashed line. png (140kB) pdf (110kB) |
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Figure 10: Comparison of prediction to data after the fit in the eight signal and four control regions. The background contributions after the global fit are shown as filled histograms. The total background before the fit is shown as a dashed blue histogram. The Higgs boson signal (mH = 125 GeV), scaled according to the results of the fit, is shown as a filled red histogram superimposed on the fitted backgrounds. The size of the combined statistical and systematic uncertainty in the sum of the signal and fitted background is indicated by the blue hatched band. The ratio of the data to the sum of the signal and fitted background is shown in the lower panel. The yields in each region are shown in Table 10. png (127kB) pdf (130kB) |
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Figure 11a: The distribution of the discriminant variables observed in data (points with bars indicating the statistical errors) and expected (histograms) in the (a) 2ℓSS, (b) 3ℓ, (c) 4ℓ (Z-enriched) and (d) 4ℓ (Z-depleted) signal regions. The background contributions after the global fit are shown as filled histograms. The total background before the fit is shown as a dashed blue histogram. The Higgs boson signal (mH = 125 GeV), scaled according to the results of the fit, is shown as a filled red histogram superimposed on the fitted backgrounds. The size of the combined statistical and systematic uncertainty in the sum of the signal and fitted background is indicated by the blue hatched band. The ratio of the data to the sum of the signal and fitted background is shown in the lower panel. png (117kB) pdf (77kB) |
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Figure 11b: The distribution of the discriminant variables observed in data (points with bars indicating the statistical errors) and expected (histograms) in the (a) 2ℓSS, (b) 3ℓ, (c) 4ℓ (Z-enriched) and (d) 4ℓ (Z-depleted) signal regions. The background contributions after the global fit are shown as filled histograms. The total background before the fit is shown as a dashed blue histogram. The Higgs boson signal (mH = 125 GeV), scaled according to the results of the fit, is shown as a filled red histogram superimposed on the fitted backgrounds. The size of the combined statistical and systematic uncertainty in the sum of the signal and fitted background is indicated by the blue hatched band. The ratio of the data to the sum of the signal and fitted background is shown in the lower panel. png (125kB) pdf (77kB) |
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Figure 11c: The distribution of the discriminant variables observed in data (points with bars indicating the statistical errors) and expected (histograms) in the (a) 2ℓSS, (b) 3ℓ, (c) 4ℓ (Z-enriched) and (d) 4ℓ (Z-depleted) signal regions. The background contributions after the global fit are shown as filled histograms. The total background before the fit is shown as a dashed blue histogram. The Higgs boson signal (mH = 125 GeV), scaled according to the results of the fit, is shown as a filled red histogram superimposed on the fitted backgrounds. The size of the combined statistical and systematic uncertainty in the sum of the signal and fitted background is indicated by the blue hatched band. The ratio of the data to the sum of the signal and fitted background is shown in the lower panel. png (55kB) pdf (92kB) |
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Figure 11d: The distribution of the discriminant variables observed in data (points with bars indicating the statistical errors) and expected (histograms) in the (a) 2ℓSS, (b) 3ℓ, (c) 4ℓ (Z-enriched) and (d) 4ℓ (Z-depleted) signal regions. The background contributions after the global fit are shown as filled histograms. The total background before the fit is shown as a dashed blue histogram. The Higgs boson signal (mH = 125 GeV), scaled according to the results of the fit, is shown as a filled red histogram superimposed on the fitted backgrounds. The size of the combined statistical and systematic uncertainty in the sum of the signal and fitted background is indicated by the blue hatched band. The ratio of the data to the sum of the signal and fitted background is shown in the lower panel. png (112kB) pdf (93kB) |
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Figure 12a: The distribution of the discriminant variable observed in data (points with bars indicating the statistical errors) and expected (histograms) in the (a) 2ℓSS+1τhad, (b) 2ℓOS+1τhad, (c) 1ℓ+2τhad and (d) 3ℓ+1τhad signal regions. The background contributions after the global fit are shown as filled histograms. The total background before the fit is shown as a dashed blue histogram. The Higgs boson signal (mH = 125 GeV), scaled according to the results of the fit, is shown as a filled red histogram superimposed on the fitted backgrounds. The size of the combined statistical and systematic uncertainty in the sum of the signal and fitted background is indicated by the blue hatched band. The ratio of the data to the sum of the signal and fitted background is shown in the lower panel. png (112kB) pdf (94kB) |
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Figure 12b: The distribution of the discriminant variable observed in data (points with bars indicating the statistical errors) and expected (histograms) in the (a) 2ℓSS+1τhad, (b) 2ℓOS+1τhad, (c) 1ℓ+2τhad and (d) 3ℓ+1τhad signal regions. The background contributions after the global fit are shown as filled histograms. The total background before the fit is shown as a dashed blue histogram. The Higgs boson signal (mH = 125 GeV), scaled according to the results of the fit, is shown as a filled red histogram superimposed on the fitted backgrounds. The size of the combined statistical and systematic uncertainty in the sum of the signal and fitted background is indicated by the blue hatched band. The ratio of the data to the sum of the signal and fitted background is shown in the lower panel. png (124kB) pdf (96kB) |
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Figure 12c: The distribution of the discriminant variable observed in data (points with bars indicating the statistical errors) and expected (histograms) in the (a) 2ℓSS+1τhad, (b) 2ℓOS+1τhad, (c) 1ℓ+2τhad and (d) 3ℓ+1τhad signal regions. The background contributions after the global fit are shown as filled histograms. The total background before the fit is shown as a dashed blue histogram. The Higgs boson signal (mH = 125 GeV), scaled according to the results of the fit, is shown as a filled red histogram superimposed on the fitted backgrounds. The size of the combined statistical and systematic uncertainty in the sum of the signal and fitted background is indicated by the blue hatched band. The ratio of the data to the sum of the signal and fitted background is shown in the lower panel. png (108kB) pdf (94kB) |
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Figure 12d: The distribution of the discriminant variable observed in data (points with bars indicating the statistical errors) and expected (histograms) in the (a) 2ℓSS+1τhad, (b) 2ℓOS+1τhad, (c) 1ℓ+2τhad and (d) 3ℓ+1τhad signal regions. The background contributions after the global fit are shown as filled histograms. The total background before the fit is shown as a dashed blue histogram. The Higgs boson signal (mH = 125 GeV), scaled according to the results of the fit, is shown as a filled red histogram superimposed on the fitted backgrounds. The size of the combined statistical and systematic uncertainty in the sum of the signal and fitted background is indicated by the blue hatched band. The ratio of the data to the sum of the signal and fitted background is shown in the lower panel. png (113kB) pdf (94kB) |
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Figure 13: The observed best-fit values of the ttH signal strength μ and their uncertainties by final-state category and combined. The individual μ values for the channels are obtained from a simultaneous fit with the signal-strength parameter for each channel floating independently. The SM prediction is μ=1. png (65kB) pdf (92kB) |
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Figure 14: Event yields as a function of log10(S/B) for data, background and a Higgs boson signal with mH = 125 GeV. The discriminant bins in all signal regions are combined into bins of log10(S/B), where S is the expected signal yield and B the background yield from the unconditional fit. The background yields are shown as the fitted values, while the signal yields are shown as both fitted values (μ=1.6) and the expectation from the SM (μ=1). The total background before the fit is shown as a dashed blue histogram. The pull (residual divided by its uncertainty) of the data with respect to the background-only prediction is shown in the lower panel, where the full red line (dashed yellow line) indicates the pull of the prediction for signal with μ=1.6 (μ=1) and background with respect to the background-only prediction. The background is also shown after the fit to data assuming zero signal contribution as well as its pull (dotted black line) with respect to the background from the nominal fit. png (102kB) pdf (14kB) |
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Figure 15: Summary of the measurements of μ from individual analyses and the combined result. "ML" refers to the multileptonic decay channels discussed in Section 8. The best-fit values of μ for the individual analyses are extracted independently, and systematic uncertainty nuisance parameters are only correlated for the combination. As no events are observed in the H→ 4ℓ analysis, a 68% confidence level (CL) upper limit on μ, computed using the CLs method [107], is reported. png (44kB) eps (17kB) pdf (5kB) |
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Figure 16: Summary of the best-fit values of μ broken down by Higgs boson decay mode. The decays H → WW* and H → ZZ* are assumed to have the same signal-strength modification factor and are shown together as VV. All systematic uncertainties are correlated as in the nominal result. png (46kB) eps (19kB) pdf (5kB) |
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Figure 17a: Two-dimensional scans of the signal strength modifiers for the processes ttH, H→ bb versus ttH, H → WW*/ZZ* (left) and ttH, H→ τ τ versus ttH, H → WW*/ZZ* (right). The two signal strengths not appearing in each plot are profiled. The decays H → WW* and H → ZZ* are assumed to have the same signal strength modification factor μVV. png (36kB) eps (11kB) pdf (6kB) |
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Figure 17b: Two-dimensional scans of the signal strength modifiers for the processes ttH, H→ bb versus ttH, H → WW*/ZZ* (left) and ttH, H→ τ τ versus ttH, H → WW*/ZZ* (right). The two signal strengths not appearing in each plot are profiled. The decays H → WW* and H → ZZ* are assumed to have the same signal strength modification factor μVV. png (39kB) eps (11kB) pdf (6kB) |
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Figure 18: Allowed regions at 68% and 95% CL in the κF–κV plane from the combination of all ttH channels. The Higgs boson is assumed to not couple to any particles beyond the Standard Model, and the H → γγ and H → gg couplings are expressed in terms of κF and κV. png (34kB) eps (11kB) pdf (5kB) |
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Figure 19: The fractional contributions of the various backgrounds to the total predicted background in each of the twelve analysis categories. The background prediction methods are described in Section 6 : "Non-prompt", "Fake τhad" and "q mis-id" refer to the data-driven background estimates (largely tt but also include other electroweak processes), and rare processes (tZ, tW, tWZ, ttWW, triboson production, ttt, ttttH, tH, rare top decay) are labelled as "Other". png (36kB) pdf (15kB) |
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Figure 20: Comparison of prediction to data before the fit in the eight signal and four control regions. The systematic uncertainties in the predicted yields are indicated by the hashed blue band. png (123kB) pdf (129kB) |
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Figure 21: Expected best fit values of the ttH signal strength μttH and their uncertainties by final state category and combined. png (63kB) pdf (92kB) |
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Figure 22: The observed best fit values of the ttH signal strength μttH and their uncertainties by final state category and combined. The individual values for the categories are obtained from fits in each channel separately. The SM prediction is μttH=1. png (68kB) pdf (93kB) |
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Figure 23: Upper limits on the ttH signal strength μttH at 95% CL by final state category and combined. The observed limits (solid lines) are compared to the expected median limits under the background-only hypothesis and under the signal-plus-background hypothesis (μ=1). png (44kB) pdf (91kB) |
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Figure 24: Observed correlations between the signal strength μttH and the nuisance parameters in the profile likelihood fit to the data. Only the nuisance parameters which exhibit a correlation larger than 20% are displayed. png (275kB) pdf (126kB) |
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Figure 25a: Comparison of data and prediction after the fit for (a) the lepton flavour composition in the 2ℓSS channel; (b) the number of jets in the 2ℓSS channel; (c) the subleading same-sign lepton pT in the 3ℓ channel; (d) the number of b-tagged jets in the 3ℓ channel; (e) the number of jets in the 2ℓSS+1τhad channel; (f) the number of jets in the 2ℓOS+1τhad channel. The last bin in all figures contains the overflow. The bottom panel displays the ratio of data to the total prediction. The hashed area represents the total uncertainty in the background. The ttH signal yield is normalised to the fitted values (μttH=1.6). png (107kB) pdf (76kB) |
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Figure 25b: Comparison of data and prediction after the fit for (a) the lepton flavour composition in the 2ℓSS channel; (b) the number of jets in the 2ℓSS channel; (c) the subleading same-sign lepton pT in the 3ℓ channel; (d) the number of b-tagged jets in the 3ℓ channel; (e) the number of jets in the 2ℓSS+1τhad channel; (f) the number of jets in the 2ℓOS+1τhad channel. The last bin in all figures contains the overflow. The bottom panel displays the ratio of data to the total prediction. The hashed area represents the total uncertainty in the background. The ttH signal yield is normalised to the fitted values (μttH=1.6). png (109kB) pdf (77kB) |
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Figure 25c: Comparison of data and prediction after the fit for (a) the lepton flavour composition in the 2ℓSS channel; (b) the number of jets in the 2ℓSS channel; (c) the subleading same-sign lepton pT in the 3ℓ channel; (d) the number of b-tagged jets in the 3ℓ channel; (e) the number of jets in the 2ℓSS+1τhad channel; (f) the number of jets in the 2ℓOS+1τhad channel. The last bin in all figures contains the overflow. The bottom panel displays the ratio of data to the total prediction. The hashed area represents the total uncertainty in the background. The ttH signal yield is normalised to the fitted values (μttH=1.6). png (138kB) pdf (77kB) |
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Figure 25d: Comparison of data and prediction after the fit for (a) the lepton flavour composition in the 2ℓSS channel; (b) the number of jets in the 2ℓSS channel; (c) the subleading same-sign lepton pT in the 3ℓ channel; (d) the number of b-tagged jets in the 3ℓ channel; (e) the number of jets in the 2ℓSS+1τhad channel; (f) the number of jets in the 2ℓOS+1τhad channel. The last bin in all figures contains the overflow. The bottom panel displays the ratio of data to the total prediction. The hashed area represents the total uncertainty in the background. The ttH signal yield is normalised to the fitted values (μttH=1.6). png (118kB) pdf (75kB) |
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Figure 25e: Comparison of data and prediction after the fit for (a) the lepton flavour composition in the 2ℓSS channel; (b) the number of jets in the 2ℓSS channel; (c) the subleading same-sign lepton pT in the 3ℓ channel; (d) the number of b-tagged jets in the 3ℓ channel; (e) the number of jets in the 2ℓSS+1τhad channel; (f) the number of jets in the 2ℓOS+1τhad channel. The last bin in all figures contains the overflow. The bottom panel displays the ratio of data to the total prediction. The hashed area represents the total uncertainty in the background. The ttH signal yield is normalised to the fitted values (μttH=1.6). png (124kB) pdf (94kB) |
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Figure 25f: Comparison of data and prediction after the fit for (a) the lepton flavour composition in the 2ℓSS channel; (b) the number of jets in the 2ℓSS channel; (c) the subleading same-sign lepton pT in the 3ℓ channel; (d) the number of b-tagged jets in the 3ℓ channel; (e) the number of jets in the 2ℓSS+1τhad channel; (f) the number of jets in the 2ℓOS+1τhad channel. The last bin in all figures contains the overflow. The bottom panel displays the ratio of data to the total prediction. The hashed area represents the total uncertainty in the background. The ttH signal yield is normalised to the fitted values (μttH=1.6). png (110kB) pdf (94kB) |
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Figure 26a: Comparison of prediction to data in all categories used by the 2ℓSS (top), 3ℓ (middle) and 2ℓSS+1τhad (bottom) cut-and-count analyses serving as a cross-check to the nominal analyses. The systematic uncertainties in the predicted yields are indicated by the hashed blue band. The signal yields are shown as the fitted values obtained with the nominal analysis (μttH=1.6). The name of the categories refer to the requirements made on the jet multiplicity, b-tagged jet multiplicity, the lepton flavour, the invariant mass of the opposite-sign pair of leptons with the smallest Δ R separation (M), the maximum |η| of the two light leptons and the pT of the subleading jet (j1). png (116kB) pdf (96kB) |
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Figure 26b: Comparison of prediction to data in all categories used by the 2ℓSS (top), 3ℓ (middle) and 2ℓSS+1τhad (bottom) cut-and-count analyses serving as a cross-check to the nominal analyses. The systematic uncertainties in the predicted yields are indicated by the hashed blue band. The signal yields are shown as the fitted values obtained with the nominal analysis (μttH=1.6). The name of the categories refer to the requirements made on the jet multiplicity, b-tagged jet multiplicity, the lepton flavour, the invariant mass of the opposite-sign pair of leptons with the smallest Δ R separation (M), the maximum |η| of the two light leptons and the pT of the subleading jet (j1). png (140kB) pdf (89kB) |
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Figure 26c: Comparison of prediction to data in all categories used by the 2ℓSS (top), 3ℓ (middle) and 2ℓSS+1τhad (bottom) cut-and-count analyses serving as a cross-check to the nominal analyses. The systematic uncertainties in the predicted yields are indicated by the hashed blue band. The signal yields are shown as the fitted values obtained with the nominal analysis (μttH=1.6). The name of the categories refer to the requirements made on the jet multiplicity, b-tagged jet multiplicity, the lepton flavour, the invariant mass of the opposite-sign pair of leptons with the smallest Δ R separation (M), the maximum |η| of the two light leptons and the pT of the subleading jet (j1). png (110kB) pdf (95kB) |
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Figure 27a: Comparison of data and prediction before the fit for the b-tagged jet multiplicity (left) and the subleading same-sign lepton pT (right) in the 3ℓ tt control regions. The last bin in all figures contains the overflow. The bottom panel displays the ratio of data to the total prediction. The hashed area represents the total uncertainty in the background. png (128kB) pdf (108kB) |
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Figure 27b: Comparison of data and prediction before the fit for the b-tagged jet multiplicity (left) and the subleading same-sign lepton pT (right) in the 3ℓ tt control regions. The last bin in all figures contains the overflow. The bottom panel displays the ratio of data to the total prediction. The hashed area represents the total uncertainty in the background. png (159kB) pdf (110kB) |
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Figure 28: Charge misassignment rates measured in data for very tight electrons, as a function of |η| for different pT ranges. The error bars include both the statistical and systematic uncertainties. png (32kB) eps (18kB) pdf (6kB) |
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Figure 29a: Efficiencies for loose (a) prompt leptons, (b) fake and non-prompt electrons and (c) fake and non-prompt muons to be very tight, as measured in data control regions. The binning and parametrisation as a function of pT and/or number of b-jets and/or the angular distance between the lepton and the closest jet, as used by the matrix method, are shown. Error bars are statistical and the yellow filled bands indicate the systematic uncertainties. png (89kB) pdf (41kB) |
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Figure 29b: Efficiencies for loose (a) prompt leptons, (b) fake and non-prompt electrons and (c) fake and non-prompt muons to be very tight, as measured in data control regions. The binning and parametrisation as a function of pT and/or number of b-jets and/or the angular distance between the lepton and the closest jet, as used by the matrix method, are shown. Error bars are statistical and the yellow filled bands indicate the systematic uncertainties. png (108kB) pdf (254kB) |
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Figure 29c: Efficiencies for loose (a) prompt leptons, (b) fake and non-prompt electrons and (c) fake and non-prompt muons to be very tight, as measured in data control regions. The binning and parametrisation as a function of pT and/or number of b-jets and/or the angular distance between the lepton and the closest jet, as used by the matrix method, are shown. Error bars are statistical and the yellow filled bands indicate the systematic uncertainties. png (75kB) pdf (44kB) |
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Figure 30a: Left (right): Ratio between the number of light leptons (τhad candidates) passing tight requirements to those passing the loose but failing tight requirements (fake factors), as a function of the lepton pT. The measurements are performed in the 2ℓSS and 2ℓSS+1τhad (2ℓOS+1τhad) control regions. Error bars are statistical and the yellow filled bands indicate the systematic uncertainties. png (89kB) pdf (18kB) |
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Figure 30b: Left (right): Ratio between the number of light leptons (τhad candidates) passing tight requirements to those passing the loose but failing tight requirements (fake factors), as a function of the lepton pT. The measurements are performed in the 2ℓSS and 2ℓSS+1τhad (2ℓOS+1τhad) control regions. Error bars are statistical and the yellow filled bands indicate the systematic uncertainties. png (111kB) pdf (73kB) |
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| Tables | ||
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Table 01: The configurations used for event generation of signal and background processes. The samples used to estimate the systematic uncertainties are indicated in brackets. "V" refers to production of an electroweak boson (W or Z/γ*). "Tune" refers to the underlying-event tuned parameters of the parton shower generator. The parton distribution function (PDF) shown in the table is the one used for the matrix element (ME). The PDF used for the parton shower is either NNPDF 2.3 LO [59] for samples using the A14 [60] tune or CTEQ6L1 [61,62] for samples using either UE-EE-5 [63] or Perugia2012 [64] tune. "MG5_AMC" refers to MadGraph5_AMC@NLO with several versions from 2.1.0 to 2.3.3 [57]; "Pythia 6" refers to version 6.427 [65]; "Pythia 8" refers to version 8.2 [50]; "Herwig++" refers to version 2.7 [66]; "MEPS" refers to the matrix-element parton shower matching method used in Sherpa [67]. Samples using Pythia 6 or Pythia 8 have heavy-flavour hadron decays modelled by EvtGen 1.2.0 [68]. All samples include leading-logarithm photon emission, either modelled by the parton shower generator or by PHOTOS [69]. png (243kB) pdf (64kB) |
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Table 02: Loose (L), loose and isolated (L†), loose, isolated and pass the non-prompt BDT (L*), tight (T) and very tight (T*) light lepton definitions. Selections for the tighter leptons are applied in addition to the looser ones. For the muons, the L*, T and T* lepton definitions are identical. png (20kB) pdf (61kB) |
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Table 03: Selection criteria applied in all channels. Same-flavour, opposite-charge lepton pairs are referred to as SFOC pairs. The common selection criteria for all channels is listed in the first line under the title "Common". png (103kB) pdf (60kB) |
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Table 04: Summary of the basic characteristics of the seven analysis channels. The lepton selection follows the definition in Table 2 and is labelled as loose (L), loose and isolated (L†), loose, isolated and pass the non-prompt BDT (L*), tight (T) and very tight (T*), respectively. The τhad selection is labelled as medium (M) and tight (T). png (12kB) pdf (53kB) |
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Table 05: Acceptance times efficiency (A×ε) for ttH signal in each analysis channel. This includes Higgs boson and top quark branching fractions, detector acceptance, and reconstruction and selection efficiency, and is computed relative to inclusive ttH production considering all Higgs boson and top decays. In the 4ℓ channel, the two numbers correspond to the Z-enriched and the Z-depleted categories. png (6kB) pdf (42kB) |
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Table 06: Variables used in the multivariate analysis (denoted by ×) for the 2ℓSS, 3ℓ, 4ℓ (Z-enriched category), 1ℓ+2τhad, 2ℓSS+1τhad and 2ℓOS+1τhad channels. For 2ℓSS and 2ℓSS+1τhad, lepton 0 and lepton 1 are the leading and subleading leptons, respectively. For 3ℓ, lepton 0 is the lepton of opposite charge to the same-charge pair, while the same-charge leptons are labelled with increasing index (lepton 1 and lepton 2) as pT decreases. The best Z-candidate dilepton invariant mass is the mass of the dilepton pair closest to the Z boson mass. The variables also used in the cross-check analyses are indicated by a *. png (96kB) pdf (46kB) |
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Table 07: Selection criteria applied to define the control regions used for the non-prompt lepton (top part) and fake τhad (bottom part) estimates. The 2ℓSS CR is used for both the 2ℓSS and 3ℓ channels, as indicated by putting 3ℓ in parenthesis. Same-flavour, opposite-charge (same-charge) lepton pairs are referred to as SFOC (SFSC) pairs. png (66kB) pdf (63kB) |
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Table 08: Summary of the non-prompt lepton and fake τhad background estimate strategies of the seven analysis channels. DD means data-driven background estimates and the techniques used are the matrix method (MM) and the fake-factor method (FF). The scale factor method (SF), which scales the estimate from simulation by a correction factor measured in data, is partially data-driven. The lower half of the table lists the selection requirements used to define the control regions. The lepton selection follows the same convention as in Table 2 and is labelled as loose (L), loose and isolated (L†), loose, isolated and pass the non-prompt BDT (L*), tight (T) and very tight (T*), respectively. Analogously, the τhad selection is labelled as medium (M) and tight (T). png (104kB) pdf (56kB) |
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Table 09: Sources of systematic uncertainty considered in the analysis. "N" means that the uncertainty is taken as normalisation-only for all processes and channels affected, whereas "S" denotes systematics that are considered shape-only in all processes and channels. "SN" means that the uncertainty is taken on both shape and normalisation. Some of the systematic uncertainties are split into several components, as indicated by the number in the rightmost column. png (73kB) pdf (53kB) |
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Table 10: Background, signal and observed yields in the twelve analysis categories in 36.1 fb-1 of data at √s = 13 TeV. Uncertainties in the background expectations due to systematic effects and simulation statistics are shown. "Non-prompt", "Fake τhad" and "q mis-id" refer to the data-driven background estimates described in section 6. Rare processes (tZ, tW, tWZ, ttWW, triboson production, ttt, tttt, tH, rare top decay) are labelled as "Other". In the top part, the pre-fit values are quoted, i.e. using the initial values of background systematic uncertainty nuisance parameters and the signal expectation from the SM. In the bottom part, the corresponding post-fit values are quoted. In the post-fit case, the prediction and uncertainties for ttH reflect the best-fit production rate of 1.6 +0.5-0.4 times the Standard Model expectation and the uncertainty in the total background estimation is smaller than for the pre-fit values due to anticorrelations between the nuisance parameters obtained in the fit. png (113kB) pdf (108kB) |
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Table 11: Summary of the basic characteristics and analysis strategies of all channels. In the 4ℓ channel, the two entries correspond to the Z-enriched and the Z-depleted categories. 1D and 5D refer to one- and five-dimensional BDTs respectively. png (52kB) pdf (41kB) |
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Table 12: Summary of the effects of the most important groups of systematic uncertainties in μ. Due to rounding effects and small correlations between the different sources of uncertainties, the total systematic uncertainty can be different from the sum in quadrature of the individual sources. png (29kB) pdf (43kB) |
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Table 13: Observed and expected best-fit values of the signal strength μ and associated significance with respect to the SM background-only hypothesis. The expected values are shown for the pre-fit background estimates. The observed significance is indicated with a - for the channels where μ is negative. png (40kB) pdf (50kB) |
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Table 14: Summary of the observed and expected μ measurements and ttH production significance from individual analyses and the combination. As no events are observed in the H → 4ℓ analysis, a 68% confidence level (CL) upper limit on μ, computed using the CLs method [107], is reported. png (14kB) pdf (56kB) |
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Table 15: Summary of the uncertainties affecting the combined value of μ. png (38kB) pdf (44kB) |
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Table 16: The cross sections used for each of the Monte Carlo simulated samples used in the analysis. The size of the QCD and PDF+αS scale uncertainties are indicated as well as the order of the cross section calculation. The uncertainties for ttγ, t Z, t W Z, and VV (→ ℓ ℓ XX) include extrapolation uncertainties into the analysis phase space. png (282kB) pdf (60kB) |
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