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A search for tt resonances using lepton plus jets events in proton-proton collisions at √s = 8 TeV with the ATLAS detector ATLAS-CONF-2015-009 17 March 2015
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| Abstract | ||
| A search for new particles that decay into top quark pairs is reported. The search is performed with the ATLAS experiment at the LHC using an integrated luminosity of 20.3 fb$^{-1}$ of proton-proton collision data collected at the centre-of-mass energy $\sqrt{s}=8$TeV. The lepton plus jets final state is used, where the top-pair decays as $W^+bW^-\bar{b}$, with one $W$ boson decaying leptonically and the other hadronically. The top quark pair invariant mass spectrum is examined for local excesses that are inconsistent with the Standard Model predictions. No evidence for a top quark pair resonance is found, and 95% confidence level limits on the production rate are determined for massive states in four benchmark models. The upper limits on the cross-section times branching ratio of a narrow $Z'$ boson decaying to top pairs range from 4.2 pb to 0.03 pb for resonance masses from 0.4 TeV to 3.0 TeV. A narrow leptophobic topcolour $Z'$ boson with mass below 1.8 TeV is excluded. Upper limits are set on the cross-section times branching ratio fo r a broad colour-octet resonance with $\Gamma/m =$ 15% decaying to $t\bar{t}$. These range from 4.8 pb to 0.09 pb for masses from 0.4 TeV to 3.0 TeV. A Kaluza-Klein excitation of the gluon in a Randall--Sundrum model is excluded for masses below 2.2 TeV. | ||
| Figures | ||
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Figure 01a: Selection efficiency times acceptance times branching ratio as a function of the true mtt for the different signals in the models considered: (a) Z', (b) gKK, (c) GKK, (d) scalar resonance. The dashed lines show the boosted-topology selection and the unbroken lines show the combined selection. The branching ratio for tt → e/μ + jets is 17%. png (75kB) eps (16kB) pdf (8kB) |
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Figure 01b: Selection efficiency times acceptance times branching ratio as a function of the true mtt for the different signals in the models considered: (a) Z', (b) gKK, (c) GKK, (d) scalar resonance. The dashed lines show the boosted-topology selection and the unbroken lines show the combined selection. The branching ratio for tt → e/μ + jets is 17%. png (75kB) eps (16kB) pdf (8kB) |
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Figure 01c: Selection efficiency times acceptance times branching ratio as a function of the true mtt for the different signals in the models considered: (a) Z', (b) gKK, (c) GKK, (d) scalar resonance. The dashed lines show the boosted-topology selection and the unbroken lines show the combined selection. The branching ratio for tt → e/μ + jets is 17%. png (75kB) eps (16kB) pdf (8kB) |
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Figure 01d: Selection efficiency times acceptance times branching ratio as a function of the true mtt for the different signals in the models considered: (a) Z', (b) gKK, (c) GKK, (d) scalar resonance. The dashed lines show the boosted-topology selection and the unbroken lines show the combined selection. The branching ratio for tt → e/μ + jets is 17%. png (75kB) eps (16kB) pdf (8kB) |
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Figure 02a: mtt for the different signal models for events satisfying the resolved-topology selection and using the resolved-topology reconstruction: (a) Z', (b) gKK, (c) GKK, (d) scalar resonance. The label `inclusive' indicates that for this figure events that pass the resolved-topology selection are included regardless of whether they pass the boosted-topology selection. png (103kB) eps (12kB) pdf (5kB) |
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Figure 02b: mtt for the different signal models for events satisfying the resolved-topology selection and using the resolved-topology reconstruction: (a) Z', (b) gKK, (c) GKK, (d) scalar resonance. The label `inclusive' indicates that for this figure events that pass the resolved-topology selection are included regardless of whether they pass the boosted-topology selection. png (103kB) eps (12kB) pdf (5kB) |
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Figure 02c: mtt for the different signal models for events satisfying the resolved-topology selection and using the resolved-topology reconstruction: (a) Z', (b) gKK, (c) GKK, (d) scalar resonance. The label `inclusive' indicates that for this figure events that pass the resolved-topology selection are included regardless of whether they pass the boosted-topology selection. png (107kB) eps (12kB) pdf (5kB) |
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Figure 02d: mtt for the different signal models for events satisfying the resolved-topology selection and using the resolved-topology reconstruction: (a) Z', (b) gKK, (c) GKK, (d) scalar resonance. The label `inclusive' indicates that for this figure events that pass the resolved-topology selection are included regardless of whether they pass the boosted-topology selection. png (107kB) eps (12kB) pdf (5kB) |
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Figure 03a: Reconstructed tt invariant mass for the different signal models for events satisfying the boosted-topology selection and using the boosted-topology reconstruction: (a) Z', (b) gKK, (c) GKK, (d) scalar resonance. png (103kB) eps (11kB) pdf (5kB) |
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Figure 03b: Reconstructed tt invariant mass for the different signal models for events satisfying the boosted-topology selection and using the boosted-topology reconstruction: (a) Z', (b) gKK, (c) GKK, (d) scalar resonance. png (115kB) eps (13kB) pdf (6kB) |
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Figure 03c: Reconstructed tt invariant mass for the different signal models for events satisfying the boosted-topology selection and using the boosted-topology reconstruction: (a) Z', (b) gKK, (c) GKK, (d) scalar resonance. png (106kB) eps (12kB) pdf (5kB) |
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Figure 03d: Reconstructed tt invariant mass for the different signal models for events satisfying the boosted-topology selection and using the boosted-topology reconstruction: (a) Z', (b) gKK, (c) GKK, (d) scalar resonance. png (106kB) eps (12kB) pdf (5kB) |
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Figure 04a: Reconstructed mass of the hadronically decaying top quark candidate, mth, semileptonically decaying top quark candidate, mtl, and hadronically decaying W-boson candidate mW after the resolved-topology selection in the electron and muon channels. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. Some background sources are too small to be visible in the figure. png (115kB) eps (27kB) pdf (8kB) |
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Figure 04b: Reconstructed mass of the hadronically decaying top quark candidate, mth, semileptonically decaying top quark candidate, mtl, and hadronically decaying W-boson candidate mW after the resolved-topology selection in the electron and muon channels. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. Some background sources are too small to be visible in the figure. png (119kB) eps (27kB) pdf (9kB) |
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Figure 04c: Reconstructed mass of the hadronically decaying top quark candidate, mth, semileptonically decaying top quark candidate, mtl, and hadronically decaying W-boson candidate mW after the resolved-topology selection in the electron and muon channels. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. Some background sources are too small to be visible in the figure. png (115kB) eps (28kB) pdf (8kB) |
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Figure 04d: Reconstructed mass of the hadronically decaying top quark candidate, mth, semileptonically decaying top quark candidate, mtl, and hadronically decaying W-boson candidate mW after the resolved-topology selection in the electron and muon channels. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. Some background sources are too small to be visible in the figure. png (114kB) eps (28kB) pdf (9kB) |
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Figure 04e: Reconstructed mass of the hadronically decaying top quark candidate, mth, semileptonically decaying top quark candidate, mtl, and hadronically decaying W-boson candidate mW after the resolved-topology selection in the electron and muon channels. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. Some background sources are too small to be visible in the figure. png (110kB) eps (24kB) pdf (8kB) |
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Figure 04f: Reconstructed mass of the hadronically decaying top quark candidate, mth, semileptonically decaying top quark candidate, mtl, and hadronically decaying W-boson candidate mW after the resolved-topology selection in the electron and muon channels. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. Some background sources are too small to be visible in the figure. png (109kB) eps (23kB) pdf (8kB) |
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Figure 05a: The invariant mass of the large-radius jets and the invariant mass of the semileptonically decaying top quark candidate, mtl, after the boosted selection. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. Some background sources are too small to be visible in the figure. png (154kB) eps (39kB) pdf (11kB) |
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Figure 05b: The invariant mass of the large-radius jets and the invariant mass of the semileptonically decaying top quark candidate, mtl, after the boosted selection. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. Some background sources are too small to be visible in the figure. png (153kB) eps (39kB) pdf (12kB) |
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Figure 05c: The invariant mass of the large-radius jets and the invariant mass of the semileptonically decaying top quark candidate, mtl, after the boosted selection. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. Some background sources are too small to be visible in the figure. png (132kB) eps (37kB) pdf (11kB) |
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Figure 05d: The invariant mass of the large-radius jets and the invariant mass of the semileptonically decaying top quark candidate, mtl, after the boosted selection. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. Some background sources are too small to be visible in the figure. png (132kB) eps (37kB) pdf (11kB) |
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Figure 06a: The transverse momentum, pT , and first kt splitting scale, √d12, of the large-radius jet after the boosted selection. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. Some background sources are too small to be visible in the figure. png (146kB) eps (37kB) pdf (10kB) |
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Figure 06b: The transverse momentum, pT , and first kt splitting scale, √d12, of the large-radius jet after the boosted selection. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. Some background sources are too small to be visible in the figure. png (144kB) eps (37kB) pdf (10kB) |
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Figure 06c: The transverse momentum, pT , and first kt splitting scale, √d12, of the large-radius jet after the boosted selection. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. Some background sources are too small to be visible in the figure. png (142kB) eps (35kB) pdf (10kB) |
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Figure 06d: The transverse momentum, pT , and first kt splitting scale, √d12, of the large-radius jet after the boosted selection. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. Some background sources are too small to be visible in the figure. png (142kB) eps (36kB) pdf (11kB) |
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Figure 07a: The mttreco spectrum for the different channels, before any nuisance parameter fit, after the resolved-topology selection. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. The red line shows the expected distribution for a hypothetical gKK of mass 2.0 TeV, width 15.3%. png (130kB) eps (40kB) pdf (11kB) |
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Figure 07b: The mttreco spectrum for the different channels, before any nuisance parameter fit, after the resolved-topology selection. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. The red line shows the expected distribution for a hypothetical gKK of mass 2.0 TeV, width 15.3%. png (131kB) eps (40kB) pdf (11kB) |
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Figure 07c: The mttreco spectrum for the different channels, before any nuisance parameter fit, after the resolved-topology selection. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. The red line shows the expected distribution for a hypothetical gKK of mass 2.0 TeV, width 15.3%. png (142kB) eps (40kB) pdf (11kB) |
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Figure 07d: The mttreco spectrum for the different channels, before any nuisance parameter fit, after the resolved-topology selection. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. The red line shows the expected distribution for a hypothetical gKK of mass 2.0 TeV, width 15.3%. png (128kB) eps (39kB) pdf (11kB) |
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Figure 07e: The mttreco spectrum for the different channels, before any nuisance parameter fit, after the resolved-topology selection. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. The red line shows the expected distribution for a hypothetical gKK of mass 2.0 TeV, width 15.3%. png (133kB) eps (40kB) pdf (11kB) |
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Figure 07f: The mttreco spectrum for the different channels, before any nuisance parameter fit, after the resolved-topology selection. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. The red line shows the expected distribution for a hypothetical gKK of mass 2.0 TeV, width 15.3%. png (132kB) eps (40kB) pdf (11kB) |
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Figure 08a: The mttreco spectrum for the different channels, before any nuisance parameter fit, after the boosted-topology selection. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. The red line shows the expected distribution for a hypothetical gKK of mass 2.0 TeV, width 15.3%. png (136kB) eps (37kB) pdf (10kB) |
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Figure 08b: The mttreco spectrum for the different channels, before any nuisance parameter fit, after the boosted-topology selection. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. The red line shows the expected distribution for a hypothetical gKK of mass 2.0 TeV, width 15.3%. png (145kB) eps (38kB) pdf (11kB) |
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Figure 08c: The mttreco spectrum for the different channels, before any nuisance parameter fit, after the boosted-topology selection. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. The red line shows the expected distribution for a hypothetical gKK of mass 2.0 TeV, width 15.3%. png (142kB) eps (38kB) pdf (11kB) |
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Figure 08d: The mttreco spectrum for the different channels, before any nuisance parameter fit, after the boosted-topology selection. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. The red line shows the expected distribution for a hypothetical gKK of mass 2.0 TeV, width 15.3%. png (156kB) eps (39kB) pdf (11kB) |
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Figure 08e: The mttreco spectrum for the different channels, before any nuisance parameter fit, after the boosted-topology selection. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. The red line shows the expected distribution for a hypothetical gKK of mass 2.0 TeV, width 15.3%. png (150kB) eps (38kB) pdf (10kB) |
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Figure 08f: The mttreco spectrum for the different channels, before any nuisance parameter fit, after the boosted-topology selection. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. The red line shows the expected distribution for a hypothetical gKK of mass 2.0 TeV, width 15.3%. png (155kB) eps (39kB) pdf (11kB) |
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Figure 09a: The mttreco distributions, before any nuisance parameter fit, summed over (a) all 6 boosted channels, (b) all 6 resolved channels, and (c) all 12 channels compared with data. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. The red line shows the expected distribution for a hypothetical gKK of mass 2.0 TeV, width 15.3%. png (149kB) eps (40kB) pdf (11kB) |
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Figure 09b: The mttreco distributions, before any nuisance parameter fit, summed over (a) all 6 boosted channels, (b) all 6 resolved channels, and (c) all 12 channels compared with data. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. The red line shows the expected distribution for a hypothetical gKK of mass 2.0 TeV, width 15.3%. png (139kB) eps (40kB) pdf (11kB) |
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Figure 09c: The mttreco distributions, before any nuisance parameter fit, summed over (a) all 6 boosted channels, (b) all 6 resolved channels, and (c) all 12 channels compared with data. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. The red line shows the expected distribution for a hypothetical gKK of mass 2.0 TeV, width 15.3%. png (145kB) eps (40kB) pdf (11kB) |
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Figure 10a: The mttreco distributions, after the nuisance-parameter fit with background-only hypothesis, summed over (a) all 6 boosted channels, (b) all 6 resolved channels, and (c) all 12 channels compared with data. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. The red line shows the expected distribution for a hypothetical gKK of mass 2.0 TeV. png (125kB) eps (39kB) pdf (10kB) |
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Figure 10b: The mttreco distributions, after the nuisance-parameter fit with background-only hypothesis, summed over (a) all 6 boosted channels, (b) all 6 resolved channels, and (c) all 12 channels compared with data. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. The red line shows the expected distribution for a hypothetical gKK of mass 2.0 TeV. png (129kB) eps (40kB) pdf (11kB) |
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Figure 10c: The mttreco distributions, after the nuisance-parameter fit with background-only hypothesis, summed over (a) all 6 boosted channels, (b) all 6 resolved channels, and (c) all 12 channels compared with data. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. The red line shows the expected distribution for a hypothetical gKK of mass 2.0 TeV. png (124kB) eps (40kB) pdf (11kB) |
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Figure 11a: Observed and expected upper limits on the cross-section times the ttbar branching ratio as a function of the mass of (a) Topcolour-assisted-technicolour Z'TC2, (b) Bulk RS Kaluza–Klein gluon, (c) Bulk RS Kaluza–Klein graviton, (d) scalar resonance. png (193kB) eps (18kB) pdf (7kB) |
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Figure 11b: Observed and expected upper limits on the cross-section times the ttbar branching ratio as a function of the mass of (a) Topcolour-assisted-technicolour Z'TC2, (b) Bulk RS Kaluza–Klein gluon, (c) Bulk RS Kaluza–Klein graviton, (d) scalar resonance. png (132kB) eps (17kB) pdf (7kB) |
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Figure 11c: Observed and expected upper limits on the cross-section times the ttbar branching ratio as a function of the mass of (a) Topcolour-assisted-technicolour Z'TC2, (b) Bulk RS Kaluza–Klein gluon, (c) Bulk RS Kaluza–Klein graviton, (d) scalar resonance. png (132kB) eps (18kB) pdf (7kB) |
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Figure 11d: Observed and expected upper limits on the cross-section times the ttbar branching ratio as a function of the mass of (a) Topcolour-assisted-technicolour Z'TC2, (b) Bulk RS Kaluza–Klein gluon, (c) Bulk RS Kaluza–Klein graviton, (d) scalar resonance. png (121kB) eps (16kB) pdf (7kB) |
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Figure 12: Observed and expected upper limits on the cross-section times the ttbar branching ratio as a function of the width of Kaluza–Klein gluon. The theoretical predictions for the production cross-section times branching ratio at the corresponding widths are also shown. png (183kB) eps (29kB) pdf (10kB) |
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| Tables | ||
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Table 01: Average impact of the dominant systematic uncertainties on the total background yield and on the estimated yield for a Z' sample with m=1.75 TeV. The electron and muon channel spectra are added. The shift is given in percent of the nominal value. Certain systematic uncertainties are not applicable to the Z' samples, which is indicated with a bar (-) in the table. png (69kB) pdf (44kB) |
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Table 02: Data and expected background event yields after the resolved and boosted selections. The quadratic sum of all systematic uncertainties on the expected background yields is also given. png (69kB) pdf (34kB) |
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