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Search for heavy particles decaying to pairs of highly-boosted top quarks using lepton-plus-jets events in proton--proton collisions at √ s = 13 TeV with the ATLAS detector ATLAS-CONF-2016-014 21 March 2016
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| Abstract | ||
| A search for new heavy particles that decay into top-antitop quark pairs ($t\bar{t}$) is performed using data collected by the ATLAS detector at the Large Hadron Collider from proton-proton collisions at a centre-of-mass energy of $13$\,\TeV{}. The integrated luminosity of the data sample is $3.2\,$fb$^{-1}$. Events consistent with top-quark pair production are selected by requiring a single isolated charged lepton, missing transverse momentum, and a high-transverse-momentum jet with substructure compatible with that of a hadronic top-quark decay. Jets identified as likely to contain $b$-hadrons are used to further control the background from other Standard Model processes. The invariant mass spectrum of the candidate top-quark pairs is searched for excesses above the background expectation. No significant deviations from the Standard Model predictions are found. Exclusion limits are set on the production cross section times branching ratio for hypothetical $Z'$ bosons decaying into $t\bar{t}$. | ||
| Figures | ||
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Figure 01: Acceptance times selection efficiency, including branching ratio, as a function of the top-antitop quark invariant mass mt t immediately before top quark decay for simulated Z' → t t events. The dashed line shows the μ+jets selection and the solid line shows the e+jets selection. png (34kB) eps (16kB) pdf (5kB) |
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Figure 02: Reconstructed top quark pair invariant mass, mt t reco, for simulated Z' → t t events satisfying the selection. The Z' in the simulated samples used here has a width of 3% of its mass. png (67kB) eps (13kB) pdf (5kB) |
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Figure 03a: The distribution of the transverse momentum of the lepton in the (a)e+jets and (b)μ+jets selections. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. png (29kB) pdf (25kB) |
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Figure 03b: The distribution of the transverse momentum of the lepton in the (a)e+jets and (b)μ+jets selections. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. png (28kB) pdf (25kB) |
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Figure 04a: The distribution of the ETmiss in the (a)e+jets and (b)μ+jets selections. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. png (28kB) pdf (24kB) |
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Figure 04b: The distribution of the ETmiss in the (a)e+jets and (b)μ+jets selections. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. png (27kB) pdf (24kB) |
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Figure 05a: The distribution of the transverse momentum of the hardest small-R jet with Δ R(ℓ, jet) < 1.5 in the (a)e+jets and (b)μ+jets selections. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. png (29kB) pdf (26kB) |
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Figure 05b: The distribution of the transverse momentum of the hardest small-R jet with Δ R(ℓ, jet) < 1.5 in the (a)e+jets and (b)μ+jets selections. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. png (28kB) pdf (25kB) |
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Figure 06a: The distribution of the transverse momentum of the large-R jet in the (a)e+jets and (b)μ+jets selections. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. png (26kB) pdf (20kB) |
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Figure 06b: The distribution of the transverse momentum of the large-R jet in the (a)e+jets and (b)μ+jets selections. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. png (26kB) pdf (20kB) |
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Figure 07a: The distribution of the reconstructed mass of the leptonic top candidate in the (a)e+jets and (b)μ+jets selections. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. png (28kB) pdf (19kB) |
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Figure 07b: The distribution of the reconstructed mass of the leptonic top candidate in the (a)e+jets and (b)μ+jets selections. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. png (28kB) pdf (19kB) |
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Figure 08a: The distribution of the mass of the large-R jet in the (a)e+jets and (b)μ+jets selections. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. png (28kB) pdf (22kB) |
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Figure 08b: The distribution of the mass of the large-R jet in the (a)e+jets and (b)μ+jets selections. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. png (29kB) pdf (22kB) |
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Figure 09a: The mrecot t distribution before the likelihood fit in the (a)e+jets and (b)μ+jets selections. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. The signal distribution for a Z'TC2 with mass 2 TeV, and width divided by mass of 1.2%, is also shown stacked on top of the background expectation. The ratio of the data to the total expectation from background processes is shown in the lower panel, open triangles indicate that the ratio point would appear outside the panel. png (27kB) pdf (22kB) |
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Figure 09b: The mrecot t distribution before the likelihood fit in the (a)e+jets and (b)μ+jets selections. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. The signal distribution for a Z'TC2 with mass 2 TeV, and width divided by mass of 1.2%, is also shown stacked on top of the background expectation. The ratio of the data to the total expectation from background processes is shown in the lower panel, open triangles indicate that the ratio point would appear outside the panel. png (28kB) pdf (22kB) |
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Figure 10a: The mrecot t distributions after a likelihood fit assuming there is no signal for the (a)e+jets and (b)μ+jets selections. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. The ratio of the data to the final fitted expectation is shown in the lower panel, open triangles indicate that the ratio point would appear outside the panel. png (27kB) pdf (21kB) |
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Figure 10b: The mrecot t distributions after a likelihood fit assuming there is no signal for the (a)e+jets and (b)μ+jets selections. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. The ratio of the data to the final fitted expectation is shown in the lower panel, open triangles indicate that the ratio point would appear outside the panel. png (27kB) pdf (22kB) |
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Figure 11: The observed and expected cross section 95% CL upper limits on the Z'TC2 signal. The theoretical predictions for the production cross section times branching ratio at the corresponding masses are also shown. png (126kB) pdf (16kB) |
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Figure 12: Post-fit pulls and impact of the 20 most relevant uncertainties on the best-fit value of the signal strength, μ, For a Z' with mass of 2 TeV. The uncertainties are ranked by the size of the impact on the fitted signal-cross section. png (132kB) eps (21kB) pdf (8kB) |
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| Tables | ||
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Table 01: Data and expected background event yields after the e+jets and μ+jets selections. The total systematic uncertainty on the expected background yields is also given. png (12kB) pdf (32kB) |
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