A search for tt resonances in lepton plus jets events with ATLAS using 14 fb-1 of proton-proton collisions at √s = 8 TeV

ATLAS-CONF-2013-052

13 May 2013

These preliminary results are superseded by the following paper:

TOPQ-2012-18
ATLAS recommends to use the results from the paper.

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Abstract
A search for new particles that decay into top quark pairs ( $t\bar{t}$) is performed with the ATLAS experiment at the LHC using an integrated luminosity of 14 fb$^{-1}$ of proton-proton collision data collected at center-of-mass energy $\sqrt{s}=8$ TeV. The lepton plus jets final state is used, where the top-pair decays as $t\bar{t} \rightarrow W^{+}bW^{-}\bar{b}$ with one $W$ boson decaying leptonically and the other hadronically. The $t\bar{t}$ system is reconstructed using both a conventional resolved jet analysis and a large-radius jet substructure analysis. The $t\bar{t}$ invariant mass spectrum is searched for local excesses deviating from the Standard Model prediction. No evidence for a $t\bar{t}$ resonance is found and 95% CL limits on the production rate are determined for massive states in two benchmark models. The upper limits on the cross section times branching ratio of a narrow $Z'$ boson decaying to top pairs range from 5.3 pb for a resonance mass of 0.5 TeV to 0.08 pb for a mass of 3 TeV. A narrow leptophobic topcolor $Z'$ boson with a mass below 1.8 TeV is excluded. Upper limits are set on the cross section times branching ratio for a broad color-octet resonance with $\Gamma/m =$15.3% decaying to $t\bar{t}$. These range from 9.6 pb for a mass of 0.5 TeV to 0.152 pb for a mass of 2.5 TeV. A Kaluza-Klein excitation of the gluon in a Randall-Sundrum model is excluded for masses below 2.0 TeV.
Figures
Figure 01:
The selection efficiency as a function of the true mtt for Z′ →tt events. The μ+jets channel is shown with gray lines and the e+jets channel with black lines. Dashed lines show the boosted selection and solid lines the total selection efficiency. The error bars indicate statistical uncertainties.
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Figure 02a:
The reconstructed tt invariant mass, mttreco, and the corresponding difference between true and reconstructed invariant mass using the resolved (a,b) and boosted selection (c,d), for a range of narrow Z′ masses.
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Figure 02b:
The reconstructed tt invariant mass, mttreco, and the corresponding difference between true and reconstructed invariant mass using the resolved (a,b) and boosted selection (c,d), for a range of narrow Z′ masses.
png (76kB)  eps (12kB)  pdf (6kB) 
Figure 02c:
The reconstructed tt invariant mass, mttreco, and the corresponding difference between true and reconstructed invariant mass using the resolved (a,b) and boosted selection (c,d), for a range of narrow Z′ masses.
png (76kB)  eps (11kB)  pdf (5kB) 
Figure 02d:
The reconstructed tt invariant mass, mttreco, and the corresponding difference between true and reconstructed invariant mass using the resolved (a,b) and boosted selection (c,d), for a range of narrow Z′ masses.
png (78kB)  eps (11kB)  pdf (5kB) 
Figure 03a:
The transverse momentum of the leading jet, after the resolved selection (e+jets channel). The shaded areas indicate the total systematic uncertainties. Some background sources are too small to be visible in the figure.
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Figure 03b:
The transverse momentum of the leading jet, after the resolved selection (μ+jets channel). The shaded areas indicate the total systematic uncertainties. Some background sources are too small to be visible in the figure.
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Figure 04a:
The transverse momentum of the hadronically decaying top quark candidate, after the boosted selection (e+jets channel). The shaded areas indicate the total systematic uncertainties. Some background sources are too small to be visible in the figure. The last bin in each histogram includes overflow events.
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Figure 04b:
The transverse momentum of the hadronically decaying top quark candidate, after the boosted selection (μ+jets channel). The shaded areas indicate the total systematic uncertainties. Some background sources are too small to be visible in the figure. The last bin in each histogram includes overflow events.
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Figure 05a:
The invariant mass of the semi-leptonically decaying top quark candidate, mt,lep, after the boosted selection (e+jets channel). The mass has been reconstructed from the narrow jet, the charged lepton and the missing transverse momentum, using a W mass constraint to obtain the longitudinal momentum of the neutrino. The shaded areas indicate the total systematic uncertainties. Some background sources are too small to be visible in the figure.
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Figure 05b:
The invariant mass of the semi-leptonically decaying top quark candidate, \mtlep, after the boosted selection (μ+jets channel). The mass has been reconstructed from the narrow jet, the charged lepton and the missing transverse momentum, using a W mass constraint to obtain the longitudinal momentum of the neutrino. The shaded areas indicate the total systematic uncertainties. Some background sources are too small to be visible in the figure.
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Figure 06a:
The mass of the hadronic top jet, mt,had, after the boosted selection (e+jets channel). The shaded areas indicate the total systematic uncertainties. Some background sources are too small to be visible in the figure.
png (89kB)  eps (31kB)  pdf (11kB) 
Figure 06b:
The mass of the hadronic top jet, mt,had, after the boosted selection (μ+jets channel). The shaded areas indicate the total systematic uncertainties. Some background sources are too small to be visible in the figure.
png (85kB)  eps (29kB)  pdf (10kB) 
Figure 07a:
The first kt splitting scale, √d12, of the hadronic top jet after the boosted selection (e+jets channel). The shaded areas indicate the total systematic uncertainties. Some background sources are too small to be visible in the figure.
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Figure 07b:
The first kt splitting scale, √d12, of the hadronic top jet after the boosted selection (μ+jets channel). The shaded areas indicate the total systematic uncertainties. Some background sources are too small to be visible in the figure.
png (87kB)  eps (29kB)  pdf (11kB) 
Figure 08a:
The tt invariant mass spectra for the e+jets channel with the resolved selection. The shaded areas indicate the total systematic uncertainties.
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Figure 08b:
The tt invariant mass spectra for the μ+jets channel with the resolved selection. The shaded areas indicate the total systematic uncertainties.
png (82kB)  eps (38kB)  pdf (12kB) 
Figure 08c:
The tt invariant mass spectra for the e+jets channel with the boosted selection. The shaded areas indicate the total systematic uncertainties.
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Figure 08d:
The tt invariant mass spectra for the μ+jets channel with the boosted selection. The shaded areas indicate the total systematic uncertainties.
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Figure 09:
The tt invariant mass spectrum, summing the spectra from the two channels and the two selection methods. The shaded areas indicate the total systematic uncertainties. Two benchmark signals are indicated on top of the background, a Z′ with m=1.5 TeV and a gKK with m=2.0 TeV. The assumed cross sections of the signals in this figure are the theoretical predictions given in Tables 3 and 4.
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Figure 10a:
Observed and expected upper cross section limits times the tt branching ratio on (a) Z′ bosons. The resolved and the boosted selections have been combined in the estimation of the limits. Both systematic and statistical uncertainties are included.
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Figure 10b:
Observed and expected upper cross section limits times the tt branching ratio on (b) Kaluza-Klein gluons. The resolved and the boosted selections have been combined in the estimation of the limits. Both systematic and statistical uncertainties are included.
png (83kB)  eps (18kB)  pdf (8kB) 
Figure 11a:
The signal efficiency versus false-identification rate of various lepton isolation discriminants, in the muon channel. The signal efficiency is evaluated from MC of (a) SM ttbar (b) 1.0 TeV Z′ (c) 1.5 TeV Z′ (d) 2.0 TeV Z′, for events passing all the other boosted selections except for isolation.
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Figure 11b:
The signal efficiency versus false-identification rate of various lepton isolation discriminants, in the muon channel. The signal efficiency is evaluated from MC of (a) SM ttbar (b) 1.0 TeV Z′ (c) 1.5 TeV Z′ (d) 2.0 TeV Z′, for events passing all the other boosted selections except for isolation.
png (100kB)  eps (13kB)  pdf (10kB) 
Figure 11c:
The signal efficiency versus false-identification rate of various lepton isolation discriminants, in the muon channel. The signal efficiency is evaluated from MC of (a) SM ttbar (b) 1.0 TeV Z′ (c) 1.5 TeV Z′ (d) 2.0 TeV Z′, for events passing all the other boosted selections except for isolation.
png (103kB)  eps (13kB)  pdf (10kB) 
Figure 11d:
The signal efficiency versus false-identification rate of various lepton isolation discriminants, in the muon channel. The signal efficiency is evaluated from MC of (a) SM ttbar (b) 1.0 TeV Z′ (c) 1.5 TeV Z′ (d) 2.0 TeV Z′, for events passing all the other boosted selections except for isolation.
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Figure 12a:
Various selection efficiencies as a function of the true mtt for the Z′ resonance. The μ+jets channel is shown with blue lines and the e+jets channel with red.
png (60kB)  eps (16kB)  pdf (6kB) 
Figure 12b:
Various selection efficiencies as a function of the true mtt for the Z′ resonance. The μ+jets channel is shown with blue lines and the e+jets channel with red.
png (64kB)  eps (17kB)  pdf (6kB) 
Figure 12c:
Various selection efficiencies as a function of the true mtt for the Z′ resonance. The μ+jets channel is shown with blue lines and the e+jets channel with red.
png (61kB)  eps (17kB)  pdf (6kB) 
Figure 12d:
Various selection efficiencies as a function of the true mtt for the Z′ resonance. The μ+jets channel is shown with blue lines and the e+jets channel with red.
png (58kB)  eps (16kB)  pdf (6kB) 
Figure 12e:
Various selection efficiencies as a function of the true mtt for the Z′ resonance. The μ+jets channel is shown with blue lines and the e+jets channel with red.
png (60kB)  eps (17kB)  pdf (6kB) 
Figure 13a:
Event display for mttreco = 2.6 TeV e+jets ttbar candidate events. The upper left panel displays a transverse (X-Y) view of detector and objects, while the lower left panel shows the longitudinal (R-z) view. In this view, jets are represented by circular sectors with their lengths proportional to the transverse energies. Green jets are reconstructed with R = 0.4, while red jets are reconstructed with R = 1. The b-tagged R = 0.4 jets are labelled with blue bars. An η-φ view of the same event is shown in the upper right panel, with the lego-plot of calorimeter energy in the lower right panel. In this plane, jets are represented by solid circles of the same color scheme, while the b-tagged ones are labelled by concentric blue circles. The red dashed circle represents the missing transverse momentum. The area of the circles are proportional to the transverse energy or momentum of the physics objects.
png (646kB)  eps (4MB)  pdf (113kB) 
Figure 13b:
Event display for mttreco = 2.5 TeV μ+jets ttbar candidate events. The upper left panel displays a transverse (X-Y) view of detector and objects, while the lower left panel shows the longitudinal (R-z) view. In this view, jets are represented by circular sectors with their lengths proportional to the transverse energies. Green jets are reconstructed with R = 0.4, while red jets are reconstructed with R = 1. The b-tagged R = 0.4 jets are labelled with blue bars. An η-φ view of the same event is shown in the upper right panel, with the lego-plot of calorimeter energy in the lower right panel. In this plane, jets are represented by solid circles of the same color scheme, while the b-tagged ones are labelled by concentric blue circles. The red dashed circle represents the missing transverse momentum. The area of the circles are proportional to the transverse energy or momentum of the physics objects.
png (644kB)  eps (4MB)  pdf (112kB) 
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2024-05-19 01:17:43