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Measurement of the W boson polarization in top quark decays with the ATLAS detector 11 May 2012 | |
| Contact: ATLAS Top conveners | |
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| e-print arXiv:1205.2484 | pdf from arXiv |
| Inspire record | - |
| Figures Auxiliary Material | - |
| Abstract | |
| This paper presents measurements of the polarization of W bosons in top quark decays, derived from ttbar events with missing transverse momentum, one charged lepton and at least four jets, or two charged leptons and at least two jets. Data from pp collisions at a centre-of-mass energy of 7 TeV were collected with the ATLAS experiment at the LHC and correspond to an integrated luminosity of 1.04 fb^-1. The measured fractions of longitudinally, left- and right-handed polarization are F_0=0.67+/-0.07, F_L=0.32+/-0.04 and F_R=0.01+/-0.05, in agreement with the Standard Model predictions. As the polarization of the W bosons in top quark decays is sensitive to the Wtb vertex Lorentz structure and couplings, the measurements were used to set limits on anomalous contributions to the Wtb couplings. | |
| Figures | |
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Figure 01a: ET spectrum of the electron in the single-electron channel. The error band shows the statistical uncertainty from the Monte Carlo simulation. The last bin in each distribution includes the overflow. png (81kB) eps (21kB) pdf (9kB) |
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Figure 01b: pT spectrum of the muon in the single-muon channel. The error band shows the statistical uncertainty from the Monte Carlo simulation. The last bin in each distribution includes the overflow. png (83kB) eps (21kB) pdf (9kB) |
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Figure 01c: Missing transverse momentum spectrum in the single-electron channel. The error band shows the statistical uncertainty from the Monte Carlo simulation. The last bin in each distribution includes the overflow. png (87kB) eps (23kB) pdf (9kB) |
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Figure 01d: pT spectrum of the leading jet in the single-muon channel. The error band shows the statistical uncertainty from the Monte Carlo simulation. The last bin in each distribution includes the overflow. png (81kB) eps (20kB) pdf (8kB) |
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Figure 02a: ET spectrum of the leading electron in the ee channel. The error band shows the statistical uncertainty from the Monte Carlo simulation. The last bin in each distribution includes the overflow. png (72kB) eps (18kB) pdf (7kB) |
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Figure 02b: pT spectrum of the leading muon in the μμ channel. The error band shows the statistical uncertainty from the Monte Carlo simulation. The last bin in each distribution includes the overflow. png (68kB) eps (18kB) pdf (7kB) |
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Figure 02c: Missing transverse momentum spectrum in the eμ channel. The error band shows the statistical uncertainty from the Monte Carlo simulation. The last bin in each distribution includes the overflow. png (75kB) eps (21kB) pdf (7kB) |
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Figure 02d: pT spectrum of the leading jet in the eμ channel. The error band shows the statistical uncertainty from the Monte Carlo simulation. The last bin in each distribution includes the overflow. png (76kB) eps (22kB) pdf (8kB) |
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Figure 03a: Distributions of cos θ* for each of the three simulated signal templates. The templates for the combined single-lepton are shown. png (50kB) eps (9kB) pdf (4kB) |
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Figure 03b: Distributions of cos θ* for each of the three simulated signal templates. The templates for the combined dilepton channels are shown. png (45kB) eps (8kB) pdf (4kB) |
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Figure 04a: Distributions of the reconstructed cos θ* used in the template method for data (markers), fitted background (dotted line), the Standard Model prediction (dashed line) and the best fit value (solid line) for the single-lepton channels. The total uncertainties on the helicity fractions for the best fit values are represented by the grey band. For the dilepton channels, each event contributed with two entries, corresponding to the two leptonic decays of the W bosons. png (81kB) eps (15kB) pdf (7kB) |
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Figure 04b: Distributions of the reconstructed cos θ* used in the template method for data (markers), fitted background (dotted line), the Standard Model prediction (dashed line) and the best fit value (solid line) for the dilepton channels. The total uncertainties on the helicity fractions for the best fit values are represented by the grey band. For the dilepton channels, each event contributed with two entries, corresponding to the two leptonic decays of the W bosons. png (76kB) eps (13kB) pdf (6kB) |
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Figure 05a: Unfolded distributions of cos θ* for the single-lepton channels. The error bars on unfolded data (markers) include both the statistical and systematic contributions. For comparison, the Standard Model NNLO QCD prediction (dashed line) and its uncertainty are also shown. png (46kB) eps (10kB) pdf (5kB) |
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Figure 05b: Unfolded distributions of cos θ* for the dilepton channels. The error bars on unfolded data (markers) include both the statistical and systematic contributions. For comparison, the Standard Model NNLO QCD prediction (dashed line) and its uncertainty are also shown. png (46kB) eps (10kB) pdf (5kB) |
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Figure 06: Overview of the four measurements of the W boson helicity fractions and the combined values. The error bars correspond to the statistical and total uncertainties. png (41kB) eps (11kB) pdf (5kB) |
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Figure 07: Allowed regions at 68% and at 95% confidence level (CL) for the Wtb anomalous couplings gR and gL. In the Standard Model, the anomalous couplings vanish at tree level. png (47kB) eps (22kB) pdf (10kB) |
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| Auxiliary figures and tables | |
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Figure
08: Standard Model distribution of cos θ*. The different contributions for each W boson polarization state in top quark decays are shown, as well as an illustration of how the angular asymmetries are defined. The right-handed component, which is highly suppressed in the Standard Model, is scaled to the same size as the left-handed component in order to be visible. png (96kB) eps (40kB) pdf (21kB) |
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09: Prior and posterior probabilities for the fit parameter Re(CuW33)/Λ2. The prior probability was chosen to be flat in the physically allowed range. The posterior probability density regions integrated symmetrically around the peak up to 68% and 95% are shown. Uncertainties on the top quark and W boson masses as well as the decay width of the top quark were included in the evaluation of the probabilities and cause the prior to bend at the edges. png (60kB) eps (21kB) pdf (7kB) |
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Figure
10: Overview of the four measurements of the W boson helicity fractions and the combined values. The error bars correspond to the statistical and total uncertainties. This is the color version of Figure 6. png (51kB) eps (11kB) pdf (5kB) |
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