TOP-14-021 - Observation of a Z and measurement of a W boson produced in association with a top quark pair in events with multiple leptons at CMS

Abstract

A measurement of top quark pair production in association with a W or Z boson is presented, using 19.5 fb-1 of 8 TeV pp collision data collected by the CMS experiment at the CERN LHC. Those final states are selected in which the W boson decays to a charged lepton and a neutrino (W → ℓν) or the Z decays to two charged leptons (Z → ℓ+-), and the top quark pair decays to jets (tt → bqq bqq), a charged lepton plus jets (tt → bℓν bqq), or two charged leptons (tt → bℓ+ν bℓ-ν). These decays produce signatures with two, three, or four isolated leptons (electrons or muons) plus b-tagged jets. The ttW cross section is measured to be σ(ttW) = 382
+117
–102
fb with an observed (expected) significance of 4.8 (3.5) standard deviations from the background hypothesis. The ttZ cross section is measured to be σ(ttZ) = 242
+65
–55
fb with an observed (expected) significance of 6.4 (5.7) standard deviations from the background hypothesis. The measured cross sections are used to find bounds for five anomalous dimension-six operators which would affect the ttW and ttZ cross sections. At 95% confidence, we find -0.14 < cuB < 0.14, -0.33 < c'HQ < -0.24 or -0.02 < c'HQ < 0.23, -0.31 < cHQ < 0.63, -0.71 < cHu < 0.37, and -0.43 < c3W < 0.43.

Results

A measurement of top quark pair production in association with a W or Z boson at 8 TeV using the CMS detector has been presented. Signatures from different decay modes of the top quark pair resulting in final states with two, three, and four leptons have been analyzed. Results from two independent ttW channels and three ttZ channels have been presented, along with combined measurements. The combined measurement of the ttW cross section in same-sign and three lepton events is σ(ttW) = 382
+117
–102
fb corresponding to a 4.8 σ deviation from the background-only hypothesis, and a 1.7 σ deviation from the standard model prediction. Combining opposite-sign, three lepton, and four lepton channels, the ttZ cross section is measured to be σ(ttZ) = 242
+65
–55
fb an observation with a significance of 6.4 standard deviations from the background-only hypothesis and in agreement with the standard model. Using these cross section measurements, limits have been placed on the vector and axial couplings of the Z boson to the top quark, and on dimension-six operators parameterizing new physics. All of the measured values are compatible with the standard model predictions, within uncertainties.

Figures

Figure 1: Feynman diagrams

Figure 1: Primary Feynman diagrams for ttW+ and ttZ production at the LHC. The charge conjugate process of ttW+ produces ttW-.

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PDF - PNG
 

Figure 2: Matching variables

Figure 2a: Distributions for the match score to a partially reconstructed hadronic tt system in same-flavor opposite-sign dilepton events with six or more jets. Signal and background yields normalized to post-fit values. The match score comes from a linear discriminant which evaluates the consistency of reconstructed leptons and jets matched to parent particles based on their invariant mass, reconstructed charge, and b-tag discriminant values. A higher score indicates a better match to a tt pair producing two b-jets and three light flavor jets.

Figure 2b: Distribution for the match score to a fully reconstructed semi-leptonic tt system in same-sign electron-muon events with at least four jets. Signal and background yields normalized to post-fit values. The match score comes from a linear discriminant which evaluates the consistency of reconstructed leptons and jets matched to parent particles based on their invariant mass, reconstructed charge, and b-tag discriminant values. A higher score indicates a better match to a tt pair producing two b-jets, a lepton and neutrino, and two light flavor jets.

Figure 2c: Distribution for the match score to a fully reconstructed semi-leptonic tt system in three lepton events with two same-flavor opposite-sign leptons close to the Z mass and at least four jets. Signal and background yields normalized to post-fit values. The match score comes from a linear discriminant which evaluates the consistency of reconstructed leptons and jets matched to parent particles based on their invariant mass, reconstructed charge, and b-tag discriminant values. A higher score indicates a better match to a tt pair producing two b-jets, a lepton and neutrino, and two light flavor jets.

Figure 3: Same-sign ttW final discriminants

Figure 3a: Post-fit plots of the final discriminant for same-sign μμ ttW channel with three jets, and four or more jets.

PDF - PNG
PDF - PNG

Figure 3b: Post-fit plots of the final discriminant for same-sign eμ ttW channel with three jets, and four or more jets.

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PDF - PNG

Figure 3c: Post-fit plots of the final discriminant for same-sign ee ttW channel with three jets, and four or more jets.

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PDF - PNG

Figure 4: 3ℓ ttW final discriminants

Figure 4: Post-fit plots of the final discriminant for the 3ℓ ttW channel with one jet, and two or more jets.

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PDF - PNG

Figure 5: Opposite-sign, 3ℓ, and 4ℓ ttZ final discriminants

Opposite sign ttZ

Figure 5a: Post-fit plots of the final discriminant for the opposite-sign ttZ channel with five jets, and six or more jets.

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PDF - PNG

3ℓ ttZ

Figure 5b: Post-fit plots of the final discriminant for the 3ℓ ttZ channel with three jets, and four or more jets.

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PDF - PNG

4ℓ ttZ

Figure 5c: Post-fit plots of the final discriminant for the 4ℓ ttZ channel with two lepton pairs consistent with a Z → ℓ+- decay, and exactly one lepton pair consistent with a Z → ℓ+- decay.

PDF - PNG
PDF - PNG

Figure 6: Properties of signal-like 3ℓ ttZ events

Figure 6a: Pre-fit distribution of the mass and pT of the lepton pair identified with the Z decay for signal-like events in the 3ℓ ttZ channel (three jets with BDT > 0.3 and four or more jets with BDT > -0.2).


Figure 6b: Pre-fit distribution of the number of jets and number of medium b-tagged jets for signal-like events in the 3ℓ ttZ channel (three jets with BDT > 0.3 and four or more jets with BDT > -0.2).


Figure 6c: Pre-fit distribution of the best matched dijet pair from a W decay and best matched trijet system from a top decay for signal-like events in the 3ℓ ttZ channel (three jets with BDT > 0.3 and four or more jets with BDT > -0.2).


Figure 7: σ(ttW) vs. σ(ttZ)

Figure 7: Profile likelihood as a function of σ(ttW) and σ(ttZ). Lines denote 1, 2, 3, 4, and 5 σ confidence levels.

Figure 8: Vector vs. axial components of top-Z coupling

Figure 8: Difference between the profile likelihood and the best fit profile likelihood functions for the relative vector and axial components of the top-Z coupling. Contours corresponding to the 1σ, 2σ, and 3σ confidence levels are shown in lines, while the best fit radius is denoted by a star.

Figure 9: Effect of dimension-six operators on σ(ttW) and σ(ttZ)

Figure 9a: Sampled coefficient values plotted in the σ(ttZ), σ(ttW) plane, for cuB.

Figure 9b: Sampled coefficient values plotted in the σ(ttZ), σ(ttW) plane, for c'HQ.

Figure 9c: Sampled coefficient values plotted in the σ(ttZ), σ(ttW) plane, for cHQ.

Figure 9d: Sampled coefficient values plotted in the σ(ttZ), σ(ttW) plane, for cHu.

Figure 9e: Sampled coefficient values plotted in the σ(ttZ), σ(ttW) plane, for c3W.

Tables

Table Caption
Table 1: Post-fit expected and observed yields after selection for same-sign ttW final states.

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Table 2: Post-fit expected and observed yields after selection for opposite-sign ttZ final states.

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Table 3: Post-fit expected and observed yields after selection for three lepton ttW and three and four lepton ttZ final states.

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Table 4: Impact of removing sets of systematic uncertainties on the expected signal strength precision for ttW and ttZ.

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Table 5: Cross section with 68% (95%) CL ranges and sensitivity for ttW.

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Table 6: Cross section with 68% (95%) CL ranges and sensitivity for ttZ.

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Table 7: Preliminary constraints from this ttZ and ttW measurement on selected dimension-six operators.

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Extra figures

Figure 10: Non-prompt validation

Figure 10a: Lower lepton pT in same-sign, μμ events with two tight leptons and exactly two jets.

Figure 10b: Lower pT lepton η in same-sign, eμ events with two tight leptons and exactly two jets.

Figure 10c: Lower lepton pT in same-sign, ee events with two tight leptons and exactly two jets.

Figure 10d: Lower same-sign lepton pT in three lepton events where two leptons match a Z → ℓ+- decay, with no jets and low MET.

Figure 11: 3ℓ ttZ matching algorithm inputs

Figure 11a: Expected distribution for the invariant mass of two jets from a W decay in 3ℓ ttZ events.

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Figure 11b: Expected distribution for the invariant mass of any two jets in 3ℓ ttZ events.

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Figure 11c: Ratio of the invariant mass of two jets from a W decay to the invariant mass of any two jets, in 3ℓ ttZ events. Renormalized so that the average ratio value is equal to 1.0 for pairs of jets from a W decay.

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Figure 12: 3ℓ ttZ BDT inputs

Figure 12a: Input variable to the ttZ vs. WZ and tt BDT in the 3ℓ ttZ channel with four or more jets. Signal and background yields normalized to post-fit values.

Figure 12b: Input variable to the ttZ vs. WZ and tt BDT in the 3ℓ ttZ channel with four or more jets. Signal and background yields normalized to post-fit values.

Figure 12c: Input variable to the ttZ vs. WZ and tt BDT in the 3ℓ ttZ channel with four or more jets. Signal and background yields normalized to post-fit values.

-- ValeryZhukov - 2015-09-14

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Topic revision: r1 - 2015-09-14 - ValeryZhukov
 
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