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CMS-PAS-TOP-17-023
Measurement of differential cross sections and charge ratios for t-channel single top quark production at 13 TeV
Abstract: A measurement is presented of differential cross sections for t-channel single top quark production in proton-proton collisions at a centre-of-mass energy of 13 TeV by the CMS experiment at the LHC. Events containing single muons or electrons and two or three jets are analysed, corresponding to an integrated luminosity of 36 fb1. The cross section is measured as a function of the top quark transverse momentum, rapidity, and polarization angle, the charged lepton transverse momentum and rapidity, and the transverse momentum of the W boson from the top quark decay. In addition, the charge ratio, defined as the ratio of the single top quark cross section to the sum of the single top quark and antiquark cross sections, is measured differentially as a function of the top quark, charged lepton, and W boson kinematic observables. The results are found to be in agreement with predictions using various next-to-leading order event generators and various sets of parton distribution functions. Additionally, the spin asymmetry, sensitive to the top quark polarisation, is determined from the differential distribution of the polarisation angle at parton level to be 0.439 ± 0.062, in agreement with the standard model prediction using the POWHEG event generator at next-to-leading order.
Figures & Tables Summary References CMS Publications
Figures

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Figure 1:
Born-level Feynman diagrams for single top quark production in the t channel: (left) 22 and (right) 23 processes. Corresponding diagrams exist for single top antiquark production.

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Figure 1-a:
Born-level Feynman diagram for single top quark production in the t channel: 22 process. Corresponding diagram exists for single top antiquark production.

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Figure 1-b:
Born-level Feynman diagram for single top quark production in the t channel: 23 process. Corresponding diagram exists for single top antiquark production.

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Figure 2:
Distributions of the transverse W boson mass in the 2 jets, 0 b-tag control region for the (left) muon and (right) electron channels after scaling the simulated and multijet templates to the result of a dedicated ML fit performed in this region. The lower plots give the ratio of the data to the fit results. The hatched band displays the post-fit uncertainties per bin after the experimental systematic uncertainties have been profiled.

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Figure 2-a:
Distribution of the transverse W boson mass in the 2 jets, 0 b-tag control region for the muon channel after scaling the simulated and multijet templates to the result of a dedicated ML fit performed in this region. The lower plot gives the ratio of the data to the fit results. The hatched band displays the post-fit uncertainties per bin after the experimental systematic uncertainties have been profiled.

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Figure 2-b:
Distribution of the transverse W boson mass in the 2 jets, 0 b-tag control region for the electron channel after scaling the simulated and multijet templates to the result of a dedicated ML fit performed in this region. The lower plot gives the ratio of the data to the fit results. The hatched band displays the post-fit uncertainties per bin after the experimental systematic uncertainties have been profiled.

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Figure 3:
Distributions of the BDT discriminants in 2 jets, 1 b-tag region: (left) BDTt-ch. trained to separate signal from background events; (right) BDTtˉt/W trained to separate W+jets from tˉt events in a background-dominated phase space. Events in muon and electron channel have been summed. The predictions have been scaled to the result of the inclusive ML fit. The parts of the distributions used in the fits are indicated in the lower panels. The hatched band displays the post-fit uncertainties per bin after the experimental systematic uncertainties have been profiled.

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Figure 3-a:
Distribution of the BDTt-ch. BDT discriminant trained to separate signal from background events in 2 jets, 1 b-tag region. Events in muon and electron channel have been summed. The predictions have been scaled to the result of the inclusive ML fit. The parts of the distribution used in the fit is indicated in the lower panel. The hatched band displays the post-fit uncertainties per bin after the experimental systematic uncertainties have been profiled.

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Figure 3-b:
Distribution of the BDTtˉt/W BDT discriminant trained to separate W+jets from tˉt events in a background-dominated phase space in 2 jets, 1 b-tag region. Events in muon and electron channel have been summed. The predictions have been scaled to the result of the inclusive ML fit. The parts of the distribution used in the fit is indicated in the lower panel. The hatched band displays the post-fit uncertainties per bin after the experimental systematic uncertainties have been profiled.

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Figure 4:
Distributions of the transverse W boson mass (left) for events with 2 jets, 1 b-tag and (right) in events with 3 jets, 2 b-tags. Events in muon and electron channel have been summed. The predictions have been scaled to the result of the inclusive ML fit. The parts of the distributions used in the fits are indicated in the lower panels. The hatched band displays the post-fit uncertainties per bin after the experimental systematic uncertainties have been profiled.

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Figure 4-a:
Distribution of the transverse W boson mass in events with 2 jets, 1 b-tag. Events in muon and electron channel have been summed. The predictions have been scaled to the result of the inclusive ML fit. The parts of the distribution used in the fit is indicated in the lower panel. The hatched band displays the post-fit uncertainties per bin after the experimental systematic uncertainties have been profiled.

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Figure 4-b:
Distribution of the transverse W boson mass in events with 3 jets, 2 b-tags. Events in muon and electron channel have been summed. The predictions have been scaled to the result of the inclusive ML fit. The parts of the distribution used in the fit is indicated in the lower panel. The hatched band displays the post-fit uncertainties per bin after the experimental systematic uncertainties have been profiled.

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Figure 5:
Distributions of the unfolding observables in a (left column) background-dominated and a (right column) signal-enriched region for events passing the 2 jets, 1 b-tag selection: (upper row) top quark pT; (middle row) charged lepton pT; (lower row) W boson pT. Events in muon and electron channel have been summed. The predictions have been scaled to the result of the inclusive ML fit. The lower plots give the ratio of the data to the fit results. The hatched band displays the post-fit uncertainties per bin after the experimental systematic uncertainties have been profiled.

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Figure 5-a:
Distribution of the top quark pT unfolding observable in a background-dominated region for events passing the 2 jets, 1 b-tag selection. Events in muon and electron channel have been summed. The predictions have been scaled to the result of the inclusive ML fit. The lower plot gives the ratio of the data to the fit results. The hatched band displays the post-fit uncertainties per bin after the experimental systematic uncertainties have been profiled.

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Figure 5-b:
Distribution of the top quark pT unfolding observable in a signal-enriched region for events passing the 2 jets, 1 b-tag selection. Events in muon and electron channel have been summed. The predictions have been scaled to the result of the inclusive ML fit. The lower plot gives the ratio of the data to the fit results. The hatched band displays the post-fit uncertainties per bin after the experimental systematic uncertainties have been profiled.

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Figure 5-c:
Distribution of the charged lepton pT unfolding observable in a background-dominated region for events passing the 2 jets, 1 b-tag selection. Events in muon and electron channel have been summed. The predictions have been scaled to the result of the inclusive ML fit. The lower plot gives the ratio of the data to the fit results. The hatched band displays the post-fit uncertainties per bin after the experimental systematic uncertainties have been profiled.

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Figure 5-d:
Distribution of the charged lepton pT unfolding observable in a signal-enriched region for events passing the 2 jets, 1 b-tag selection. Events in muon and electron channel have been summed. The predictions have been scaled to the result of the inclusive ML fit. The lower plot gives the ratio of the data to the fit results. The hatched band displays the post-fit uncertainties per bin after the experimental systematic uncertainties have been profiled.

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Figure 5-e:
Distribution of the W boson pT unfolding observable in a background-dominated region for events passing the 2 jets, 1 b-tag selection. Events in muon and electron channel have been summed. The predictions have been scaled to the result of the inclusive ML fit. The lower plot gives the ratio of the data to the fit results. The hatched band displays the post-fit uncertainties per bin after the experimental systematic uncertainties have been profiled.

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Figure 5-f:
Distribution of the W boson pT unfolding observable in a signal-enriched region for events passing the 2 jets, 1 b-tag selection. Events in muon and electron channel have been summed. The predictions have been scaled to the result of the inclusive ML fit. The lower plot gives the ratio of the data to the fit results. The hatched band displays the post-fit uncertainties per bin after the experimental systematic uncertainties have been profiled.

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Figure 6:
Distributions of the unfolding observables in a (left column) background-dominated and a (right column) signal-enriched region for events passing the 2 jets, 1 b-tag selection: (upper row) top quark rapidity; (middle row) charged lepton rapidity; (lower row) top quark polarisation angle. Events in muon and electron channel have been summed. The predictions have been scaled to the result of the inclusive ML fit. The lower plots give the ratio of the data to the fit results. The hatched band displays the post-fit uncertainties per bin after the experimental systematic uncertainties have been profiled.

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Figure 6-a:
Distribution of the top quark rapidity polarisation in a background-dominated region for events passing the 2 jets, 1 b-tag selection. Events in muon and electron channel have been summed. The predictions have been scaled to the result of the inclusive ML fit. The lower plot gives the ratio of the data to the fit results. The hatched band displays the post-fit uncertainties per bin after the experimental systematic uncertainties have been profiled.

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Figure 6-b:
Distribution of the charged lepton rapidity observable in a signal-enriched region for events passing the 2 jets, 1 b-tag selection. Events in muon and electron channel have been summed. The predictions have been scaled to the result of the inclusive ML fit. The lower plot gives the ratio of the data to the fit results. The hatched band displays the post-fit uncertainties per bin after the experimental systematic uncertainties have been profiled.

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Figure 6-c:
Distribution of the charged lepton rapidity observable in a background-dominated region for events passing the 2 jets, 1 b-tag selection. Events in muon and electron channel have been summed. The predictions have been scaled to the result of the inclusive ML fit. The lower plot gives the ratio of the data to the fit results. The hatched band displays the post-fit uncertainties per bin after the experimental systematic uncertainties have been profiled.

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Figure 6-d:
Distribution of the charged lepton rapidity observable in a signal-enriched region for events passing the 2 jets, 1 b-tag selection. Events in muon and electron channel have been summed. The predictions have been scaled to the result of the inclusive ML fit. The lower plot gives the ratio of the data to the fit results. The hatched band displays the post-fit uncertainties per bin after the experimental systematic uncertainties have been profiled.

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Figure 6-e:
Distribution of the top quark polarisation angle observable in a background-dominated region for events passing the 2 jets, 1 b-tag selection. Events in muon and electron channel have been summed. The predictions have been scaled to the result of the inclusive ML fit. The lower plot gives the ratio of the data to the fit results. The hatched band displays the post-fit uncertainties per bin after the experimental systematic uncertainties have been profiled.

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Figure 6-f:
Distribution of the top quark polarisation angle observable in a signal-enriched region for events passing the 2 jets, 1 b-tag selection. Events in muon and electron channel have been summed. The predictions have been scaled to the result of the inclusive ML fit. The lower plot gives the ratio of the data to the fit results. The hatched band displays the post-fit uncertainties per bin after the experimental systematic uncertainties have been profiled.

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Figure 7:
Differential t-channel single top quark cross sections at parton level: (upper row) top quark pT and rapidity; (middle row) charged lepton pT and rapidity; (lower left) W boson pT; (lower right) top quark polarisation angle. The inner tick marks denote the unfolded post-fit uncertainty after the experimental systematic uncertainties have been profiled.

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Figure 7-a:
Differential t-channel single top quark cross section at parton level, as a function of the top quark pT. The inner tick marks denote the unfolded post-fit uncertainty after the experimental systematic uncertainties have been profiled.

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Figure 7-b:
Differential t-channel single top quark cross section at parton level, as a function of the top quark rapidity. The inner tick marks denote the unfolded post-fit uncertainty after the experimental systematic uncertainties have been profiled.

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Figure 7-c:
Differential t-channel single top quark cross section at parton level, as a function of the charged lepton pT. The inner tick marks denote the unfolded post-fit uncertainty after the experimental systematic uncertainties have been profiled.

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Figure 7-d:
Differential t-channel single top quark cross section at parton level, as a function of the charged lepton rapidity. The inner tick marks denote the unfolded post-fit uncertainty after the experimental systematic uncertainties have been profiled.

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Figure 7-e:
Differential t-channel single top quark cross section at parton level, as a function of the W boson pT. The inner tick marks denote the unfolded post-fit uncertainty after the experimental systematic uncertainties have been profiled.

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Figure 7-f:
Differential t-channel single top quark cross section at parton level, as a function of the top quark polarisation angle. The inner tick marks denote the unfolded post-fit uncertainty after the experimental systematic uncertainties have been profiled.

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Figure 8:
Differential t-channel single top quark cross sections at particle level: (upper row) top quark pT and rapidity; (middle row) charged lepton pT and rapidity; (lower left) W boson pT; (lower right) top quark polarisation angle. The inner tick marks denote the unfolded post-fit uncertainty after the experimental systematic uncertainties have been profiled.

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Figure 8-a:
Differential t-channel single top quark cross section at particle level as a function of the top quark pT. The inner tick marks denote the unfolded post-fit uncertainty after the experimental systematic uncertainties have been profiled.

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Figure 8-b:
Differential t-channel single top quark cross section at particle level as a function of the top quark rapidity. The inner tick marks denote the unfolded post-fit uncertainty after the experimental systematic uncertainties have been profiled.

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Figure 8-c:
Differential t-channel single top quark cross section at particle level as a function of the charged lepton pT. The inner tick marks denote the unfolded post-fit uncertainty after the experimental systematic uncertainties have been profiled.

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Figure 8-d:
Differential t-channel single top quark cross section at particle level as a function of the charged lepton rapidity. The inner tick marks denote the unfolded post-fit uncertainty after the experimental systematic uncertainties have been profiled.

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Figure 8-e:
Differential t-channel single top quark cross section at particle level as a function of the W boson pT. The inner tick marks denote the unfolded post-fit uncertainty after the experimental systematic uncertainties have been profiled.

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Figure 8-f:
Differential t-channel single top quark cross section at particle level as a function of the top quark polarisation angle. The inner tick marks denote the unfolded post-fit uncertainty after the experimental systematic uncertainties have been profiled.

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Figure 9:
Normalised differential t-channel single top quark cross sections at parton level: (upper row) top quark pT and rapidity; (middle row) charged lepton pT and rapidity; (lower left) W boson pT; (lower right) top quark polarisation angle. The inner tick marks denote the unfolded post-fit uncertainty after the experimental systematic uncertainties have been profiled.

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Figure 9-a:
Normalised differential t-channel single top quark cross section at parton level as a function of the top quark pT. The inner tick marks denote the unfolded post-fit uncertainty after the experimental systematic uncertainties have been profiled.

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Figure 9-b:
Normalised differential t-channel single top quark cross section at parton level as a function of thetop quark rapidity. The inner tick marks denote the unfolded post-fit uncertainty after the experimental systematic uncertainties have been profiled.

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Figure 9-c:
Normalised differential t-channel single top quark cross section at parton level as a function of the charged lepton pT. The inner tick marks denote the unfolded post-fit uncertainty after the experimental systematic uncertainties have been profiled.

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Figure 9-d:
Normalised differential t-channel single top quark cross section at parton level as a function of the charged lepton rapidity. The inner tick marks denote the unfolded post-fit uncertainty after the experimental systematic uncertainties have been profiled.

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Figure 9-e:
Normalised differential t-channel single top quark cross section at parton level as a function of the W boson pT. The inner tick marks denote the unfolded post-fit uncertainty after the experimental systematic uncertainties have been profiled.

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Figure 9-f:
Normalised differential t-channel single top quark cross section at parton level as a function of the top quark polarisation angle. The inner tick marks denote the unfolded post-fit uncertainty after the experimental systematic uncertainties have been profiled.

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Figure 10:
Normalised differential t-channel single top quark cross sections at particle level: (upper row) top quark pT and rapidity; (middle row) charged lepton pT and rapidity; (lower left) W boson pT; (lower right) top quark polarisation angle. The inner tick marks denote the unfolded post-fit uncertainty after the experimental systematic uncertainties have been profiled.

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Figure 10-a:
Normalised differential t-channel single top quark cross section at particle level, as a function of the top quark pT. The inner tick marks denote the unfolded post-fit uncertainty after the experimental systematic uncertainties have been profiled.

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Figure 10-b:
Normalised differential t-channel single top quark cross section at particle level, as a function of the top quark rapidity. The inner tick marks denote the unfolded post-fit uncertainty after the experimental systematic uncertainties have been profiled.

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Figure 10-c:
Normalised differential t-channel single top quark cross section at particle level, as a function of the charged lepton pT. The inner tick marks denote the unfolded post-fit uncertainty after the experimental systematic uncertainties have been profiled.

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Figure 10-d:
Normalised differential t-channel single top quark cross section at particle level, as a function of the charged lepton rapidity. The inner tick marks denote the unfolded post-fit uncertainty after the experimental systematic uncertainties have been profiled.

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Figure 10-e:
Normalised differential t-channel single top quark cross section at particle level, as a function of the W boson pT. The inner tick marks denote the unfolded post-fit uncertainty after the experimental systematic uncertainties have been profiled.

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Figure 10-f:
Normalised differential t-channel single top quark cross section at particle level, as a function of the top quark polarisation angle. The inner tick marks denote the unfolded post-fit uncertainty after the experimental systematic uncertainties have been profiled.

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Figure 11:
Differential t-channel single top quark charge ratios at parton level: (upper row) top quark pT and rapidity; (middle row) charged lepton pT and rapidity; (lower row) W boson pT. The inner tick marks denote the unfolded post-fit uncertainty after the experimental systematic uncertainties have been profiled.

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Figure 11-a:
Differential t-channel single top quark charge ratio at parton level, as a function of the top quark pT. The inner tick marks denote the unfolded post-fit uncertainty after the experimental systematic uncertainties have been profiled.

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Figure 11-b:
Differential t-channel single top quark charge ratio at parton level, as a function of the top quark rapidity. The inner tick marks denote the unfolded post-fit uncertainty after the experimental systematic uncertainties have been profiled.

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Figure 11-c:
Differential t-channel single top quark charge ratio at parton level, as a function of the charged lepton pT. The inner tick marks denote the unfolded post-fit uncertainty after the experimental systematic uncertainties have been profiled.

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Figure 11-d:
Differential t-channel single top quark charge ratio at parton level, as a function of the top quark rapidity. The inner tick marks denote the unfolded post-fit uncertainty after the experimental systematic uncertainties have been profiled.

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Figure 11-e:
Differential t-channel single top quark charge ratio at parton level, as a function of the W boson pT. The inner tick marks denote the unfolded post-fit uncertainty after the experimental systematic uncertainties have been profiled.

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Figure 12:
Differential t-channel single top quark charge ratios at particle level: (upper row) top quark pT and rapidity; (middle row) charged lepton pT and rapidity; (lower row) W boson pT. The inner tick marks denote the unfolded post-fit uncertainty after the experimental systematic uncertainties have been profiled.

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Figure 12-a:
Differential t-channel single top quark charge ratio at particle level, as a function of the top quark pT. The inner tick marks denote the unfolded post-fit uncertainty after the experimental systematic uncertainties have been profiled.

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Figure 12-b:
Differential t-channel single top quark charge ratio at particle level, as a function of the top quark rapidity. The inner tick marks denote the unfolded post-fit uncertainty after the experimental systematic uncertainties have been profiled.

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Figure 12-c:
Differential t-channel single top quark charge ratio at particle level, as a function of the charged lepton pT. The inner tick marks denote the unfolded post-fit uncertainty after the experimental systematic uncertainties have been profiled.

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Figure 12-d:
Differential t-channel single top quark charge ratio at particle level, as a function of the charged lepton rapidity. The inner tick marks denote the unfolded post-fit uncertainty after the experimental systematic uncertainties have been profiled.

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Figure 12-e:
Differential t-channel single top quark charge ratio at particle level, as a function of the W boson pT. The inner tick marks denote the unfolded post-fit uncertainty after the experimental systematic uncertainties have been profiled.
Tables

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Table 1:
Measured and observed event yields in the 2j1b region for each lepton channel and charge. The uncertainties in the yields denote the post-fit uncertainties per process after the experimental systematic uncertainties have been profiled.
Summary
Differential cross sections for t-channel single top quark production in proton-proton collisions at a centre-of-mass energy of 13 TeV have been measured by the CMS experiment at the LHC using a sample of proton-proton collision events, corresponding to an integrated luminosity of 36 fb1. The cross sections are determined as a function of the top quark transverse momentum, rapidity, and polarisation angle, the charged lepton transverse momentum and rapidity, and the transverse momentum of the W boson from the top quark decay. In addition, the differential charge ratio σt/σt+ˉt has been measured as a function of the top quark transverse momentum and rapidity, the charged lepton transverse momentum and rapidity, and the transverse momentum of the W boson from the top quark decay. Data events containing a single electron or muon and two or three jets are used. The single top quark and antiquark yields are determined through maximum-likelihood fits to the data distributions. The differential cross sections are then found at the parton and particle levels by unfolding estimated signal yields. The results are compared to various next-to-leading order (NLO) predictions and found to be in good agreement. Also, the differential charge ratios are compared against various parton distribution function sets and all are found within uncertainties. Lastly, the top quark spin asymmetry, which is sensitive to the top quark polarisation, has been measured using the differential cross section as a function of the top quark polarisation angle at the parton level. The measured spin asymmetry value of 0.439 ± 0.062 is in good agreement with the standard model prediction, found using the POWHEG event generator at NLO.
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Compact Muon Solenoid
LHC, CERN