TOP-12-035
Measurement of the ratio R=B(t → Wb) / B(t → Wq)

Abstract

The ratio of the top quark branching fractions R=B(t→Wb)/B(t→Wq), where q can be a d, s, or a b quark, is measured in the t‾t dilepton final state. The measurement is performed with 16.7 fb-1 of proton-proton collision data at √s=8 TeV collected with the CMS detector. Data-driven strategies are used to constrain the main background contributions to the data sample. By counting the number of b -jets identified per event, the ratio of branching fractions is determined to be

R=1.023+0.036-0.034(stat+syst)
in good agreement with the SM prediction. A lower limit R>0.945 at 95% CL is obtained after requiring that R≤1. Assuming unitarity of the CKM matrix and its three-generation structure, the matrix element
|Vtb|=1.011+0.018-0.017(stat.+syst.)
is measured and |Vtb|>0.972 is obtained at 95% CL.


The complete physics analysis summary is available in CDS. Follow this link.
Click on the figures below to get the pdf version.

Tables

Table Caption
Event yields after the full event selection. The first uncertainty quoted is from the size of the samples (either data or simulation) used for the estimate, and the second is the remaining systematic uncertainty. tt +V is used to designated associated production of vector bosons with a tt pair.
Fraction of tt events (ftt) and relative contribution from single top (kst) in the selected dilepton samples as determined from a profile likelihood fit. The total uncertainty is shown.
Fraction of lepton-jet pairs correctly matched to top quark decays in the selected events. The fractions estimated in data and expected from MC simulation are reported separately while the last column shows the ratio of the two. Statistical and systematic uncertainties are included.
Summary of the systematic uncertainties affecting the measurement of R. The values of the uncertainties are relative to the value of R obtained from the fit.

Figures

Figure Caption
Diagrammatic representation of the tt production via gluon-gluon fusion and subsequent decay in a WbWq pair. The successful selection of two (left), one (center) or no (right) jets matched from top decays is pictured by the different diagrams according to the number of extra ISR jets.
Number of b-tagged jets per event for the different tt dilepton channels. For each final state, separate subsets are shown corresponding to events with two, three or four jets. The simulated tt and single top events correspond to a scenario with R=1. The bottom plot shows the ratio between data and expectations. The shaded band represents uncertainty on the background contribution.
Expected lepton-jet invariant mass distribution for tt dilepton events: the shapes for the correct and misassigned lepton-jet invariant mass pairs are compared.
Expected contributions to the misassigned lepton-jet invariant mass pair distribution. The unmatched contribution comprises both ISR jets in tt events as well as jets selected in background events. The misassignment model derived in simulation is superimposed as a closure test of the modeling of the overall spectrum.
Lepton-jet invariant mass spectrum reconstructed in data for the same-flavor (left) and opposite-flavor (right) channels. The comparison with the correct and misassigned components of the spectrum is included. The misassigned lepton-jet pairs are modeled with an empirical model.
Background-subtracted lepton-jet invariant mass distribution compared to the expectations for the signal.
Fraction of events with 2 (light gray), 1 (gray) or 0 (dark gray) top quark decays selected in the events as determined from data. The values are shown for the different event categories considered in the analysis.
Probability model for the observation of different b-tag multiplicities in dilepton events with two, three or four jets as a function of R.
Variation of the profile likelihood used to extract R from the data. The variation observed in the exclusive dilepton channels is depicted by the dashed lines. The inset shows the inclusive b-tag multiplicity distribution and the result of the fit.
Limit bands at different CL on R. The range of the observed R values allowed at different confidence levels are shown as colored bands for each true value of R.
Chronological summary of the different measurements of R performed at the Tevatron and the LHC. The shaded region represents the lower endpoint for the confidence interval at 95% confidence level on R obtained after imposing the physical constraint R≤1

Event displays

Event display Caption
Display in the ρ-φ plane of a eμ event candidate selected in 8 TeV data. The kinematics of the main objects used in the event selection are highlighted: two isolated leptons and two AK5 particle flow jets. The reconstructed ETmiss with the PF algorithm is also displayed for reference even if not used in the selection of eμ events.
Display in the ρ-z plane of the same eμ event candidate. Besides the two isolated lepton candidates the characteristics of a displaced vertex and of a secondary soft muon present in the two b-tagged jets are highlighted.
Topic attachments
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PDFpdf Diagram_0.pdf r1 manage 20.7 K 2013-02-21 - 12:17 PedroSilva  
PNGpng Diagram_0.png r1 manage 25.6 K 2013-02-21 - 12:17 PedroSilva  
PDFpdf Diagram_1.pdf r1 manage 19.5 K 2013-02-21 - 12:21 PedroSilva  
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PDFpdf Diagram_2.pdf r1 manage 20.5 K 2013-03-05 - 22:01 PedroSilva  
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PDFpdf FC.pdf r2 r1 manage 17.9 K 2013-02-21 - 21:47 PedroSilva  
PNGpng FC.png r2 r1 manage 16.6 K 2013-02-21 - 21:47 PedroSilva  
PDFpdf InclusiveFitPLR_data.pdf r2 r1 manage 18.2 K 2013-02-21 - 21:47 PedroSilva  
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PDFpdf RhoPhiView_193541_492_314062344.pdf r1 manage 132.9 K 2013-02-22 - 10:31 PedroSilva  
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PDFpdf Table1.pdf r2 r1 manage 61.4 K 2013-02-21 - 11:31 PedroSilva  
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PDFpdf Table2.pdf r1 manage 43.8 K 2013-02-21 - 11:31 PedroSilva  
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PDFpdf Table3.pdf r2 r1 manage 63.1 K 2013-02-26 - 16:54 PedroSilva  
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