TOP-13-016 Measurement of the ttbb cross section and the ratio ttbb/ttjj in the lepton+jets final state at 8 TeV with the CMS detector.


The associated production of a top-quark pair with at least two additional jets is studied in the single lepton+jets final state for an integrated luminosity of 19.6 fb-1 at 8 TeV. The jets from the top-quark decay are identified using a constrained kinematic fit and multivariate classifiers in different categories split by the jet multiplicity. The contributions of ttjj and ttbb with at least two additional jets of any flavor or b jets to the inclusive tt+jets production are extracted with a simultaneous template fit of b-tag discriminants. The measured cross sections $\sigma(\rm ttbb)  = 271.0 \pm 10 \pm 3.0(\rm stat) \pm  32.2(\rm syst) \pm 7.0(\rm lumi)~\rm fb$ and $\sigma(\rm ttjj) = 23.1 \pm 2.3(\rm stat) \pm 2.9(\rm syst) \pm 0.6(\rm lumi)~\rm pb$ and the cross section ratio $\sigma(\rm ttbb)/ \sigma(\rm ttjj) = 0.0117 \pm 0.0040(\rm stat) \pm 0.0003(\rm syst)$ correspond to the phase space of additional jets defined at generator-level as: PT > 40 GeV/c, |η| < 2.5, ΔRjj > 0.5. The flavor of the generated jets is defined by the flavor of the leading quark in the jet. The obtained results are in good agreement with NLO calculations and the CMS measurement in the dilepton channel when the same jet definition is used.

Public Document

The CMS Physics Analysis Summary is available on CDS.


A measurement of the ttbb and ttjj cross sections and their cross section ratio in pp collisions at a centre-of-mass energy of $\sqrt{s}$=8 TeV has been performed. The analysis is carried out on a data set corresponding to 19.6 fb-1 of integrated luminosity, using events in the lepton+jets final state. The event selection at reconstruction-level is: at least one primary vertex; one single 'tight' lepton with $P_T^{e,\mu}>30$ GeV/c and veto against additional 'loose' leptons; at least four jets with $ P_T^{j}>50$ GeV/c; at least two b-tagged jets. In the hardB definition the flavor of the generated jets is defined by the flavor of the leading quark inside the jet cone. In the hadronB definition the flavor is defined by the presence of a B hadron in the list of jet constituents. The hardB definition is similar to the parton-level in analytical calculations. In this case jets with collinear gluon radiation and subsequent splitting g->bb are treated as light jets. The particle-level hadronB definition is used in many CMS analyses. The B hadron in this definition has no requirement on momentum and can be produced in PS inside the jet. We perform the analysis using both definitions and compare the results. Most figures and tables are presented for the hardB definition, the results for the hadronB are similar. For the phase space of two additional jets at generator-level (PT > 40 GeV/c, |η| < 2.5, ΔRjj > 0.5) the measured cross sections are:
res1.png and


Figure 1: Jet Multiplicities

BJetSel_HistoNumJets.png BJetSel_HistoNumBJets.png Fig1. Multiplicity of all jets(left) and b-tagged jets(right) after the event selection. The MC event yields are pre-fit values. The hatched errors and the green band in the Data/MC ratio include the statistical errors in data and MC samples and the systematic uncertainties in the MC simulation.

Right: PDF PNG

Figure 2: M(top) from Kinematic Fit

m_4j_2bj_KinFitSel_HistoInvMassTopHad_nolog.png m_5j_2bj_KinFitSel_HistoInvMassTopHad_nolog.png Fig.2 Reconstructed invariant mass of the hadronic top-quark of the best ranked jet combination identified by the constrained kinematic fit in the muon N(jet) = 4, 5, 6, ≥7 jet categories (from left to right, top to bottom). The errors include the statistical and systematic uncertainties.

Right: PDF PNG
m_6j_2bj_KinFitSel_HistoInvMassTopHad_nolog.png m_7j_2bj_KinFitSel_HistoInvMassTopHad_nolog.png Fig.2
Right: PDF PNG

Figure 3: B-tag discriminants in the muon channel

m_4j_2bj_SubCh_0RemainBJets_PreSel_Histo_ttbar_CVS_3.png m_5j_2bj_SubCh_0RemainBJets_PreSel_Histo_ttbar_CVS_3.png m_5j_2bj_SubCh_1RemainBJets_PreSel_Histo_CVS_remainingJetsAll.png Fig.3 Pre-fit distributions of the CSV b-tag discriminant for different jet multiplicity categories in the muon channel. The errors include the statistical and systematic uncertainties. The (Njet,Nba) categories from left to right and from top to bottom: (4,0), (5,0), (5,≥1), (6,0),(6,≥1), (≥7,0), (≥7,≥1).

Center: PDF PNG
Right: PDF PNG
m_6j_2bj_SubCh_0RemainBJets_PreSel_Histo_ttbar_CVS_3.png m_6j_2bj_SubCh_1RemainBJets_PreSel_Histo_CVS_remainingJetsAll.png m_7j_2bj_SubCh_0RemainBJets_PreSel_Histo_ttbar_CVS_3.png Fig.3
Center: PDF PNG
Right: PDF PNG
m_7j_2bj_SubCh_1RemainBJets_PreSel_Histo_CVS_remainingJetsAll.png     Fig.3

Figure 4: B-tag discriminants in the electron channel

e_4j_2bj_SubCh_0RemainBJets_PreSel_Histo_ttbar_CVS_3.png e_5j_2bj_SubCh_0RemainBJets_PreSel_Histo_ttbar_CVS_3.png e_5j_2bj_SubCh_1RemainBJets_PreSel_Histo_CVS_remainingJetsAll.png Fig.4 Pre-fit distributions of the CSV b-tag discriminant for different jet multiplicity categories in the electron channel. The errors include the statistical and systematic uncertainties. The (Njet,Nba) categories from left to right and from top to bottom: (4,0), (5,0), (5,≥1), (6,0),(6,≥1), (≥7,0), (≥7,≥1).

Center: PDF PNG
Right: PDF PNG
e_6j_2bj_SubCh_0RemainBJets_PreSel_Histo_ttbar_CVS_3.png e_6j_2bj_SubCh_1RemainBJets_PreSel_Histo_CVS_remainingJetsAll.png e_7j_2bj_SubCh_0RemainBJets_PreSel_Histo_ttbar_CVS_3.png Fig.4
Center: PDF PNG
Right: PDF PNG
e_7j_2bj_SubCh_1RemainBJets_PreSel_Histo_CVS_remainingJetsAll.png     Fig.4

Figure 5: Event yields before and after the fit

dataToMC_8TeV_category_yield_preFit.png dataToMC_8TeV_category_yield_postFitS.png Fig.5 Event yields in different jet categories before(left) and after(right) the fit using hardB jet flavor definition. The green band in the Data/MC ratio plot corresponds to the one sigma standard deviations of statistical+systematic uncertainties. For the post-fit yields also the two sigma deviations are shown as a yellow band.

Right: PDF PNG


Table Caption
tab1.png Table 1. Total number of the pre-fit MC simulated and data events after the selection.
tab2.png Table 2. Number of the pre-fit MC simulated and data events in different jet categories.
tab3.png Table 3. Systematic uncertainties considered in the analysis and their influence on the ttbb, ttjj fit fractions $f$ and selection efficiencies $\epsilon$ using hardB jet definition.
tab4a.png tab4b.png Table 4. Normalization factors Npostfit/Nprefit of different tt+jets components with respect to the MADGRAPH+PYTHIA predictions using the hardB (left) and the hadronB (right) jet definition.
Right: PDF PNG
tab5.png Table 5. Comparison of this measurement in the single lepton+jets channel with the result in the dilepton channel and theoretical calculation using hardB and hadronB jet flavor definitions. The uncertainties in the MADGRAPH+PYTHIA numbers are calculated by varying Q-scales of the inclusive tt+jets production up and down.

AndreasMeyer - 2015-07-23

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