TOP-12-018
Measurement of jet multiplicity top pair events


Abstract

The normalized differential ttbar cross section in jet multiplicity bins is presented, measured in proton-proton collisions using 5.0 fb-1 of data collected at sqrt{s} = 7 TeV. The analysis is performed in the electron + jets and the muon + jets channels. The jet multiplicity distribution is particularly sensitive to extra jets radiation. A comparison of the data with different Monte Carlo generators is shown. After background subtraction, the data are in agreement with ttbar signal Monte Carlo distributions. Additionally, the measurement of the top quark pair differential cross-section in the number of radiated additional hard partons in the muon + jets channel is presented. The measured fractions of events with ttbar + 0, 1, and ≥ 2 additional partons are in good agreement with different Monte Carlo predictions.

Figures

Control Plots: Jet-Multiplicity Distributions

Figure Caption
N_Jets_e.png Reconstructed jet multiplicity distribution for jets with transverse momenta of pT > 35 GeV in data, background estimated processes (QCD multijet and W+jets), and MC signal and remaining background processes in the electron + jets channel. The errors on the data points indicate the statistical uncertainties. [Get pdf version]
N_Jets_mu.png Reconstructed jet multiplicity distribution for jets with transverse momenta of pT > 35 GeV in data, background estimated processes (QCD multijet and W+jets), and MC signal and remaining background processes in the muon + jets channel. The errors on the data points indicate the statistical uncertainties. [Get pdf version]

Combined Normalized Differential Cross Sections as a Function of Jet Multiplicity

Figure Caption
diffXS_comb_Gen.png Normalized differential cross-section as a function of jet multiplicity for jets with pT > 35 GeV. The data are compared with predictions from MADGRAPH+Pythia, MC@NLO+Herwig and POWHEG+Pythia. The errors on the data points indicate the statistical (inner bars) and the total uncertainty. [Get pdf version]
diffXS_comb_Theo.png Normalized differential cross-section as a function of jet multiplicity for jets with pT > 35 GeV. The figures show the behaviour of MADGRAPH generator when varying the Q2 and matching scales. The errors on the data points indicate the statistical (inner bars) and the total uncertainty. [Get pdf version]

Normalized Differential Cross Section as a Function of Additional Parton Multiplicity (μ + jets channel)

Figure Caption
profile_fit_genJets.png

Distribution of the jet multiplicity for selected events in data compared to estimated samples. The signal events are generated by MadGraph and the parton-showering and the hadronization are performed by Pythia6.
The ttbar sample is split in 3 parts using generator information. Depending on the number of jets (pT > 30 GeV and η < 2.4) not matching any of the 4 generated quarks produced by the top pair decay (2b-jets from top quarks decays and 2 light jets from W boson decaying hadronically), events are categorized in ttbar + 0, 1, or ≥ 2 additional partons. The matching is based on a ΔR cut at 0.5.
Single top, Z+jets, and diboson events are given by MC predictions. W+jets events are produced by MC but the normalization of the sample is corrected using a charge asymmetry method basing on the asymmetry measured in data. A QCD model is taken from data inverting the relative isolation of the muon required for the selection. The normalization of this sample is finally estimated by a fit of the QCD model to data in a sideband region (missing transverse energy in the 0 or 1 b-tagged jet only). [Get pdf version]

profile_fit_genJets.png

Result of the template fit performed with the hypothesis quality test χ, drawn with events from data, the three parts of the ttbar MC sample and the remaining backgrounds.The variable χ is calculated for each event, performing a full event reconstruction. This reconstruction assigns four of the available jets to top quark pair decay products in a certain number of reconstruction hypothesis. Only hypothesis where b-tagged jets are assigned as b-jets are considered. The invariant mass of three top quark pair decay products, leptonic top quark, hadronic top quark, and hadronic W boson, can be calculated with the assigned jets and the muon and the missing transverse energy measured in the event. The quadratic sum of the weighted difference between the reconstructed invariant mass of the three products and the true one (measured with MC true information) provides for each event a χ value. The hypothesis providing the lowest χ value is taken.
Only the normalization of the three ttbar parts is free for the fit to data, the background samples are constrained. The fit is performed splitting events additionally in three jet-multiplicity bins to improve the sensitivity of the fit. Finally the result of the fit for the three ttbar parts are applied to the predicted cross-sections of each part to provide the final measurement.[Get pdf version]

unfoldXS_Gen.png Normalized differential cross-section as a function of additional parton multiplicity for jets with pT > 30 GeV. The data are compared with predictions from MADGRAPH+Pythia, MC@NLO+Herwig and POWHEG+Pythia. The errors on the data points indicate the statistical (inner bars) and the total uncertainty. [Get pdf version]
unfoldXS_Theo.png Normalized differential cross-section as a function of additional parton multiplicity for jets with pT > 30 GeV. The figures show the behaviour of MADGRAPH generator when varying the Q2 and matching scales. The errors on the data points indicate the statistical (inner bars) and the total uncertainty. [Get pdf version]

-- AlexisDescroix - 06-Nov-2012

Topic attachments
I Attachment History Action Size Date Who Comment
PDFpdf N_Jets_e.pdf r1 manage 16.1 K 2012-11-14 - 14:13 AlexisDescroix  
PNGpng N_Jets_e.png r1 manage 26.1 K 2012-11-14 - 14:13 AlexisDescroix  
PDFpdf N_Jets_log.pdf r4 r3 r2 r1 manage 16.6 K 2012-11-14 - 15:06 AlexisDescroix  
PNGpng N_Jets_log.png r5 r4 r3 r2 r1 manage 28.2 K 2012-11-14 - 15:06 AlexisDescroix  
PDFpdf N_Jets_mu.pdf r1 manage 16.2 K 2012-11-14 - 14:13 AlexisDescroix  
PNGpng N_Jets_mu.png r1 manage 26.2 K 2012-11-14 - 14:13 AlexisDescroix  
PDFpdf diffXS_comb_Gen.pdf r3 r2 r1 manage 15.2 K 2012-11-14 - 14:52 AlexisDescroix  
PNGpng diffXS_comb_Gen.png r4 r3 r2 r1 manage 14.6 K 2012-11-14 - 14:52 AlexisDescroix  
PDFpdf diffXS_comb_Theo.pdf r4 r3 r2 r1 manage 15.6 K 2012-11-14 - 14:52 AlexisDescroix  
PNGpng diffXS_comb_Theo.png r4 r3 r2 r1 manage 17.4 K 2012-11-14 - 14:52 AlexisDescroix  
PDFpdf profile_fit_genJets.pdf r1 manage 30.2 K 2012-11-14 - 14:17 AlexisDescroix  
PNGpng profile_fit_genJets.png r3 r2 r1 manage 708.2 K 2012-11-14 - 14:17 AlexisDescroix  
PDFpdf unfoldXS_Gen.pdf r5 r4 r3 r2 r1 manage 15.3 K 2012-11-14 - 14:52 AlexisDescroix  
PNGpng unfoldXS_Gen.png r5 r4 r3 r2 r1 manage 16.1 K 2012-11-14 - 14:52 AlexisDescroix  
PDFpdf unfoldXS_Theo.pdf r5 r4 r3 r2 r1 manage 15.6 K 2012-11-14 - 14:52 AlexisDescroix  
PNGpng unfoldXS_Theo.png r5 r4 r3 r2 r1 manage 18.6 K 2012-11-14 - 14:52 AlexisDescroix  
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Topic revision: r11 - 2012-11-16 - RobertoTenchini
 
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