The normalized differential ttbar cross section in jet multiplicity bins is presented, measured in protonproton 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 crosssection 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.
Figure  Caption 

Reconstructed jet multiplicity distribution for jets with transverse momenta of p_{T} > 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]  
Reconstructed jet multiplicity distribution for jets with transverse momenta of p_{T} > 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] 
Figure  Caption 

Normalized differential crosssection as a function of jet multiplicity for jets with p_{T} > 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]  
Normalized differential crosssection as a function of jet multiplicity for jets with p_{T} > 35 GeV. The figures show the behaviour of MADGRAPH generator when varying the Q^{2} and matching scales. The errors on the data points indicate the statistical (inner bars) and the total uncertainty. [Get pdf version] 
Figure  Caption  

Distribution of the jet multiplicity for selected events in data compared to estimated samples. The signal events are generated by MadGraph and the partonshowering and the hadronization are performed by Pythia6.


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 btagged jets are assigned as bjets 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. 

Normalized differential crosssection as a function of additional parton multiplicity for jets with p_{T} > 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]  
Normalized differential crosssection as a function of additional parton multiplicity for jets with p_{T} > 30 GeV. The figures show the behaviour of MADGRAPH generator when varying the Q^{2} and matching scales. The errors on the data points indicate the statistical (inner bars) and the total uncertainty. [Get pdf version] 
 AlexisDescroix  06Nov2012