Reconstructed jet multiplicities from the top-quark pair decays and associated jets in pp collisions at √s = 7 TeV measured with the ATLAS detector at the LHC

The multiplicity distribution of reconstructed jets in ttbar events is presented, measured in proton-proton collisions using 0.70 fb−1 of data collected at √s = 7 TeV. The analysis is performed in the single lepton channel, with one electron or muon in the final state. After background subtraction, the data are in agreement with ttbar signal Monte Carlo distributions. The jet multiplicity distribution is particularly sensitive to initial state radiation (ISR). A comparison of the data with different Monte Carlo ISR variations is shown. Within the uncertainties, dominated by the Jet Energy Scale, no distinction between the models can be made.