The first measurement of the cross section of the top quark pair in association with an extra hard jet (ttbar+jet) has been performed with 4.1 fb-1 of data collected at CDF. The measurement is an important test of perturbative QCD, as NLO effects play an important role in the calculation of the theoretical cross section. In addition, it is also important for future measurements at the LHC, for which almost half of the top quark events will be produced with extra jets. Therefore, this process will be a substantial background for many new physics signals. The measurement is performed using SecVtx tagged events in the lepton plus jet channel. A data-driven approach is used to predict the background content, and a 2D likelihood is formed to simultaneously measure the ttbar+jet and ttbar without extra jet cross sections. The measured result is σ ttbar+jet = 1.6±0.2(stat)±0.5(syst)pb which is in agreement with the recent NLO SM prediction 1.791(1)+0.16-0.31pb. In order to elucidate the kinematic profile of the extra jet, an isolation algorithm has been developed. The algorithm has extracted correctly the extra jet out from the final state jets ∼ 60 % of the time. This allowed for correcting the measured distributions of the extra jet for purity/efficiency in order to compare them with the MC distributions. The differences in the kinematics of the extra jet using different SecVtx requirements and different MC models (PYTHIA and MCFM) have been studied, and found to be in agreement with the SM prediction.
Read
