Abstract
A measurement of the production cross-section for top quark pairs($\ttbar$) in $pp$ collisions at $\sqrt{s}=7 \TeV$ is presented using data recorded with the ATLAS detector at the Large Hadron Collider. Events are selected in two different topologies: single lepton (electron $e$ or muon $\mu$) with large missing transverse energy and at least four jets, and dilepton ($ee$, $\mu\mu$ or $e\mu$) with large missing transverse energy and at least two jets. In a data sample of 2.9 pb-1, 37 candidate events are observed in the single-lepton topology and 9 events in the dilepton topology. The corresponding expected backgrounds from non-$\ttbar$ Standard Model processes are estimated using data-driven methods and determined to be $12.2 \pm 3.9$ events and $2.5 \pm 0.6$ events, respectively. The kinematic properties of the selected events are consistent with SM $\ttbar$ production. The inclusive top quark pair production cross-section is measured to be $\sigmattbar=145 \pm 31 ^{+42}_{-27}$ pb where the first uncertainty is statistical and the second systematic. The measurement agrees with perturbative QCD calculations.
Highlights
The observation of top quark pair production is one of the milestones for the early LHC physics programme
The measurement of the mistag fraction is performed on an inclusive jet sample and includes two methods, one which uses the invariant mass spectrum of tracks associated to reconstructed secondary vertices to separate light- and heavy-flavour jets and one which is based on the rate at which secondary vertices with negative decay-length significance are present in the data
The fit methods make different assumptions about the signal and background and serve as good crosschecks; their cross-sections are shown in Table 5 and are in good agreement with those obtained from the baseline counting method
Summary
The observation of top quark pair (tt) production is one of the milestones for the early LHC physics programme. The measurement of the top quark pair production cross-section (σtt) in the various decay channels is interesting for several reasons. The main background contributions are determined using data-driven methods, since the theoretical uncertainties on the normalisation of these backgrounds are relatively large For both single-lepton and dilepton channels, alternative methods of signal extraction and/or background estimation are explored. Two template shape fitting methods, which use additional signal regions to exploit the kinematic information in the events, are developed for the single-lepton mode. In this paper these two fitting methods serve as cross-checks of the counting method. The methods provide alternative data-driven estimates of backgrounds and are expected to become more precise when more data become available
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