Abstract
This letter presents a measurement of the top quark pair (tt) production charge asymmetry Ac using 4.7 fb-1 of proton-proton collisions at a centre-of-mass energy of 7 TeV collected by the ATLAS detector at the LHC. A tt-enriched sample of events with a single lepton (electron or muon), missing transverse momentum and at least four high transverse momentum jets, of which at least one is tagged as coming from a b-quark, is selected. A likelihood fit is used to reconstruct the tt event kinematics. A Bayesian unfolding procedure is employed to estimate Ac at the parton-level. The measured value of the tt production charge asymmetry is 0.006 +/- 0.010, where the uncertainty includes both the statistical and the systematic components. Differential Ac measurements as a function of the invariant mass, the rapidity and the transverse momentum of the tt-system are also presented. In addition, Ac is measured for a subset of events with large tt velocity, where physics beyond the Standard Model could contribute. All measurements are consistent with the Standard Model predictions.
Highlights
Background estimationThe main backgrounds affecting the measurement come from W bosons produced in association with jets (W +jets), single-top, Z+jets, production of W/Z bosons pairs and multi-jet events with background leptons.3 The W +jets and multi-jets contributions are evaluated using a data-driven approach
Differential AC measurements as a function of the invariant mass, the rapidity and the transverse momentum of the ttsystem are presented
Single-top, Z+jets and diboson production are evaluated using simulated samples normalised to the approximate NNLO cross section for single-top events, NNLO cross section for inclusive Z events, and NLO cross section for diboson events, respectively
Summary
Background estimationThe main backgrounds affecting the measurement come from W bosons produced in association with jets (W +jets), single-top, Z+jets, production of W/Z bosons pairs and multi-jet events with background leptons. The W +jets and multi-jets contributions are evaluated using a data-driven approach. The main backgrounds affecting the measurement come from W bosons produced in association with jets (W +jets), single-top, Z+jets, production of W/Z bosons pairs and multi-jet events with background leptons.. The W +jets and multi-jets contributions are evaluated using a data-driven approach. Single-top, Z+jets and diboson production are evaluated using simulated samples normalised to the approximate NNLO cross section for single-top events, NNLO cross section for inclusive Z events, and NLO cross section for diboson events, respectively. The dominant W +jets background is asymmetric in ∆|y| and a data-driven technique is used to estimate its normalisation. The approach used is based on the fact that the production rate of W ++jets is larger than that of W −+jets. To a good approximation, processes other than W +jets give equal numbers of positively and negatively charged leptons, the formula
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