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

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Summary

Introduction

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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