Measurement of the charge asymmetry in top quark pair production in pp collisions at $\sqrt s = 7 $ TeV using the ATLAS detector

TL;DR

This paper measures the charge asymmetry in top-antitop quark production at 7 TeV using ATLAS data, applying unfolding techniques to compare with Standard Model predictions.

Contribution

It provides the first measurement of the top charge asymmetry at 7 TeV with Bayesian unfolding to correct for detector effects.

Findings

Measured combined charge asymmetry: -0.024 ± 0.016 (stat) ± 0.023 (syst)

Results are consistent with Standard Model predictions

Used kinematic likelihood reconstruction and Bayesian unfolding

Abstract

We present a measurement of the charge asymmetry in top-antitop production using data corresponding to an integrated luminosity of $0.70 fb^{-1}$ of proton-proton collisions at $\sqrt s = 7$ TeV collected by the ATLAS detector. The top pair events decaying semileptonically (lepton+jets channel) to either an electron or muon, missing transverse energy and at least four jets are selected. The reconstruction of the $t\bar{t}$ events was performed using a kinematic likelihood approach. The difference of absolute values of top and antitop rapidities is used to define the charge asymmetry: $A_{C} = (N(|\Delta Y|>0) - N(|\Delta Y|<0)) / (N(|\Delta Y>0) + N(|\Delta Y|<0))$. To allow comparisons with theory calculations, a Bayesian unfolding technique is applied to correct the measured $|\Delta Y|$ distributions for acceptance and detector effects. The top charge asymmetry in both channels (e and mu) after correction is measured to be: $A_{C} = -0.009 \pm 0.023 (stat) \pm 0.032 (syst)$ (e+jets channel) and $A_{C} = -0.028 \pm 0.019 (stat) \pm 0.022 (syst)$ ($\mu$+jets channel) giving a combined result of : $A_{C} = -0.024 \pm 0.016 (stat) \pm 0.023 (syst)$. These results are compatible with the Standard Model predictions of $A_{C}=0.006$.