Measurement of the charge asymmetry in top quark pair production in $pp$ collision data at $\sqrt{s} = 7\,$TeV using the ATLAS detector

A measurement of the charge asymmetry in the production of top quark pairs in the semileptonic decay channel has been performed. A dataset corresponding to an integrated luminosity of $1.04\,{\rm fb}^{-1}$, obtained at a centre-of-mass energy of $\sqrt{s} = 7\,{\rm TeV}$ with the ATLAS experiment at the LHC, was used. After performing a selection of events with one isolated lepton, at least four jets and missing transverse energy, a kinematic fit was performed to reconstruct the $t\bar{t}$ event topology. The charge asymmetry was determined using the differential distribution of the reconstructed observable $|y_t| - |y_{\bar{t}}|$, where $y_t$ and $y_{\bar{t}}$ denote the top and antitop quark rapidities, respectively. An unfolding procedure was applied to correct for detector acceptance and resolution effects and to obtain the corresponding distribution at parton level. The total charge asymmetry after unfolding was measured to be \begin{eqnarray*} A_C^{\text{unf}} = -0.018 \pm 0.028\,\text{(stat.)} \pm 0.023\,\text{(syst.)} \end{eqnarray*} in agreement with the Standard Model prediction. In addition, a simultaneous unfolding in $|y_t| - |y_{\bar{t}}|$ and the invariant $t\bar{t}$ mass, $M_{t \bar{t}}$, was performed.