This Letter presents measurements of the polarization of the top quark in top-antitop quark pair events, using $4.7{\mathrm{fb}}^{-1}$ of proton-proton collision data recorded with the ATLAS detector at the Large Hadron Collider at $\sqrt{s}=7\mathrm{TeV}$. Final states containing one or two isolated leptons (electrons or muons) and jets are considered. Two measurements of ${\alpha }_{\ell }P$, the product of the leptonic spin-analyzing power and the top quark polarization, are performed assuming that the polarization is introduced by either a $CP$ conserving or a maximally $CP$ violating production process. The measurements obtained, ${\alpha }_{\ell }{P}_{\mathrm{CPC}}=-0.035\pm 0.014(\mathrm{stat})\pm 0.037(\mathrm{syst})$ and ${\alpha }_{\ell }{P}_{\mathrm{CPV}}=0.020\pm 0.016(\mathrm{stat}{)}_{-0.017}^{+0.013}(\mathrm{syst})$, are in good agreement with the standard model prediction of negligible top quark polarization.

• Received 24 July 2013

DOI:https://doi.org/10.1103/PhysRevLett.111.232002

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© 2013 CERN, for the ATLAS Collaboration