Measurement of the longitudinal polarization of the top quark in top-antitop events using the ATLAS detector.

This thesis presents measurements of the longitudinal polarization of the top quark in top-antitop quark pair events, using 4.7/fb of protonproton collision data recorded with the ATLAS detector at the Large Hadron Collider at Sqrt(S)= 7 TeV. The final state considered contains one isolated lepton (electron or muon) and jets. Two measurements of the product of the charged lepton's spin-analyzing power and its degree of longitudinal polarization, \alpha_l`P, are performed assuming that the longitudinal polarization of the top quark is induced by either a CP conserving process or a maximally CP violating process. The resulting measurements, \alpha_l`PCPC = -0:034\pm 0:017 (stat:)+0:038 -0:037 (syst:) and \alpha_l`PCPV = 0:023\pm 0:019 (stat:)+0:012 -0:011 (syst:) are in good agreement with the standard model expectation of negligible longitudinal polarization of the top quark in top-antitop quark pair production.