Observation of Spin Correlations in \ttbar\ events at \boldmath{$\sqrt{s}$} = 7~TeV using the ATLAS detector.

This thesis presents measurements of the the spin correlation strength in top anti-top quark pair production at the LHC using the ATLAS detector. The data used corresponds to 4.6 fb?1 of integrated luminosity taken during 2011 at the LHC at a center of mass energy of 7 TeV. The spin correlation is studied utilising different observables with different sensitivities to the production mechanism, in particular to gluon-gluon fusion in the like or unlike helicity state, quark anti- quark annihilation in the unlike helicity state, or a combination of the three. In addition cuts are made on the invariant mass of the ttbar system to enhance or suppress contributions from different initial state production mechanisms. The analysis presented is a precision test of both ttbar production and decay in the SM. These measurements are compared to the most current theoretical predictions. No deviation from the SM expectation was observed. In a subset of the data, corresponding to an integrated luminosity of 2.1 fb?1, the hypothesis of zero spin correlation is excluded at 5.1 standard deviations.