First Measurements of Spin Correlation Using Semi-leptonic $t\bar{t}$ Events at ATLAS

The top quark decays before it hadronizes. Before its spin state can be changed in a process of strong interaction, it is directly transferred to the top quark decay products. The top quark spin can be deduced by studying angular distributions of the decay products. The Standard Model predicts the top/anti-top quark ($t\bar{t}$) pairs to have correlated spins. The degree is sensitive to the spin and the production mechanisms of the top quark. Measuring the spin correlation allows to test the predictions. New physics effects can be reflected in deviations from the prediction. The measurement of the spin correlation of $t\bar{t}$ pairs, produced at the LHC with a center-of-mass energy of $\sqrt{s} = 7$ TeV and reconstructed with the ATLAS detector, is presented. The dataset corresponds to an integrated luminosity of 4.6 $\textrm{fb}^{-1}$. $t\bar{t}$ pairs are reconstructed in the $\ell$+jets channel using a kinematic likelihood fit offering the identification of light up- and down-type quarks from the $t \rightarrow bW \rightarrow bq\bar{q}'$ decay. The spin correlation is measured via the distribution of the azimuthal angle $\Delta \phi$ between two top quark spin analyzers in the laboratory frame. It is expressed as the degree of $t\bar{t}$ spin correlation predicted by the Standard Model, $f_{\textrm{SM}}$. The result of $f_{\textrm{SM}}= 1.12 \pm 0.11\,\text{(stat.)} \pm 0.22\,\text{(syst.)}$ is consistent with the Standard Model prediction of $f_{\textrm{SM}}= 1.0$.