Measurements of top-quark pair spin correlations in the $e\mu$ channel at $\sqrt{s} = 13$~TeV using pp collisions in the ATLAS detector

A measurement of observables sensitive to spin correlations in \ttbar production is presented, using 36.1 fb$^{-1}$ of $pp$ collision data at $\sqrt{s} = 13$~TeV recorded with the ATLAS detector at the Large Hadron Collider. Differential cross-sections are measured in events with exactly one electron and muon with opposite electric charge as a function of the azimuthal opening angle in the laboratory frame between the electron and muon candidates. The azimuthal opening angle is also measured as a function of the invariant mass of the \ttbar system. These results allow the measured degree of spin correlation to be compared to the Standard Model prediction which is found to deviate from the data by 3.2 standard deviations.