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A measurement of observables sensitive to spin correlations in $t\bar{t}$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 HadronCollider. Differential cross-sections are measured in events with exactly oneelectron and one muon with opposite-sign electric charge as a function of theazimuthal opening angle and the absolute difference in pseudorapidity betweenthe electron and muon candidates in the laboratory frame. The azimuthal openingangle is also measured as a function of the invariant mass of the $t\bar{t}$system. The measured differential cross-sections are compared to predictions byseveral NLO Monte Carlo generators and fixed-order calculations. The observeddegree of spin correlation is somewhat higher than predicted by the generatorsused. The data are consistent with the prediction of one of the fixed-ordercalculations at NLO, but agree less well with higher-order predictions. Usingthese leptonic observables, a search is performed for pair production ofsupersymmetric top squarks decaying into Standard Model top quarks and lightneutralinos. Top squark masses between 170 and 230 GeV are largely excluded atthe 95% confidence level for kinematically allowed values of the neutralinomass.

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