A measurement of spin correlation in $t\overline{t}$ production is presented using data collected with the ATLAS detector at the Large Hadron Collider in proton-proton collisions at a center-of-mass energy of 8 TeV, corresponding to an integrated luminosity of $20.3{\mathrm{fb}}^{-1}$. The correlation between the top and antitop quark spins is extracted from dilepton $t\overline{t}$ events by using the difference in the azimuthal angle between the two charged leptons in the laboratory frame. In the helicity basis the measured degree of correlation corresponds to $A_{\text{helicity}}=0.38\pm 0.04$, in agreement with the standard model prediction. A search is performed for pair production of top squarks with masses close to the top quark mass decaying to predominantly right-handed top quarks and a light neutralino, the lightest supersymmetric particle. Top squarks with masses between the top quark mass and 191 GeV are excluded at the 95% confidence level.

• Received 16 December 2014

DOI:https://doi.org/10.1103/PhysRevLett.114.142001

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© 2015 CERN, for the ATLAS Collaboration