Measurement of spin correlation in top–antitop quark events and search for top squark pair production in proton–proton collisions at $\sqrt{s}$=8 TeV using the ATLAS detector

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