Measurements of top-quark properties at 8 TeV using the ATLAS detector at the LHC

This work presents the measurements of the charge asymmetry and top quark spin observables in $t\bar{t}$ events based on 20.3 ${\rm fb}^{-1}$ of $\sqrt{s} = 8$ TeV proton--proton collisions recorded with the ATLAS detector at the LHC. These analyses are performed in the dilepton final state, characterized by the presence of two isolated leptons (electrons or muons). Two different reconstruction algorithms are studied and applied to reconstruct the $t\bar{t}$ system. For the measurements of the charge asymmetry, inclusive and differential measurements are performed using top-quark and lepton based observables. The differential measurements are performed as a function of the transverse momentum, mass, and boost of the $t\bar{t}$ system. For the top quark spin measurements, 15 observables, each sensitive to a different coefficient of the spin density matrix of $t\bar{t}$ production, are measured independently. Ten of these observables are measured for the first time. These measurements represent a full characterization of the spin density matrix. The measurements are corrected to particle and parton level by using the Fully Bayesian Unfolding method. At particle level, all measurements are performed in a fiducial region, while at parton level they are performed in the full phase space. No significant deviation from the Standard Model is observed for any of the measurements performed.