Disentangling Quantum Mechanics in Top Quark Pairs at the ATLAS Experiment

This thesis presents sensitivity studies for possible measurements of quantum entanglement and quantum discord in top quark pairs at $\sqrt{s}=13\,$TeV with the ATLAS experiment using simulations of proton-proton collisions corresponding to an integrated luminosity of $140\,\text{fb}^{-1}$. Quantum entanglement can be found in inseparable correlated systems. Top quark pairs are predicted to be generated in a maximally entangled state close to the production threshold. In the semileptonic decay channel, the angular separation between the decay products of the top quarks is used as an indicator of quantum entanglement. The expected result for the entanglement marker is $D = - 0.598 \pm 0.006 (\text{stat.}) \pm 0.017(\text{syst.}) $ for $M_{t\bar{t}} < 390\,$GeV, which is more than five standard deviations from the entanglement limit. Quantum discord quantifies the quantumness of a correlation, also found in separable states. The full spin density matrix is determined in the dileptonic decay channel to calculate quantum discord. It is separately determined for the top and the anti-top quark. The expected results for the top quark and anti-top quark discord inclusively are $\mathcal{D}_{t} = \mathcal{D}_{\bar{t}} = 0.07\pm 0.4 $, which are $1.7$ standard deviations away from zero. The sensitivity is significantly impacted by systematic signal modelling uncertainties.