Quantum information tools at the LHC: top-quark pair studies and toponium extraction

This thesis explores spin correlations and quantum information observables in top quark pair production at the LHC, treating the tt¯ system as an ideal two-qubit laboratory. A flexible framework is implemented to reconstruct the spin density matrix in tt¯ events in multiple channels at parton level, with extension to particle level studies for semileptonic final states. From the spin density matrix, quantum observables related to the tt¯ system are reconstructed. The framework is then applied to investigate the sensitivity of kinematic, spin and quantum observables to toponium formation near threshold, using both event-averaged and per-event variables. Bound-state effects are found to modify the spin correlation matrix, enhancing entanglement measures while reducing quantum magic. Among per-event observables, kinematic variables provide the strongest discrimination, with the top quark momentum in the tt¯ rest frame emerging as the most sensitive to toponium effects. A BDT combining kinematic and quantum-information-inspired variables further improves separation compared to any single observable, with QI-inspired variables providing a mild but non-negligible improvement to the separation performance. Detector-level-like cuts have been applied to the events to obtain a more realistic analysis, resulting in only a modest reduction of performance. Overall, this work establishes a solid foundation for future ATLAS and CMS measurements of quantum properties and bound-state effects in tt¯ production.