Quantum entanglement at the energy frontier: Measurement of spin entanglement in top-antitop quark pairs with the ATLAS experiment

Abstract

In this thesis, measurements of quantum entanglement between the spins of pair-produced top and antitop quarks are presented. The sensitivity of these measurements is found to be highest in two distinct kinematic regions. The first region is characterised by low values of the invariant mass of the top-quark pair system, close to the production threshold, where the top-quark pairs are expected to be predominantly produced in a maximally entangled spin-singlet state. The second region corresponds to very high invariant mass, where the top quarks are highly boosted and are predicted to form a maximally entangled spin-triplet state. Accordingly, two complementary analyses are performed, targeting entanglement measurements in these distinct regions of phase space.