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Entanglement in Top Quark Pair Production Experiments
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Collider events are the sets of particles produced in high energy collisions.
They are usually analysed through kinematic distributions, which describe how quantities such as energies, momenta and angles are distributed, and through production rates, which measure how often a given process occurs.
This thesis uses a complementary description based on the spin state of the particles produced in the final state. The spin information is reconstructed as a density matrix, which encodes the quantum state of the selected particles and allows the use of tools from quantum information.
The study starts from the bipartite spin system pp → t¯t, where two protons collide and produce a top quark and an antitop quark. In this case, the concurrence quantifies the entanglement between the two particles. The analysis is then extended to three-particle final states, including the Standard Model processes pp → t¯tγ, with an additional photon, and pp → t¯tZ, with an additional Z boson. A benchmark Beyond the Standard Model (BSM) process, pp → t¯tv1, is also considered, where v1 is a massive vector
mediator. The events are simulated with MadGraph, a program that generates particle-collision events from scattering amplitudes, and the analysis is performed at tree level, meaning that only the leading-order diagrams without loop corrections are included. For this purpose, this thesis also extends the MadGraph density matrix extraction to three selected particles in the final state. The reconstructed spin states are then studied through their purity, reduced bipartite entanglement and multipartite entanglement
diagnostics.
The results show that proton-proton samples can hide different underlying spin structures. The separated q¯q and gg production channels display different degrees of mixedness and different multipartite behaviour. In the proposed BSM process, a χ2 analysis over the mt¯t distribution shows that quantities extracted from the spin density matrix, such as spin correlations and entanglement measures, can be sensitive to the new contribution.
Overall, the results show that density matrices provide access to quantum state information in simulated collider processes, extending the analysis beyond the standard bipartite t¯t setting.
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Màster Oficial de Ciència i Tecnologia Quàntiques / Quantum Science and Technology, Facultat de Física, Universitat de Barcelona. Curs: 2025-2026. Tutors: Alba Cervera Lierta, Diego Blas
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BELMONTE GIMÉNEZ, Blanca. Entanglement in Top Quark Pair Production Experiments. [consulted: 24 of September of 2026]. Available at: https://hdl.handle.net/2445/230960