Abstract:
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In recent years, there has been growing interest in probing the quantum properties of particle states produced at colliders. Phenomena such as quantum entanglement, one of the cornerstones of quantum mechanics, offer a new perspective on collider physics. Recently, the ATLAS and CMS experiments at the LHC reported the first observations of entanglement in top–antitop top-antitop pairs produced in proton–proton collisions. These measurements open up an intriguing new avenue: using quantum-information observables as probes of physics beyond the Standard Model (BSM). Entangled systems exhibit correlations that cannot be reproduced by classical descriptions. A particularly striking manifestation of this is Bell’s inequality, which constrains theories based on local hidden variables. Quantum-mechanical systems can violate these bounds, providing a direct test of the non-classical nature of their correlations. At colliders, analogous tests can be formulated using the Clauser–Horne–Shimony–Holt (CHSH) inequality and related quantum-information observables.
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In this talk, I will introduce the basic concepts of quantum information relevant to collider physics and describe the top–antitop system as a bipartite quantum state. I will then discuss the prospects for measuring quantum-information observables in top-antitop production and explore how these observables can be used as sensitive probes of new physics. As a concrete example, I will consider a chiral U(1) extension of the Standard Model containing a new Z’ boson. The chiral interactions of the Z’ modify the helicity amplitudes governing top–antitop production and, consequently, the structure of the top-antitop spin-density matrix. I will discuss how these modifications are reflected in entanglement, Bell-type correlations, and related quantum observables, and how they may provide complementary handles for uncovering BSM physics at colliders.