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arXiv:2402.14725 · Physics Letters B

Full quantum tomography of top quark decays

J. A. Aguilar–Saavedra

Published Jul 4, 2024• 13 citations
arXivHTMLPDF

Abstract

Quantum tomography in high-energy physics processes has usually been restricted to the spin degrees of freedom. We address the case of top quark decays t → W b , in which the orbital angular momentum ( L ) and the spins of W and b are intertwined into a 54-dimensional LWb density operator. The entanglement between L and the W or b spin is large and could be determined for decays of single top quarks produced at the Large Hadron Collider with Run 2 data. With the foreseen statistical and systematic uncertainties, the significance is well above 5 σ from the separability hypothesis for L - W entanglement, and 3.2 σ for L - b . These would be the first entanglement measurements between orbital and spin angular momenta in high-energy physics. Likewise, the genuine tripartite entanglement between L and the two spins could be established with more than 5 σ . The method presented paves the way for similar measurements in other processes.

DOI
10.1016/j.physletb.2024.138849

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Quantum tomography in high-energy physics processes has usually been restricted to the spin degrees of freedom. We address the case of top quark decays t → W b , in which the orbital angular momentum ( L ) and the spins of W and b are intertwined into a 54-dimensional LWb density operator. The entanglement between L and the W or b spin is large and could be determined for decays of single top quarks produced at the Large Hadron Collider with Run 2 data. With the foreseen statistical and systematic uncertainties, the significance is well above 5 σ from the separability hypothesis for L - W entanglement, and 3.2 σ for L - b . These would be the first entanglement measurements between orbital and spin angular momenta in high-energy physics. Likewise, the genuine tripartite entanglement between L and the two spins could be established with more than 5 σ . The method presented paves the way for similar measurements in other processes.
Abstract Entanglement is a central subject in quantum mechanics. Due to its genuine relativistic behavior and fundamental nature, high-energy colliders are attractive systems for the experimental study of fundamental aspects of quantum mechanics. We propose the detection of entanglement between the spins of top–antitop–quark pairs at the LHC, representing the first proposal of entanglement detection in a pair of quarks, and also the entanglement observation at the highest energy scale so far. We show that entanglement can be observed by direct measurement of the angular separation between the leptons arising from the decay of the top–antitop pair. The detection can be achieved with high statistical significance, using the current data recorded during Run 2 at the LHC. In addition, we develop a simple protocol for the quantum tomography of the top–antitop pair. This experimental technique reconstructs the quantum state of the system, providing a new experimental tool to test theoretical predictions. Our work explicitly implements canonical experimental techniques in quantum information in a two-qubit high-energy system, paving the way to use high-energy colliders to also study quantum information aspects.
The decays of the Higgs boson produce a state in which the spins of the decay products and the orbital angular momentum ( L ) are highly entangled. We obtain the tripartite density operator, as well as reduced operators, for the decay into two weak bosons. For H → Z Z in the four-lepton final state we also estimate the statistical sensitivity at the Large Hadron Collider and future upgrades, using a binned method to reconstruct the density operators from distributions. With the expected Run 3 data, establishing genuine tripartite entanglement would be possible beyond the 5 σ level. The violation of Bell inequalities involving the spins of the two weak bosons could also be established beyond the 5 σ level. Published by the American Physical Society 2024
Tripartite entanglement in H→ZZ,WWH \to ZZ,WWH→ZZ,WW decays
The decays of the Higgs boson produce a state in which the spins of the decay products and the orbital angular momentum (LLL) are highly entangled. We obtain the tripartite density operator, as well as reduced operators, for the decay into two weak bosons. For H→ZZH \to ZZH→ZZ in the four-lepton final state we also estimate the statistical sensitivity at the Large Hadron Collider and future upgrades, using a binned method to reconstruct the density operators from distributions. With the expected Run 3 data, establishing genuine tripartite entanglement would be possible beyond the 5σ5σ5σ level. The violation of Bell inequalities involving the spins of the two weak bosons could also be established beyond the 5σ5σ5σ level.
In recent years, an interest in the study of quantum information has grown within the high-energy particle physics community. The possibility to establish the presence of entanglement at particle colliders, such as the Large Hadron Collider (LHC) at CERN, is a novel and thrilling research direction, offering the opportunity to push quantum mechanics to its limits by using the heaviest fundamental particle: the top quark. With this in mind, we propose to use measurements of quantum entanglement to probe the behaviour of fundamental interactions and to search for New Physics at high energy within the Standard Model Effective Field Theory paradigm. Inspired by recent proposals to measure entanglement of top quark pairs produced at the LHC, we examine how the existence of new interactions between fundamental particles modify the Standard Model expectations. By performing analytical calculations, we unveil a non-trivial pattern of effects depending on the kinematical phase space configurations.
The Large Hadron Collider provides a unique opportunity to study quantum entanglement and violation of Bell inequalities at the highest energy available today. In this paper, we will investigate these quantum correlations with top quark pair production, which represents a system of two-qubits. The semileptonic top pair channel has a factor of 6 increase in statistics and easier reconstruction with respect to the dileptonic channel. Although measuring the spin polarization of the hadronic top quark is known to be challenging, our study indicates that it is feasible to reconstruct the spin density matrix of the two-qubit system using an optimal hadronic polarimeter. This is achieved with the aid of jet substructure techniques and NN-inspired reconstruction methods, which improve the mapping between subjets and quarks. We find that entanglement can already be observed at more than the 5 σ level with existing data, and violation of Bell inequalities may be probed above the 4 σ level at the HL-LHC with 3 ab − 1 of data. Published by the American Physical Society 2024
It has been shown that entanglement and Bell nonlocality, which are key concepts in quantum mechanics, can be probed in high-energy colliders via processes of fundamental particle scattering. In fact, the ATLAS and CMS Collaborations have measured entanglement using top-quark pairs produced in proton-proton collisions at the LHC. Recently, it was shown that spin correlations can be measured in pairs of bottom quarks at the LHC, despite the fact that bottom quarks, unlike top quarks, hadronize before decaying. Here, we demonstrate that quantum correlations can also be studied using bottom-quark pairs, and analyze the feasibility of the observation of entanglement and Bell nonlocality in several collider experiments. Given the low mass of the bottom quark relative to typical energies accessible at the LHC, many of the bottom-quark pairs are in the ultrarelativistic regime, where they can exhibit strong spin entanglement. We find that entanglement of bottom-quark pairs may be measurable even with the LHC Run 2 data, especially with the CMS B parking dataset, while observation of Bell nonlocality may become feasible at the high-luminosity phase of the LHC.