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arXiv:2205.00542 · The European Physical Journal C

Improved tests of entanglement and Bell inequalities with LHC tops

J. A. Aguilar–Saavedra, J.A. Casas

Published Aug 3, 2022• 103 citations
arXivHTMLPDF

Abstract

Abstract We discuss quantum entanglement in top pair production at the LHC. Near the $$t {\bar{t}}$$ t t ¯ threshold, entanglement observables are enhanced by suppressing the contribution of $$q {\bar{q}}$$ q q ¯ subprocesses, which is achieved by a simple cut on the velocity of the $$t {\bar{t}}$$ t t ¯ system in the laboratory frame. Furthermore, we design new observables that directly measure the relevant combinations of $$t{\bar{t}}$$ t t ¯ spin correlation coefficients involved in the measurement of entanglement and Bell inequalities. As a result, the statistical sensitivity is enhanced, up to a factor of 7 for Bell inequalities near threshold.

DOI
10.1140/epjc/s10052-022-10630-4

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Abstract We discuss quantum entanglement in top pair production at the LHC. Near the ttˉt {\bar{t}}ttˉ t t ¯ threshold, entanglement observables are enhanced by suppressing the contribution of qqˉq {\bar{q}}qqˉ​ q q ¯ subprocesses, which is achieved by a simple cut on the velocity of the ttˉt {\bar{t}}ttˉ t t ¯ system in the laboratory frame. Furthermore, we design new observables that directly measure the relevant combinations of ttˉt{\bar{t}}ttˉ t t ¯ spin correlation coefficients involved in the measurement of entanglement and Bell inequalities. As a result, the statistical sensitivity is enhanced, up to a factor of 7 for Bell inequalities near threshold.
Abstract We present the prospects of detecting quantum entanglement and the violation of Bell inequalities in ttˉt\bar{t}ttˉ t t ¯ events at the LHC. We introduce a unique set of observables suitable for both measurements, and then perform the corresponding analyses using simulated events in the dilepton final state, reconstructing up to the unfolded level. We find that entanglement can be established at better than 5σ5 \sigma 5σ 5 σ both at threshold as well as at high pTp_TpT​ p T already in the LHC Run 2 dataset. On the other hand, only very high- pTp_TpT​ p T events are sensitive to a violation of Bell inequalities, making it significantly harder to observe experimentally. By employing a sensitive and robust observable, two different unfolding methods and independent statistical approaches, we conclude that, at variance with previous estimates, testing Bell inequalities will be challenging even in the high luminosity LHC run.
Correlations between the spins of top-quark pairs produced at a collider can be used to probe quantum entanglement at energies never explored so far. We show how the measurement of a single observable can provide a test of the violation of a Bell inequality at the 98% C.L. with the statistical uncertainty of the data already collected at the Large Hadron Collider, and at the 99.99% C.L. with the higher luminosity of the next run. Detector acceptance, efficiency, and migration effects are taken into account. The test relies on the spin correlations alone and does not require the determination of probabilities-in contrast to all other tests of Bell inequalities.
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
A top quark and an antitop quark produced together at colliders have correlated spins. These spins constitute a quantum state that can exhibit entanglement and violate Bell’s inequality. In realistic collider experiments, most analyses allow the axes, as well the Lorentz frame, to vary event by event, thus introducing a dependence on the choice of event-dependent basis leading us to adopt “fictitious states,” rather than genuine quantum states. The basis dependence of fictitious states allows for an optimization procedure, which makes the usage of fictitious states advantageous in measuring entanglement and Bell inequality violation. In this work, we show analytically that the basis that diagonalizes the spin-spin correlations is optimal for maximizing spin correlations, entanglement, and Bell inequality violation. We show that the optimal basis is approximately the same as the fixed beam basis (or the rotated beam basis) near the t t ¯ production threshold, while it approaches the helicity basis far above threshold. Using this basis, we present the sensitivity for entanglement and Bell inequality violation in t t ¯ events at the Large Hadron Collider (LHC) and a future e + e − collider. Since observing Bell inequality violation appears to be quite challenging experimentally, and requires a large dataset in collider experiments, choosing the optimal basis is crucially important to observe Bell inequality violation. Our method and general approach are equally applicable to other systems beyond t t ¯ , including interactions beyond the Standard Model.
A bstract Quantum entanglement is a fundamental property of quantum mechanics. Recently, studies have explored entanglement in the tt‾ t\overline{t} tt t t ¯ system at the Large Hadron Collider (LHC) when both the top quark and anti-top quark decay leptonically. Entanglement is detected via correlations between the polarizations of the top and anti-top and these polarizations are measured through the angles of the decay products of the top and anti-top. In this work, we propose searching for evidence of quantum entanglement in the semi-leptonic decay channel where the final state includes one lepton, one neutrino, two b -flavor tagged jets, and two light jets from the W decay. We find that this channel is both easier to reconstruct and has a larger effective quantity of data than the fully leptonic channel. As a result, the semi-leptonic channel is 60% more sensitive to quantum entanglement and a factor of 3 more sensitive to Bell inequality violation, compared to the leptonic channel. In 139 fb − 1 (3 ab −1 ) of data at the LHC (HL-LHC), it should be feasible to measure entanglement at a precision of ≲ 3% (0 . 7%). Detecting Bell inequality violation, on the other hand, is more challenging. With 300 fb −1 (3 ab −1 ) of integrated luminosity at the LHC Run-3 (HL-LHC), we expect a sensitivity of 1 . 3 σ (4 . 1 σ ). In our study, we utilize a realistic parametric fitting procedure to optimally recover the true angular distributions from detector effects. Compared to unfolding this procedure yields more stable results.
We investigate quantum steering and Bell nonlocality in top-quark pair production at the LHC near threshold. The toponium contribution strengthens the spin-singlet component and substantially enhances the entanglement between the two quark spins. With current LHC data, quantum steering appears observable with a statistical significance around 10σ10σ10σ. For Bell nonlocality, the statistical significance can also be high close to threshold, reaching about 9σ9σ9σ, although the feasibility of such a measurement will depend crucially on the control of systematic uncertainties.