Quantum tops at the LHC: from entanglement to Bell inequalities

Abstract

We present the prospects of detecting quantum entanglement and the violation of Bell inequalities in $$t\bar{t}$$ 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 $$5 \sigma $$ 5 σ both at threshold as well as at high $$p_T$$ p T already in the LHC Run 2 dataset. On the other hand, only very high-$$p_T$$ 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.

Quantum state tomography, entanglement detection and Bell violation prospects in weak decays of massive particles

“…That process (operators → angular distributions) is frequently simulated in the field of particle physics both using analytical calculations and Monte Carlo simulations. 6 Experimentalists analysing decay data wish to invert that process. Their goal is to analyse a set of angular decay distributions, and from those distributions obtain the spin density matrix of the originating quantum system.…”

“…For example a method has been proposed [4,5] for determining the spin density matrix of pairs of top and anti-top quarks at the LHC. The method has been used to study in simulation how one could measure the entanglement of the pair [5,6]. Entanglement detection and Bell violation have been investigated for simulated bipartite particle systems of qubits represented by top-quark, tau-lepton or photon pairs [7], and the quantum discord and steering in t t systems has also been investigated [8].…”

Comment on "Spin correlations in elastic $e^+ e^-$ scattering in QED"

“…To describe the Weyl spinors in (20) and (21), it is enough to rotate ϕ 1 from ( 23) around the y-axis. Denoting as χ 1 and χ 2 the rotation angles that describe the spin direction of the electron and positron, and using the expression (27), we obtain the Weyl spinor…”

“…Here we are interested in studying entanglement in scattering processes from QED. Recent work on this and related tests of Bell-type inequalities at colliders can be found in [19][20][21][22][23][24][25][26][27].…”

Laboratory-frame tests of quantum entanglement in $H \to WW$

“…And the proposed tests often yield, as a byproduct, new observables that might also be useful in searches for physics beyond the SM. Several studies in this regard have been performed for the entangled state of a top quark-antiquark pair [8][9][10][11][12][13][14][15].…”