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Entanglement is an intrinsic property of quantum mechanics and is predicted to be exhibited in the particles produced at the Large Hadron Collider. A measurement of the extent of entanglement in top quark-antiquark (tt) events produced in proton-proton collisions at a center-of-mass energy of 13 TeV is performed with the data recorded by the CMS experiment at the CERN LHC in 2016, and corresponding to an integrated luminosity of 36. 3 fb^-1. The events are selected based on the presence of two leptons with opposite charges and high transverse momentum. An entanglement-sensitive observable D is derived from the top quark spin-dependent parts of the tt production density matrix and measured in the region of the tt production threshold. Values of D -1/3 are evidence of entanglement and D is observed (expected) to be -0. 480 ^+0. 026-₀. ₀₂₉ (-0. 467 ^+0. 026-₀. ₀₂₉) at the parton level. With an observed significance of 5. 1 standard deviations with respect to the non-entangled hypothesis, this provides observation of quantum mechanical entanglement within tt pairs in this phase space. This measurement provides a new probe of quantum mechanics at the highest energies ever produced.
Hayrapetyan et al. (Thu,) studied this question.
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