Observation of quantum entanglement in top quark pair production in proton–proton collisions at ; <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML">; <mml:mrow>; <mml:msqrt>; <mml:mi>s</mml:mi>; </mml:msqrt>; <mml:mo>=</mml:mo>; <mml:mn>13</mml:mn>; </mml:mrow>; </mml:math>; TeV | CU Experts

Abstract; 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 (; ; ; ; ; t; ; ; ; ; t; ; ¯; ; ; ; ; ) 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 ; ; ; ; ; t; ; ; ; ; t; ; ¯; ; ; ; ; production density matrix and measured in the region of the ; ; ; ; ; t; ; ; ; ; t; ; ¯; ; ; ; ; production threshold. Values of ; ; ; ; D; <; −; 1; ; /; ; 3; ; ; are evidence of entanglement and D is observed (expected) to be ; ; ; ; −; ; 0.480; ; −; 0.029; ; ; +; 0.026; ; ; ; ; (; ; ; ; −; ; 0.467; ; −; 0.029; ; ; +; 0.026; ; ; ; ; ) 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 ; ; ; ; ; t; ; ; ; ; t; ; ¯; ; ; ; ; pairs in this phase space. This measurement provides a new probe of quantum mechanics at the highest energies ever produced.

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