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. © CERN, for the benefit of the CMS Collaboration.