Probing entanglement in top quark production with the CMS detector

Entanglement is an intrinsic property of quantum mechanics and its measurement probes the current understanding of the underlying quantum nature of elementary particles at a fundamental level. A measurement of the extent of entanglement in top quark-antiquark ($\mathrm{t\bar{t}}$) events produced in proton-proton collisions at a center-of-mass energy of 13 TeV is performed on the data recorded by the CMS experiment at the CERN LHC in 2016, corresponding to an integrated luminosity of 35.9 $\mathrm{fb^{-1}}$. The events are selected based on the presence of two oppositely charged high transverse momentum leptons. An entanglement-sensitive observable $D$ is derived from the top quark spin-dependent parts of the $\mathrm{t\bar{t}}$ production density matrix. Values of $D$ smaller than $-1/3$ are evidence of entanglement and, within a particular phase space, $D$ is measured to be $-0.478^{+0.025}_{-0.027}$. With an expected (observed) significance of 5.1 (5.7) standard deviations, this provides observation for quantum mechanical entanglement within $\mathrm{t\bar{t}}$ pairs in this phase space. This measurement provides a new quantum probe of the inner workings of the standard model at the highest energy ever tested.