CMS-PAS-TOP-23-001

CMS-PAS-TOP-23-001
Probing entanglement in top quark production with the CMS detector
CMS Collaboration
27 March 2024

Abstract: 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 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.
Links: CDS record (PDF) ; Physics Briefing ; CADI line (restricted) ; These preliminary results are superseded in this paper, Submitted to ROPP. The superseded preliminary plots can be found here.

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