CMS-PAS-TOP-23-007

CMS-PAS-TOP-23-007
Measurements of polarization, spin correlations, and entanglement in top quark pairs using lepton+jets events from pp collisions at $ \sqrt{s}= $ 13 TeV
CMS Collaboration
13 June 2024

Abstract: Measurements of the polarization and spin correlations in top quark pairs ($ \mathrm{t\bar t} $) are presented using events with a single electron or muon and jets in the final state. The measurements are based on proton-proton collision data from the LHC at $ \sqrt{s}= $ 13 TeV collected by the CMS experiment, corresponding to an integrated luminosity of 138 fb$ ^{-1} $. All coefficients of the polarization vectors and the spin correlation matrix are extracted simultaneously by performing a binned likelihood fit to the data. The measurement is performed in bins of additional observables such as the mass of the $ \mathrm{t\bar t} $ system and the top quark scattering angle. Inclusive coefficients are obtained by combining the results of all fitted bins. From the measured spin correlations, conclusions on the $ \mathrm{t\bar t} $ spin entanglement are drawn. The standard model predicts entangled spin $ \mathrm{t\bar t} $ states at the production threshold and at high masses of the $ \mathrm{t\bar t} $ system. Entanglement is observed for the first time in events with high $ \mathrm{t\bar t} $ mass, with an observed (expected) significance of 6.7 (5.6) standard deviations. The observed level of entanglement cannot be explained by classical exchange of information between the two particles alone. The observed (expected) significance for entanglement attributable to space-like separated $ \mathrm{t\bar t} $ pairs is 5.4 (4.1) standard deviations.
Links: CDS record (PDF) ; Physics Briefing ; CADI line (restricted) ; These preliminary results are superseded in this paper, Submitted to PRD. The superseded preliminary plots can be found here.

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