Measurements of polarization and spin correlation and observation of entanglement in top quark pairs using lepton+jets events from proton-proton collisions at $\sqrt{s}$ = 13 TeV

Abstract

Measurements of the polarization and spin correlation in top quark pairs ($\mathrm{t}\bar{\mathrm{t}}$) 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 inclusively and in bins of additional observables, such as the mass of the $\mathrm{t}\bar{\mathrm{t}}$ system and the top quark scattering angle in the $\mathrm{t}\bar{\mathrm{t}}$ rest frame. The measured polarization and spin correlation are in agreement with the standard model. From the measured spin correlation, conclusions on the $\mathrm{t}\bar{\mathrm{t}}$ spin entanglement are drawn by applying the Peres-Horodecki criterion. The standard model predicts entangled spins for $\mathrm{t}\bar{\mathrm{t}}$ states at the production threshold and at high masses of the $\mathrm{t}\bar{\mathrm{t}}$ system. Entanglement is observed for the first time in events at high $\mathrm{t}\bar{\mathrm{t}}$ mass, where a large fraction of the $\mathrm{t}\bar{\mathrm{t}}$ decays are space-like separated, with an expected and observed significance of above 5 standard deviations.

Replaced with the published version. Added the journal reference and the DOI. All the figures and tables, including additional supplementary figures, can be found at http://cms-results.web.cern.ch/cms-results/public-results/publications/TOP-23-007 (CMS Public Pages)

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