UC San Diego

Measurements of the polarization and spin correlation in top quark pairs ( ) 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 collected by the CMS experiment, corresponding to an integrated luminosity of . 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 system and the top quark scattering angle in the rest frame. The measured polarization and spin correlation are in agreement with the standard model. From the measured spin correlation, conclusions on the spin entanglement are drawn by applying the Peres-Horodecki criterion. The standard model predicts entangled spins for states at the production threshold and at high masses of the system. Entanglement is observed for the first time in events at high mass, where a large fraction of the decays are spacelike separated, with an expected and observed significance of above 5 standard deviations. © 2024 CERN, for the CMS Collaboration 2024 CERN