Measurements of polarization and spin correlation and observation of entanglement in top quark pairs using lepton+jets events from proton-proton collisions at βˆšπ‘  =13  TeV

Jain, Sandhya;Benecke, Anna;Bethani, Agni;Bruno, Giacomo;CMS;et.al.
(2024) Physical review D β€” Vol. 110 (2024)

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Abstract
Measurements of the polarization and spin correlation in top quark pairs (𝑑⁒𝑑) 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 βˆšπ‘  =13  TeV collected by the CMS experiment, corresponding to an integrated luminosity of 138  fbβˆ’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 𝑑⁒𝑑 system and the top quark scattering angle in the 𝑑⁒𝑑 rest frame. The measured polarization and spin correlation are in agreement with the standard model. From the measured spin correlation, conclusions on the 𝑑⁒𝑑 spin entanglement are drawn by applying the Peres-Horodecki criterion. The standard model predicts entangled spins for 𝑑⁒𝑑 states at the production threshold and at high masses of the 𝑑⁒𝑑 system. Entanglement is observed for the first time in events at high 𝑑⁒𝑑 mass, where a large fraction of the 𝑑⁒𝑑 decays are spacelike separated, with an expected and observed significance of above 5 standard deviations.
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Jain, S., Benecke, A., Bethani, A., Bruno, G., Caputo, C., de Favereau de Jeneret, J., Delaere, C., Donertas, I. S., Giammanco, A., Guzel, A. O., Lemaitre, V., Lidrych, J., Mastrapasqua, P., Tran, T. T., Turkcapar, S., CMS, & et al. (2024). Measurements of polarization and spin correlation and observation of entanglement in top quark pairs using lepton+jets events from proton-proton collisions at βˆšπ‘  =13  TeV. Physical review D, 110. https://doi.org/10.1103/PhysRevD.110.112016 (Original work published 2024)