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

TL;DR

This paper reports the first observation of entanglement in top quark pairs at high mass using polarization and spin correlation measurements from proton-proton collisions at 13 TeV, confirming standard model predictions.

Contribution

It introduces a novel measurement of top quark spin entanglement in high-mass events, using a comprehensive likelihood fit to polarization and spin correlation data from CMS.

Findings

Polarization and spin correlation measurements agree with the standard model.

Entanglement in top quark pairs observed with significance above 5 sigma.

First observation of top quark spin entanglement at high $ ext{t}ar{ ext{t}}$ mass.

Abstract

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