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We report the highest-energy observation of entanglement, in top$-$antitopquark events produced at the Large Hadron Collider, using a proton$-$protoncollision data set with a center-of-mass energy of $\sqrt{s}=13$ TeV and anintegrated luminosity of 140 fb$^{-1}$ recorded with the ATLAS experiment. Spinentanglement is detected from the measurement of a single observable $D$,inferred from the angle between the charged leptons in their parent top- andantitop-quark rest frames. The observable is measured in a narrow intervalaround the top$-$antitop quark production threshold, where the entanglementdetection is expected to be significant. It is reported in a fiducial phasespace defined with stable particles to minimize the uncertainties that stemfrom limitations of the Monte Carlo event generators and the parton showermodel in modelling top-quark pair production. The entanglement marker ismeasured to be $D=-0.547 \pm 0.002~\text{(stat.)} \pm 0.021~\text{(syst.)}$ for$340 < m_{t\bar{t}} < 380 $ GeV. The observed result is more than five standarddeviations from a scenario without entanglement and hence constitutes both thefirst observation of entanglement in a pair of quarks and the highest-energyobservation of entanglement to date.

Observation of quantum entanglement in top-quark pairs using the ATLAS detector