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Measurements of Z-Boson Pair Entanglement in Decays of Higgs Bosons at the ATLAS Experiment

et al.
Creative Commons 'BY' version 4.0 license
Abstract

Entanglement is a key property of quantum systems. In this Letter the first measurements of quantum entanglement between spins in pairs of bosons are reported, using proton-proton collision data from the Large Hadron Collider (LHC) at center-of-mass energies of 13 and 13.6 TeV, recorded with the ATLAS detector. Measurements of angular observables sensitive to spin-density-matrix elements in the process yield coefficients and , consistent with their Standard Model predictions. A complementary hypothesis test using the full angular distribution, and relying on several Standard Model assumptions in the decays, provides substantially higher sensitivity to quantum correlations and disfavors the separable-state hypothesis at a significance of 4.7 standard deviations (expected ) relative to the entangled Standard Model hypothesis. These results provide strong evidence of quantum entanglement between massive bosons (spin qutrits) at the electroweak scale.

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