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

Entanglement is a key property of quantum systems. In this Letter the first measurements of quantum entanglement between spins in pairs of Z 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 Z Z * spin-density-matrix elements in the H → Z Z * → ℓ + ℓ − ℓ + ℓ − process yield coefficients C 2 , 1 , 2 , − 1 = − 0.71 ± 0.45 and C 2 , 2 , 2 , − 2 = 0.08 ± 0.44 , 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 4.9 σ ) relative to the entangled Standard Model hypothesis. These results provide strong evidence of quantum entanglement between massive bosons (spin qutrits) at the electroweak scale.