In the depths of the Large Hadron Collider, pairs of heavy Z bosons are born, their spins inextricably linked. Scientists of the ATLAS collaboration have for the first time measured this quantum entanglement directly, using Higgs boson decays.
The experiment relies on the process H → ZZ* → four leptons. The two Z bosons, each with spin 1, behave as qutrits — three-level quantum systems. Their polarizations are correlated in such a way that measuring one instantly determines the state of the other, even if the particles fly off in different directions within the detector.
An analysis of the angular distributions of the decay products made it possible to extract the coefficients of the spin density matrix. The resulting values C_{2,1,2,−1} = −0,71 ± 0,45 and C_{2,2,2,−2} = 0,08 ± 0,44 agree with the predictions of the Standard Model. An additional test based on the full angular distribution rejected the hypothesis of a separable state with a significance of 4,7 standard deviations.
Entanglement had previously been observed in photons, electrons and quarks. Now it has been confirmed for massive vector bosons at energies of the electroweak interaction — on the order of 100 GeV. This is a fundamentally different regime: here the weak forces with their chiral structure are at work.
The result opens the way to studying quantum correlations in other Higgs decay channels and in future LHC runs. Such measurements could shed light on how quantum mechanics manifests itself in fundamental interactions at high energies.
The discovery shows that entanglement is not an exotic quirk of the microworld, but an inherent property of particles born in humanity's most powerful accelerators.
