Two massive Z bosons produced in the decay of a Higgs boson at the Large Hadron Collider turned out to be quantum-entangled — and this was measured directly for the first time at the electroweak energy scale.
The ATLAS collaboration at the LHC at CERN (Geneva, Switzerland) analyzed proton collision data and isolated events in which the Higgs decays into a pair of Z bosons. Each Z boson then decays into a pair of leptons or quarks, leaving characteristic traces in the detector. The scientists reconstructed the spins and momenta of the particles and then applied entanglement criteria to their correlations. The results were published in Physical Review Letters in September 2026.
Entanglement here means that the properties of one Z boson are instantly linked to the properties of the second, regardless of the distance between them at the moment of measurement. Imagine two coins tossed simultaneously: while they are in the air, each can land heads or tails, but as soon as one lands heads, the other will necessarily turn up tails — and this is not a coincidence, but a rigid connection established at the very moment the pair was created.
Ordinary intuition suggests that massive particles such as Z bosons quickly lose their quantum properties due to interaction with the environment. However, in Higgs decays, entanglement persists long enough for it to be registered. This opens a new regime for tests of fundamental interactions: it is now possible to test quantum mechanics not only on photons or electrons, but also on heavy vector bosons.
The measurement confirms that the Standard Model works in the quantum regime even for massive gauge bosons. In the future, such data will help refine the boundaries of the applicability of quantum field theory and may influence the search for new physical phenomena beyond the Standard Model.
The results of the ATLAS collaboration, published in Physical Review Letters, show that quantum entanglement can be studied in the most unexpected corners of high-energy physics.
