ATLAS detects entanglement in Higgs decay involving virtual Z boson
The ATLAS Collaboration analyzed combined Run 2 and Run 3 LHC data and extracted spin-density matrix elements from roughly 400 Higgs→ZZ*→4ℓ decays, finding that the two Z bosons are quantumly entangled as qutrits. The most sensitive likelihood-ratio test disfavors a separable state at 4.7 sigma, providing strong evidence rather than a formal discovery. Crucially, at least one Z is off-shell, so the measurement operationally links a virtual particle to entanglement, complicating the ontology of virtual particles. The High-Luminosity LHC upgrade and Run 4 beginning in 2030 should supply the data needed to reach discovery and map the full 9×9 matrix.
ATLAS measured qutrit entanglement in Higgs decays from about 400 events.
Context
Scientists used ATLAS data from LHC Run 2 and Run 3 to study Higgs decays. They identified about 400 usable H→ZZ*→4ℓ events and found entanglement evidence at 4.7 sigma. The LHC is being upgraded to the High-Luminosity LHC and will resume…
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