ATLAS reports evidence that a real Z boson and a virtual Z boson produced in a Higgs-boson decay are quantum-entangled. The reported significance is 4.7σ, strong evidence below the 5σ convention traditionally used to call a particle-physics result an observation.
Quantum entanglement means the particles’ joint quantum state cannot be described as two independent states. Their measurement results can be correlated, but those correlations cannot be used to send a controllable message faster than light.
How ATLAS reconstructs the Z bosons
ATLAS infers the bosons’ properties from the particles produced when they decay. In the events studied, each Z decays into an electron pair or a muon pair. The directions of the four resulting leptons help researchers reconstruct the bosons’ properties, including the virtual Z’s spin. Neither Z boson reaches the detector intact; a real Z has a lifetime of about 3 × 10⁻²⁵ seconds.
The virtual Z’s role follows from the decay’s energy. A Higgs boson has a mass of about 125 GeV, while two real Z bosons, at about 91 GeV each, would total about 182 GeV. The reported decay therefore involves one real Z and one virtual Z with a lower effective mass. Its properties are inferred from the decay products rather than detected directly.
Study coauthor Juan Antonio Aguilar-Saavedra says the four-lepton channel occurs about once in every 10,000 Higgs events:
What the reported 4.7σ means
The 4.7σ figure describes the reported significance for rejecting the hypothesis that the Z bosons are not entangled. The traditional 5σ convention for calling a particle-physics result an observation puts the finding below that threshold. The result is strong evidence, not an observation at 5σ.
Why a Z boson is called a qutrit
A Z boson has spin 1 and three possible spin states. In quantum-information terminology, a system with three possible states is a qutrit; a two-state system is a qubit. The ATLAS result is also reported as the first evidence of entanglement between qutrits formed by elementary particles.
The analysis infers the virtual Z’s spin from its decay products. Whether virtual particles exist as independent entities remains a separate philosophical question.
