Einstein’s ‘spooky action’ survives one of the most extreme tests yet: CERN finds strong evidence that heavy Z bosons can become quantum-entangled
Einstein famously found quantum mechanics so strange that he once referred to quantum entanglement as “spooky action at a distance”. More than a century later, one of the strangest predictions of quantum theory has now been tested in an environment far removed from the relatively low-energy systems where entanglement is usually studied.Physicists working with the ATLAS and CMS experiments at CERN’s Large Hadron Collider (LHC) have found strong evidence that two extremely short-lived, massive particles known as Z bosons can become quantum-entangled. The result provides a new demonstration that quantum correlations can survive even at the extreme energies produced in particle collisions.The finding, reported by CERN in September 2026, involves Z bosons created when a Higgs boson decays. Researchers reconstructed the properties of the short-lived particles from what they left behind in the detectors and used statistical analysis to determine whether their behaviour was consistent with quantum entanglement.
What exactly is ‘spooky action ’?
Quantum entanglement occurs when two quantum systems share a linked state, meaning measurements of one are correlated with measurements of the other in ways that cannot be explained by treating the particles as completely independent.This was one of the aspects of quantum mechanics that troubled Einstein and other physicists. They questioned whether particles might possess hidden properties that determined their behaviour before measurement, rather than nature genuinely following the unusual rules of quantum mechanics.Experiments over the decades have repeatedly supported quantum mechanics and demonstrated entanglement in different physical systems. What makes the latest CERN result notable is the extreme environment in which the effect was observed.The researchers were not watching two Z bosons sitting apart and exchanging information. Instead, they examined the quantum correlations encoded in particles produced together during a Higgs boson decay.
Why Z bosons matter
The Z boson is one of the fundamental particles responsible for carrying the weak nuclear force, one of the four fundamental forces of nature. It is also extremely unstable, decaying almost immediately after being produced.That short lifetime means scientists cannot simply capture a Z boson and directly measure it later. Instead, they reconstruct what happened by studying its decay products.In this case, the researchers focused on Higgs boson decays in which a Higgs produces two Z bosons, written as H → ZZ*. The Z bosons subsequently decay into pairs of leptons, such as electrons or muons. Those particles can be detected by ATLAS and CMS, allowing scientists to work backwards and infer properties of the original Z bosons.The process is particularly useful because the Higgs boson has zero spin. Z bosons, meanwhile, can have three possible spin states: -1, 0 and +1.When two Z bosons are produced from a Higgs decay, their combined spin must be consistent with the Higgs boson’s spin. This creates specific quantum correlations between them. Researchers can examine those correlations to determine whether the pair behaves as an entangled quantum system.
The particles disappear almost instantly
One of the most unusual aspects of the experiment is that the Z bosons themselves do not survive long enough to be directly observed. Instead, ATLAS and CMS detected the electrons and muons produced by their decay. From the directions and other properties of these particles, researchers reconstructed information about the Z bosons’ spins.They then analysed the resulting angular distributions and other observables statistically. The ATLAS measurement, published in Physical Review Letters on September 11, 2026, reported that a hypothesis assuming the two Z bosons were separable was disfavoured at a significance of 4.7 standard deviations relative to the entangled Standard Model hypothesis. The expected significance was 4.9 standard deviations.In particle physics, a result at this level is described as strong evidence, rather than the discovery standard of 5 standard deviations. The distinction matters because the measurement is based on statistical evidence and relies on several Standard Model assumptions in the decay analysis.
An extreme new setting for quantum entanglement
Quantum entanglement has previously been observed in many different systems, but most demonstrations have involved relatively low-energy particles or other quantum systems. CERN’s result pushes the phenomenon into a dramatically different regime.Z bosons are massive particles, and the collisions at the LHC occur at extraordinarily high energies. The observation therefore gives physicists another way to study quantum mechanics in conditions that are difficult to reproduce anywhere else.The ATLAS and CMS collaborations had previously investigated entanglement in other short-lived particles. Researchers had observed quantum entanglement involving top quarks, which are the heaviest known fundamental particles. The Z-boson result extends such studies to massive vector bosons and provides another opportunity to examine how quantum mechanics operates at the electroweak scale.
Could it reveal new physics?
The significance of the result goes beyond confirming that quantum mechanics continues to behave strangely.Because the Z boson is directly involved in the weak nuclear force, studying its quantum properties could provide new information about how fundamental interactions work at high energies. The Higgs boson is particularly useful in this context because its decay can create pairs of Z bosons whose quantum correlations can be reconstructed.Scientists now expect much larger datasets to make such measurements increasingly precise. CERN says the future High-Luminosity LHC, or HiLumi LHC, will dramatically increase the number of collisions available for study, potentially allowing researchers to investigate quantum entanglement and other quantum phenomena at even higher energies and with greater statistical precision.For Einstein, entanglement was one of the features that made quantum mechanics appear deeply counterintuitive. At CERN, scientists are now studying that same strange behaviour not with thought experiments, but through the fleeting traces left by some of the most energetic particle collisions humans can create.The Z bosons exist for only an instant. Yet in that tiny window, their quantum states can retain correlations that offer physicists another glimpse into the rules governing the subatomic world.
