Now, scientists have found the first strong evidence of entanglement between a pair of Z bosons<\/a> produced in the decay of a Higgs boson<\/a> \u2013 their spins so inextricably linked that one cannot be fully described without the other.<\/p>\n “The spins of the two Z bosons are extremely entangled, considerably more so than in the top-antitop case that was measured previously,” said physicist Juan Antonio Aguilar-Saavedra<\/a> of the Institute of Theoretical Physics in Spain, translated from a Spanish-language video. <\/p>\n Quantum entanglement<\/a> is one of the strangest things two particles can do.<\/p>\n It happens when their properties become so closely linked that they can no longer be described independently of one another, even when separated by vast distances.<\/p>\n When you measure something about one of the particles, you instantly know something about the other. It was this deeply counterintuitive feature of quantum mechanics that prompted Einstein’s “spooky” description.<\/p>\n Scientists have been producing and measuring entangled particles such as atoms and photons for decades, more recently hoping to exploit them for quantum communications and quantum computing<\/a>.\u00a0<\/p>\n In 2024, the ATLAS Collaboration reported the first observation of quantum entanglement between pairs of top quarks produced at the LHC.\u00a0<\/p>\n Z bosons do things a little differently.<\/p>\n Unlike top quarks, which have two possible spin states, Z bosons have three \u2013 a distinction that, in the language of quantum information, makes a Z boson<\/a> a qutrit, rather than a two-state qubit.<\/p>\n “And that makes another important difference, because this is the first time entanglement has been measured with elementary particles that are qutrits,” Aguilar-Saavedra explained<\/a>. <\/p>\n The Higgs boson \u2013 created by smashing protons together at high speeds \u2013 is a particularly useful pathway to a pair of entangled Z bosons.<\/p>\n Because the Higgs has no spin of its own, the two Z bosons can’t simply emerge with any old combination of spins. Their spins have to fit together in ways that preserve the zero spin of the particle they came from.<\/p>\n There are several ways this can happen \u2013 and quantum mechanics allows those possibilities to exist simultaneously, as a shared state between the two Z bosons. It is this shared state that can leave them entangled.<\/p>\n There’s a catch, though. A Higgs boson has a mass of around 125 GeV, while a Z boson weighs in at around 91 GeV. There simply isn’t enough energy available for the Higgs to produce two ordinary Z bosons at once.<\/p>\n So at least one of them has to be virtual.<\/p>\n Virtual particles are strange even by quantum-physics standards. They emerge fleetingly during collider interactions, but can’t be observed as free particles in the way their “real” counterparts can.<\/p>\n That raises an intriguing question: if one of the Z bosons in the pair is virtual, can it still participate in something as fundamentally quantum as entanglement? The new result suggests that it can.<\/p>\n Of course, there’s another fly in the collider ointment \u2013 a Z boson lasts for only about 3 x 10-25 seconds before it decays. Not even the “real” Z hangs around long enough for its spin to be measured directly.<\/p>\n But the Z bosons leave clues behind.<\/p>\n In the decays the collaboration studied, each Z produces two charged particles called leptons – either electrons or their heavier cousins, muons<\/a>. That gives physicists four particles whose paths through the detector they can measure.<\/p>\n And that’s the critical point \u2013 because the directions those four particles travel contain information about the spins of the Z bosons that produced them.<\/p>\n The researchers could work backward from those directions, like rewinding a tape, to reconstruct the spins of the Z boson decays that birthed them \u2013 a process that has a vast difficulty gulf between “said” and “done”.<\/p>\n That’s partially because the particular chain of decays \u2013 a Higgs into two Zs into four leptons \u2013 is incredibly rare. Even with years of particle-collision data, the researchers only had about 400 events to work from.<\/p>\n But they were enough.<\/p>\n With the reconstructed spins of their Z bosons, the researchers looked for the signature of entanglement \u2013 a pattern that can be identified in the ATLAS data.<\/p>\n The result strongly suggested entanglement.<\/p>\n In their most sensitive test, the team found that the data favored the entangled state over the non-entangled alternative with a statistical significance of 4.7 sigma \u2013 tantalizingly close to the 5-sigma threshold particle physicists traditionally demand before declaring a discovery.<\/p>\n While not quite meeting that bar, the result does constitute strong evidence that the two Z bosons really were entangled.<\/p>\n But the experiment also raises another, more intriguing question about the virtual Z itself.<\/p>\n Physicists don’t entirely agree on what it means to say a virtual particle “exists”. Unlike ordinary particles, virtual particles can’t be directly observed; some physicists regard them primarily as mathematical ingredients used to describe interactions.<\/p>\n Related: The Large Hadron Collider May Already Have Detected Hints of Dark Matter<\/a><\/p>\n Yet here, a virtual Z appears to behave in a strikingly particle-like way \u2013 it carries spin information that can become entangled with another Z.<\/p>\n “One can ask: do virtual particles exist, or are they a construct of our minds that we use to calculate things?” Aguilar-Saavedra said<\/a>. <\/p>\n He cautioned that the result doesn’t settle that philosophical question, but said it could “shed a little light on whether virtual particles are particles or not.”<\/p>\n “If it walks like a duck and quacks like a duck, then what we’re looking at must be a duck,” he said<\/a>. <\/p>\n “In this case, a virtual duck.”<\/p>\n The results have been published in Physical Review Letters<\/a>.<\/p>\n This article was fact-checked by Rebecca Dyer<\/a> and edited by Rebecca Dyer<\/a>. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know<\/a>.<\/p>\n","protected":false},"excerpt":{"rendered":"Something spooky has been spotted in CERN’s Large Hadron Collider. 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