Something spooky has been detected at CERN’s Large Hadron Collider. In the cascade of debris from high-energy collisions at the world’s most powerful particle collider, physicists have found compelling evidence of a phenomenon Albert Einstein called “spooky action at a distance.” It’s not the first time quantum entanglement has been detected at the LHC, but this time, it’s in a new place. Now, scientists have found the first solid evidence of entanglement between a pair of Z bosons produced in the decay of a Higgs boson: their spins are so inextricably linked that one cannot be fully described without the other. “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 of Spain’s Institute of Theoretical Physics, translated from a video in Spanish. frameborder=”0″ enable=”accelerometer; autoplay; writing on clipboard; encrypted media; gyroscope; picture in picture; web-share” referrerpolicy=”strict-origin-when-cross-origin” enablefullscreen> Quantum entanglement is one of the strangest things two particles can do. It occurs when their properties become so closely linked that they can no longer be described independently of each other, even when they are separated by large distances. 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 drove the Einstein’s “spooky” description. Scientists have been producing and measuring entangled particles like atoms and photons for decades, and more recently hope to exploit them for quantum communications and quantum computing. In 2024, the ATLAS collaboration reported the first observation of quantum entanglement between pairs of top quarks produced at the LHC. Z bosons do things a little differently. “And that makes another important difference, because this is the first time that entanglement has been measured with elementary particles that are qutrits,” Aguilar-Saavedra explained referrerpolicy=”strict-origin-when-cross-origin” enablefullscreen> The Higgs boson, created by colliding protons at high speeds, is a particularly useful path to an entangled pair of Z bosons. has its own spin, the two Z bosons cannot simply emerge with any old combination of spins. Their spins have to fit together in a way that preserves the zero spin of the particle they come from, and quantum mechanics allows for those possibilities to exist simultaneously, as a shared state between the two Z bosons. It is this shared state that can leave them entangled. 125 GeV, while a Z boson weighs about 91 GeV. They emerge fleetingly during collider interactions, but they cannot be observed as free particles as their “real” counterparts do. Z bosons of the pair is virtual, can it still participate in something as fundamentally quantum as entanglement? The new result suggests that it can. Of course, there is another fly in the ointment of the collider: a Z boson lasts only about 3 x 10-25 seconds before decaying, each Z produces. two charged particles called leptons: either electrons or their heavier cousins, muons. That gives physicists four particles whose paths through the detector they can measure. And that’s the critical point, because the directions those four particles travel contain information about the spins of the Z bosons that produced them, a process that has a huge gulf of difficulty between “said” and “fact.” Higgs at two Zs in four leptons is incredibly rare. Even with years of data from particle collisions, the researchers only had about 400 events to work with. But with the reconstructed spins of their Z bosons, the researchers looked for the signature of entanglement, a pattern that can be identified in the ATLAS data. that the data favored the entangled state over the non-entangled alternative with a statistical significance of 4.7 sigma—tantalizingly close to the 5-sigma threshold that particle physicists traditionally demand before declaring a discovery. While it falls short of that bar, the result constitutes strong evidence that the two Z bosons were indeed entangled. But the experiment also raises another, more intriguing question about virtual Z itself. “exists.” Unlike ordinary particles, virtual particles cannot be directly observed; some physicists consider them primarily mathematical ingredients used to describe interactions. Related: The Large Hadron Collider may have already detected hints of dark matter. However, here, a virtual Z appears to behave in a surprisingly particle-like way: It carries spin information that can be entangled with another Z. “One may wonder: do virtual particles exist, or are they a construct of our mind that we use to calculate things?” Aguilar Saavedra cautioned that the result does not resolve that philosophical question, but said it could “shed some light on whether virtual particles are particles or not.” “If it walks like a duck and quacks like a duck, then what we are seeing must be a duck,” he said. an error, please let us know.
