Scientists spot spooky quantum entanglement in Z bosons at the Large Hadron Collider

Scientists at the Large Hadron Collider (LHC) have spotted spooky quantum entanglement in Z bosons, particles produced from a Higgs boson, for the first time. This isn’t the first time quantum entanglement has been observed in particles generated at the LHC, but it is surely in a different set of particles, and one that also has a virtual counterpart. Quantum entanglement is a phenomenon observed in quantum physics that even stumped the great Albert Einstein. Two particles are said to be quantum entangled when their properties are so closely linked that the state of one is dependent on the state of the other. This holds even when the two particles are separated by a large distance from each other, prompting Einstein to call this behavior ‘spooky’. Even though we do not completely understand this, we have been able to leverage it to build technologies like quantum computing and communications. These might still be in their infancy but promise a brand new tomorrow that will be drastically different from the present. How little we know about this behavior is evident in the fact that scientists at the CERN are still making new discoveries about them. The ATLAS experiment at CERN first detected quantum entanglement in a pair of top quarks produced by the LHC in 2024. The recent discovery of quantum entanglement in Z bosons is a bit different because, unlike top quarks, Z bosons have three spin states, also known as qutrits. Z bosons are produced briefly at the LHC when a Higgs boson splits in its journey of further decay. However, since a Higgs boson does not have a spin of its own, the two Z bosons formed during its decay cannot have any odd combinations of spins. Their spins cancel each other out, resulting in the zero spin for the Higgs boson. It is likely that the two Z bosons have a shared state that makes this possible. However, there is a major issue in their production. A Higgs boson has a mass of 125 GeV, while Z bosons weigh 91 GeV. This makes it impossible for a Higgs boson to produce two Z bosons at once, which leaves only one option: one of the Z bosons is virtual. Quantum entanglement gets spookier Even as quantum physicists regularly deal with spooky particle behavior, virtual particles are a strange phenomenon for them to deal with as well. These particles emerge during particle collisions but can’t be observed like other particles. So, the question was whether they can actually participate in entanglement. Confirming this scientifically was even tougher, since Z bosons themselves last for 3 x 10-25 seconds. So, even the real boson is barely available to measure its spin. So how could scientists confirm the spin of its virtual counterpart? The researchers turned to clues available from the decay of Z bosons. The decay of each Z boson produces two charged particles, either leptons or muons, giving potentially four particles that can be detected. The direction of travel of these particles gives scientists information they can use to work backward and determine the spins of the bosons that created them. While this sounds simple, the research team at CERN had a hard time finding sufficient data to study it. Even after sifting through years of particle collision data, they could only find 400 events where Z bosons split into four leptons. The data showed a preference for an entangled state over a non-entangled state with a sigma, or statistical significance, of 4.7, close to the number five that scientists prefer to achieve before making a major claim. There is more work to be done in this area and much to be discovered about the virtual particles and how they behave. The results were published in Physical Review Letters.Get the latest in engineering, tech, space & science - delivered daily to your inbox.Ameya is a science writer based in Hyderabad, India. A Molecular Biologist at heart, he traded the micropipette to write about science during the pandemic and does not want to go back. He likes to write about genetics, microbes, technology, and public policy.