This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Physicists at the BESIII Collaboration have breathed new life into a decades-old test of one of the Standard Model's most important ideas, using a technique that had gone almost untouched by experimenters for 35 years. In new research published in Nature, the researchers examined a small table of numbers called the CKM matrix, which governs how one type of fundamental particle can transform into another.
By tracking pairs of unstable particles called lambda hyperons, linked through quantum entanglement, the team produced a fresh, independent measurement that lines up with what the Standard Model predicts. To date, the Standard Model is physicists' best working theory of the particles that make up matter and the forces that govern them. All hadronic matter is made up of quarks, which come in six types, or "flavors": up, down, strange, charm, top and bottom.
According to the Standard Model, quarks can switch flavor, with the CKM matrix providing a rulebook of probabilities for how likely each switch will occur. A core prediction of the theory is that these probabilities must sum to exactly one. If they don't, something must be missing from the theory.
One entry in the rulebook, named VUS, describes how likely a strange quark is to turn into an up quark. For years, the best measurements of VUS have come from particles called kaons, and they have shown a small but persistent mismatch with theory. Because those measurements all rely on similar assumptions, an independent cross-check has long been needed.
One such test can be made using lambda hyperons, heavier relatives of the neutron. When one of these particles decays, a strange quark inside it converts into an up quark, and occasionally the hyperon spits out an electron alongside an undetectable particle called a neutrino. This process directly reflects the size of VUS, but previous experiments haven't been able to pin down enough detail about the decay to extract a precise value.
To address the challenge, the BESIII Collaboration produced hyperons in pairs linked by quantum entanglement, so that measuring one twin reveals hidden information about the other. Colliding electrons with their antimatter counterparts occasionally created a particle that decayed into a lambda hyperon and its antimatter partner, entangled and spinning in step. By studying the decay of one particle, the team could reconstruct otherwise invisible details of the other, sifting through 10 billion collision events in the process.
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