BESIII team uses quantum entanglement to measure the quark-mixing parameter |Vus| from Λ beta decay

A team of particle physicists has used quantum entanglement to extract a fundamental parameter of the Standard Model. Researchers led by Fudan University professor Luo Tao, working within the BESIII collaboration at the Beijing Electron–Positron Collider II in China, reported the first collider-based determination of |Vus|, the Cabibbo–Kobayashi–Maskawa (CKM) matrix element describing the transition of strange to up quarks.

The measurement, published on 2 September 2026 in Nature, focuses on the semileptonic decay of the Λ hyperon into a proton, an electron and an electron antineutrino. Instead of relying on a single decaying hyperon, the method exploits Λ–antiΛ pairs produced at the J/ψ resonance, whose spins are quantum entangled. By analyzing the polarization and joint angular distribution of both sides of the decay, the team says it recovered information from the unobservable antineutrino and from the partner antiΛ that would otherwise be lost.

The study reports measurements of the axial-vector and weak-magnetism couplings and first determinations of the absolute branching fraction and weak-electricity coupling for this decay. Combining the results with recent lattice quantum chromodynamics calculations gives |Vus| = 0.2339 ± 0.0041, which the authors describe as a model-independent determination consistent with CKM unitarity. The authors also say they extracted the product of |Vus| and a dominant form-factor combination for the first time for any semileptonic baryon decay.

The statistical samples were small but reportedly competitive. Chinese news coverage says the team screened 10 billion J/ψ decay events and retained only 1,854 signal events, roughly one-twentieth the sample used in the previous most precise measurement by a Fermilab fixed-target experiment more than 30 years ago. Luo's team said the key parameters were nonetheless measured with precision comparable to the Fermilab result because the entangled two-sided analysis boosted single-event sensitivity.

The work addresses a long-standing gap. Scientists have sought more independent measurements of |Vus| because determinations from kaon and tau decays show tensions that may hint at physics beyond the Standard Model. The authors argue that exploiting polarization and entanglement in baryon semileptonic decays provides a foundation for a systematic programme that could reach precision comparable to kaon decays and deliver an independent test of the Standard Model.

The attribution and framing rely mainly on the Nature paper itself. Independent Chinese science media, including ScienceNet citing China Science Daily and a Shanghai outlet quoting Luo and BESIII spokesperson Li Haibo, provided additional detail on the six-and-a-half-year effort and the role of Fudan University, but these accounts appear to draw on the same research group and are not independent verification of the physics results.