Exploring baryon semileptonic decays through polarization and entanglement
- M. Ablikim
- M. N. Achasov
- P. Adlarson
- X. C. Ai
- R. Aliberti
- A. Amoroso
- Q. An
- Y. Bai
- O. Bakina
- Y. Ban
- H.-R. Bao
- V. Batozskaya
- K. Begzsuren
- N. Berger
- M. Berlowski
- M. Bertani
- D. Bettoni
- F. Bianchi
- E. Bianco
- A. Bortone
- I. Boyko
- R. A. Briere
- A. Brueggemann
- H. Cai
- M. H. Cai
- X. Cai
- A. Calcaterra
- G. F. Cao
- N. Cao
- S. A. Cetin
- X. Y. Chai
- J. F. Chang
- G. R. Che
- Y. Z. Che
- C. H. Chen
- Chao Chen
- G. Chen
- H. S. Chen
- H. Y. Chen
- M. L. Chen
- S.-J. Chen
- S. L. Chen
- S. M. Chen
- T. Chen
- X. R. Chen
- X. T. Chen
- X. Y. Chen
- Y. B. Chen
- Y. Q. Chen
- Z. Chen
2026-09-02
The unitarity of the Cabibbo–Kobayashi–Maskawa (CKM) matrix is a cornerstone of the standard model (SM). Precise tests of this unitarity require independent determinations of its elements, such as | V us |, which governs the transition between strange and up quarks. Current measurements from kaon and tau decays show tensions that may hint at physics beyond the SM 1–3 . Hyperon semileptonic decays provide alternative probes but have remained largely untapped because previous experiments lacked sufficient kinematic information, making the measurements insensitive to the relevant form factors 4 . Here we report measurements of the axial-vector and weak-magnetism couplings, as well as the first determinations of the absolute branching fraction and weak-electricity coupling in $$\Lambda \to {{\rm{pe}}}^{-}{\bar{\nu }}_{{\rm{e}}}$$ Λ → pe − ν ¯ e , achieved by exploiting the polarization and quantum entanglement of $$\Lambda \bar{\Lambda }$$ Λ Λ ¯ pairs produced at the J/ψ resonance. Combining our results with recent lattice quantum chromodynamics (QCD) calculations 5 gives | V us | LQCD = 0.2339 ± 0.0041, a model-independent determination consistent with CKM unitarity. By pioneering the exploitation of polarization and quantum entanglement in baryon semileptonic decays, our method enhances single-event sensitivity and is broadly applicable to other baryon semileptonic decays, establishing the foundation for a systematic research programme that can achieve precision comparable with that of kaon decays and provide a stringent independent test of the SM.