Exploring baryon semileptonic decays through polarization and entanglement

Abstract 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.