Exploring baryon semileptonic decays through polarization and entanglement

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, 2–3}. 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 factors⁴. 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 Λ→pe−ν¯e, achieved by exploiting the polarization and quantum entanglement of ΛΛ¯ pairs produced at the J/ψ resonance. Combining our results with recent lattice quantum chromodynamics (QCD) calculations⁵ 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.