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 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 <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>Λ</mi> <mo>→</mo> <msup> <mrow><mrow><mi>pe</mi></mrow> </mrow> <mrow><mo>-</mo></mrow> </msup> <msub> <mrow> <mrow><mover><mi>ν</mi> <mo>¯</mo></mover> </mrow> </mrow> <mrow><mrow><mi>e</mi></mrow> </mrow> </msub> </math> , achieved by exploiting the polarization and quantum entanglement of <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>Λ</mi> <mrow><mover><mi>Λ</mi> <mo>¯</mo></mover> </mrow> </math> 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.