Exploring baryon semileptonic decays through polarization and entanglement
Abstract
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.
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Authors: 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., C. H. Chen, Chao Chen, G. Chen, H. S. Chen, Hanxi Chen, M. L. Chen, S. L. Chen, T. Chen, X. R. Chen, X. T. Chen, Y. B. Chen, Y. Q. Chen, Z. Chen, Z. J. Chen, S. K. Choi, X. Chu, G. Cibinetto, F. Cossio, J. Cottee-Meldrum, J. J. Cui, H. L. Dai, J. P. Dai, A. Dbeyssi, R. E. de Boer, D. Dedovich, C. Q. Deng, Z. Y. Deng, A. Denig, I. Denysenko, M. Destefanis, F. MORI, B. Ding, X. X. Ding, Y. X. Ding, J. Dong, L. Y. Dong, M. Y. Dong, X. Dong, M. C. Du, S. X. Du, Y. Y. Duan, Z. H. Duan, P. Egorov, G. F. Fan, J. J. Fan, Y. H. Fan, J. Fang, S. S. Fang, W. X. Fang, Y. Q. Fang, L. Fava, F. Feldbauer, G. Felici, C. Q. Feng, J. H. Feng, L. Feng, Q. X. Feng
Institutions: Peking University, Fudan University, Sun Yat-sen University, University of Bristol, University of Jinan, University of Chinese Academy of Sciences, Moscow Institute of Physics and Technology, Istinye University, Chung-Ang University, Soochow University, Siberian Branch of the Russian Academy of Sciences, Wuhan University, Lanzhou University, University of Münster, Tsinghua University, Uppsala University, Nanjing University, Joint Institute for Nuclear Research, Henan University, Zhengzhou University, Liaoning University, Johannes Gutenberg University Mainz, China University of Geosciences, University of South China, University of Science and Technology of China, Jilin University, Hunan Normal University, University of Turin, Ruhr University Bochum, Inner Mongolia University, Università degli Studi del Piemonte Orientale “Amedeo Avogadro”, Southeast University, Hunan University, Nankai University, Yunnan University, Shandong University, Mongolian Academy of Sciences, Istituto Nazionale di Fisica Nucleare, Sezione di Ferrara, North China Electric Power University, Carnegie Mellon University, Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali di Frascati, Institute of High Energy Physics, Henan Normal University, Guangxi University of Science and Technology, Helmholtz Institute Mainz, Institute of Modern Physics, Novosibirsk State University, National Centre for Nuclear Research, Budker Institute of Nuclear Physics, Institute of Physics and Technology