On September 2, Nature published a paper titled “Exploring baryon semi-leptonic decays through polarization and entanglement,” reporting a major breakthrough in the study of baryon beta decay achieved by the BESIII (Beijing Spectrometer III) international collaboration, based at the BEPCII (Beijing Electron-Positron Collider). Pan Xiang, Associate Research Fellow from the School of Physics, SEU, engaged in this research as one of the principal contributors to the paper.
The Standard Model of particle physics is the theoretical framework that describes elementary particles and the strong, weak, and electromagnetic interactions among them. Searching for experimental phenomena beyond the predictions of the Standard Model is one of the most frontier topics in physics today. In the Standard Model, the CKM matrix describes the transition probabilities of quarks changing their flavors via weak interactions and satisfies unitarity. However, current experiments on K mesons and tau leptons have revealed discrepancies in the measurement of the CKM matrix element |Vus|, which are inconsistent with unitarity, calling urgently for new independent cross-checks.
Hyperons refer to a class of baryons containing strange quarks. Their beta decay Λ → p e⁻ ν̅ₑ provides an ideal window for precisely measuring |Vus|. Fig. 1 shows a schematic diagram of the Λ → pe⁻ν̅ₑ decay. However, for a long time, this process has been limited by the insufficient capability of fixed-target experiments to extract decay dynamic information, leaving its physical potential largely untapped. As a result, no new experimental progress has been made in this decay channel for more than three decades.
The BESIII experiment is a large international collaboration involving nearly 700 scientists from 15 countries, focusing on precision physics in the tau-charm energy region. It has achieved a series of world-leading results in hadron spectroscopy, charm physics, tau physics, and the search for new physics. SEU officially joined the collaboration in 2015.
In the BESIII experiment, electron-positron collisions annihilate to produce J/ψ particles, which then decay into a pair of quantum-entangled Λ hyperons and anti-Λ hyperons with strictly correlated spins. Leveraging this unique advantage, the research team systematically analyzed 10 billion decay events and precisely measured the absolute branching fraction, dynamic parameters, and the CKM matrix element |Vus| of the Λ → pe⁻ν̅ₑ decay. Thanks to the use of polarization and quantum entanglement information, as well as a unique joint angular distribution multivariate fitting method that enhances detection sensitivity and information utilization, this study achieved the same measurement precision as the Fermilab experiment conducted 30 years ago, but with only one-twentieth of the data volume. Moreover, it realized the first measurement of the weak-electromagnetic coupling parameters. Fig. 2 shows the |Vus| result independently measured in this work, which is consistent with the CKM unitarity prediction. This method has now been extended to decay measurements of other baryons at BESIII and is expected to provide key analytical tools for the PANDA experiment at FAIR and the next-generation Super Tau-Charm Facility.
This work was completed in collaboration by Wang Shun (de facto the first author), Distinguished Associate Professor at the School of Mathematics and Physics, Southwest University of Science and Technology; Luo Tao (de facto THE corresponding author), Professor at the Institute of Modern Physics, Fudan University; Pan Xiang, Associate Research Fellow at the School of Physics, Southeast University; Varvara Batozoskaya (Institute of High Energy Physics, Chinese Academy of Sciences & National Centre for Nuclear Research, Poland); Andrzej Kupsc (Uppsala University, Sweden & National Centre for Nuclear Research, Poland); and Patrik Adlarson (Uppsala University, Sweden). Among them, Pan Xiang made significant contributions to the measurement of the absolute branching fraction of the Λ → pe⁻ν̅ₑ decay and is one of the principal contributors to the paper. In accordance with the internationally adopted authorship rules for large experimental collaborations in high-energy physics, the authors are listed in alphabetical order.
Paper's link: https://www.nature.com/articles/s41586-026-10818-8
Source: School of Physics, SEU
Translated by: Melody Zhang
Proofread by: Dai Li
Edied by: Leah Li
