The BESIII Collaboration recently reported “Measurement of the decay Ξ⁰→Λ_γ with entangled Ξ⁰Ξ⁰ pairs”, which has been published in Science Bulletin [Sci.Bull. 70 (2025) 454-459].
Since the discovery of hyperons in the 1950s, their decay properties have attracted great attention. The radiative decay of hyperons involves an intricate interplay of three fundamental interactions. In 1964, Yasuo Hara proposed a theoretical explanation for hyperon radiative decays, but experimental observations deviated from his predictions, posing a major puzzle in the region of extremely low four-momentum transfer. Over the past six decades, various theoretical models have been proposed in an attempt to provide a unified explanation for different hyperon radiative decays. However, a fully satisfactory theory has yet to emerge, indicating that many mysteries remain hidden in this field.
At the same time, hyperon radiative decays serve as a sensitive probe for new physics. They offer the potential to observe significant CP-violating effects and provide constraints on hyperon Dalitz decays. Achieving these scientific objectives requires new and more precise measurements of hyperon radiative decays. Hyperon radiative decays are rare processes, with branching fractions suppressed by three orders of magnitude compared to hadronic final states, making their measurement highly challenging. Traditional fixed-target experiments can only measure the relative branching ratios of hyperon radiative decays, and the unknown polarization of hyperons further limits the precision of decay asymmetry parameters.
With a dataset of 10 billion J/ψ decays collected by BESIII experiment, in which hyperon-antihyperon pairs are quantum-entangled. Utilizing a unique double-tagging method, the experiment precisely identified the hyperon radiative decay process Ξ⁰→Λ_γ and measured its absolute branching fraction. Furthermore, by exploiting the spin entanglement and angular distribution of final-state particles in an electron-positron collider, the single-event sensitivity was improved significantly compared to fixed-target experiments. The experiment also provided a high-precision measurement of the decay asymmetry parameter and constructed a CP-violation observable. No significant CP-violating effect was observed within a precision of 10^-2. 

Figure: (a) Comparison of the BESIII measurement of the decay asymmetry parameter and branching fraction for Ξ⁰→Λ_γ with various theoretical predictions. (b) Summary of CP asymmetry measurements in weak radiative decays of mesons and baryons.

Reference: Science Bulletin 70 (2025) 454-459
Journal publication：https://doi.org/10.1016/j.scib.2024.12.019