The BESIII Collaboration has reported the most precise test to date of CP symmetry in the neutral decays of the Λ hyperon. The result has been published in Physical Review Letters [Phys. Rev. Lett. 136, 251802 (2026)].

Charge-parity (CP) violation is essential for understanding why the Universe is dominated by matter rather than antimatter. It has been observed in the K, B, and D meson systems and in the Λb baryon system, with all results consistent with Standard Model predictions. However, the known CP-violating effects are still far too small to explain the matter-antimatter asymmetry, motivating searches for new sources of CP violation in baryon decays.

Hyperon decays provide a sensitive laboratory for such studies. In two-body weak decays of spin-1/2 hyperons, CP symmetry can be tested by comparing the decay asymmetry parameters of hyperons and antihyperons. Improved measurements of Λ decay parameters also help clarify the long-standing S- and P-wave puzzle and the role of the ΔI = 3/2 transition in hyperon decays.

Using a data sample of (10087±44)×10⁶ J/ψ events collected with the BESIII, the collaboration performed a full angular analysis of the process J/ψ→ΛΛbar→nπ⁰+pπ⁺+c.c. The decay parameters for Λ→nπ⁰ and Λbar→nπ⁰ were determined to be 0.668±0.007±0.002 and -0.677±0.007±0.003, improving previous precisions by 43% and 27%, respectively. The CP asymmetry in the neutral Λ decay was measured to be -0.006±0.007±0.002, which is consistent with CP conservation and represents the most precise test of CP symmetry in Λ neutral decays to date, improving the previous precision by 29%. In addition, BESIII measured the ratios 0.884±0.013±0.006 and 0.885±0.013±0.004, which are smaller than unity by more than 5σ, confirming ΔI = 3/2 transition contributions in Λ and Λbar decays. These precise measurements provide important input for theoretical calculations of isospin amplitudes and CP asymmetry in hyperon decays.