The BESIII Collaboration has reported the most precise determination to date of the strong and weak phase differences in Ξ⁻ hyperon decays, along with new constraints on CP symmetry violation. The result has been published in Physical Review Letters [Phys. Rev. Lett. 136, 201802 (2026)].
Figure 1 Illustration of the Ξ⁻→Λπ⁻ with Λ→pπ⁻ process in the Ξ⁻ rest frame.
One of the main unresolved puzzles in fundamental physics is explaining the vast abundance of matter over antimatter in the universe. Sakharov's theory of baryogenesis requires processes that violate the combined charge conjugation and parity (CP) symmetry. While CP violation has been firmly established in meson decays, its observation in baryon decays remains rare, with the LHCb collaboration only recently reporting CP violation in a beauty baryon decay. Baryons, possessing an additional spin degree of freedom, offer a complementary avenue to test the Standard Model (SM) and search for new sources of CP violation.
Using a dataset of (10,087 ± 44) × 10⁶ J/ψ events collected with the BESIII detector at the BEPCII collider, the collaboration performed a search for CP violation with an entangled system of Ξ⁻Ξ̅⁺ pairs. A nine-dimensional helicity amplitude analysis was employed to fit the full decay chain e⁺e⁻→J/ψ→Ξ⁻Ξ̅⁺→Λπ⁻Λ̅π⁺→pπ⁻π⁻p̅π⁺π⁺. The strong phase difference, a measure of final-state interactions, is determined to be (δ_P - δ_S)=(0.3±1.2±0.2)×10⁻² rad, and the weak phase difference, which quantifies the CP-violating contribution, is (ξ_P - ξ_S)=(-0.2±1.2±0.1) ×10⁻² rad. Both results are consistent with zero and represent the most precise measurements achieved so far. The precision on the weak phase difference is now at the level of 10⁻² rad, still about two orders of magnitude above the SM expectation of -(2.1 ± 1.7) × 10⁻⁴ rad.
Figure 2 Strong and weak phase differences in hyperon processes.
The extracted decay parameters yield the CP asymmetry observables ACPΞ = (-7.8 ± 4.8 ± 0.8) × 10⁻³ and ΔϕCPΞ = (0.6 ± 5.1 ± 0.2) × 10⁻³ rad, both consistent with CP conservation. In addition, independent measurements of the Λ decay parameter yield a CP asymmetry ACPΛ = (-2.9 ± 4.3 ± 0.7) × 10⁻³, the most precise determination to date.
The results significantly improve upon previous BESIII measurements and challenge some theoretical predictions for the strong phase difference. While no evidence for CP violation is found, the improved precision places tighter constraints on beyond-Standard-Model contributions to hyperon decays. Probing non-zero phase differences will require substantially larger data samples, a goal that future facilities such as the next-generation super tau-charm factory aim to achieve.
