BESIII Collaboration has performed a precise measurement of CP symmetry in the decay of Σ+ hyperon to neutron. This research has been published in Physics Review Letters on Nov. 9th, 2023 [Phys. Rev. Lett. 131, 191802 (2023)].
Charge-parity (CP) violation is one of Sakharov's three essential conditions for understanding the matter-antimatter asymmetry in the Universe. While the CP violation has been observed in the decays of K, B, and D mesons, the current standard model of particle physics, described by the Kobayashi-Maskawa mechanism, does not fully explain the dominance of matter over antimatter in the Universe. Therefore, it is crucial to search for new sources of CP violation, particularly in the hyperon sector.
The nonleptonic decay of spin-1/2 Σ+ hyperon is suitable for studying CP violation. In such decay, the decay asymmetry parameters α, β, and γ are defined in terms of the S-wave (parity violating) and P-wave (parity conserving) amplitudes’ contributions. Only two of them are independent, and the corresponding decay asymmetry parameter of the anti-hyperon is denoted as αbar. By using the formula ACP=(α+αbar)/(α-αbar), which tests CP symmetry, a nonzero value of A_CP would indicate CP violation.
Based on 10 billion J/ψ events collected at the BESIII detector, the five-dimensional angular analysis of the processes of J/ψ→Σ+Σbar- (Σ+→pπ0,Σbar-→nbarπ- and Σ+→nπ+,Σbar-→pbarπ0) has been performed. The results of decay parameters αJ/ψ and ΔΦJ/ψ are consistent with the previous measurements but with improved precision. The nonzero value of ΔΦ(J/ψ) in the J/ψ→Σ+Σbar- decay, which implies the existence of polarization, and is confirmed with two different Σ+ decay channels, J/ψ→Σ+Σbar-→pπ0nbarπ-(nπ+pbarπ0) and J/ψ→Σ+Σbar-→pπ0 pbarπ0. The researcher observed a non-zero α+ value for the first time, which differs from zero by 16σ. Besides, this study represents the first investigation of CP symmetry in the hyperon to neutron decay, and the result is consistent with CP conservation.