The BESIII collaboration has presented the most precise measurements of asymmetry and decay parameters with entangled Ξ0- Ξbar0 pairs. The results are published in the journal Physical Review D as a Letter and an Editor’s Suggestion on Aug. 23, 2023 ( PHYSICAL REVIEW D 108, L031106 (2023)).
According to the Big Bang theory, at the very beginning of the Universe, matter and antimatter were believed to have existed in equal amounts. However, the current Universe is observed to be dominated by matter. In 1967, Soviet physicist Sakharov proposed three conditions to explain this matter dominance, one of which involves the violation of charge conjugation and parity (CP) symmetry. Currently, small violations of CP symmetry have been discovered in the K, B, and D meson systems. However, these violations are not sufficient to account for the imbalance between matter and antimatter. Therefore, it is crucial to continue searching for new sources of CP violation in baryon decays.
For ground-state hyperons with 1/2 spins, a promising signature of CP violation is the difference between the final state distributions of hyperons and antihyperons in their parity-violating two-body weak decays. In many previous experiments, CP tests have relied on the products of weak and strong phase differences, making it difficult to distinguish the contributions of weak interactions from those of strong interactions. However, the sequential decays of double-strange hyperon, Ξ0, allow for a separation of strong and weak phase differences.
Based on 10 billion J/ψ events collected at BESIII, a detailed study of quantum entangled Ξ0-Ξbar0 pairs has been performed through the process e+e-→ J/ψ → Ξ0- Ξbar0, Ξ0→Λ(→pπ-)π0, Ξbar0→Λbar(→pbar π+)π0, Fig. 1 shows the decay plane of this whole process. About 320000 quantum-entangled Ξ0-Ξbar0 pairs are reconstructed, and a 9-dimensional likelihood fit is performed to simultaneously determine the decay parameters of Ξ0→Λπ0 and Ξbar0→Λbar π0 , as well as the decay parameters of Λ→pπ-,Λbar → pbar π+. Using the decay parameters, three CP tests are performed, which are the most precise ones in the hyperon sector. The CP symmetry is found to be conserved in the Ξ0 decay with an accuracy of 10-3. The weak and strong phase differences in Ξ0 decay are directly measured for the first time. These are the most precise results for any weakly-decaying baryon and are crucial for understanding CP violation sources beyond the Standard Model.
Fig. 1. Decay plane of e+e-→ J/ψ → Ξ0-Ξbar0, Ξ0→Λ(→pπ-)π0, Ξbar0→Λbar(→pbar π+)π0.
The PRD(L) publication:
https://journals.aps.org/prd/abstract/10.1103/PhysRevD.108.L031106
DOI: 10.1103/PhysRevD.108.L031106