The BESIII collaboration has presented the most precise measurements ofasymmetry and decay parameters with entangledΞ⁰- Ξbar⁰pairs. 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 CPsymmetry 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 CPviolation in baryon decays.

For ground-state hyperons with 1/2 spins, a promising signature of CPviolation is the difference between the final state distributions of hyperons and antihyperons in their parity-violating two-body weak decays.In many previous experiments, CPtests 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/ψevents collected at BESIII, a detailed study of quantum entangledΞ0-Ξbar0pairs has been performed through the processe⁺e^-→J/ψ→Ξ⁰- Ξbar⁰,Ξ⁰→Λ(→pπ^-)π⁰,Ξbar⁰→Λbar(→pbarπ⁺)π⁰, Fig. 1 shows the decay plane of this whole process. About 320000 quantum-entangledΞ⁰-Ξbar⁰pairs are reconstructed, and a 9-dimensional likelihood fit is performed to simultaneously determine the decay parameters ofΞ⁰→Λπ⁰andΞbar⁰→Λbarπ⁰, as well as the decay parameters ofΛ→pπ^-,Λbar → pbar π⁺. Using the decay parameters, three CP tests are performed, which are the most precise ones in the hyperon sector. The CPsymmetry is found to be conserved in theΞ⁰decay with anaccuracy of 10^-3. The weak and strong phase differences inΞ⁰decayare 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 ofe⁺e^-→J/ψ→Ξ⁰-Ξbar⁰,Ξ⁰→Λ(→pπ^-)π⁰,Ξbar⁰→Λbar(→pbarπ⁺)π⁰.

The PRD(L) publication:

https://journals.aps.org/prd/abstract/10.1103/PhysRevD.108.L031106

DOI: 10.1103/PhysRevD.108.L031106