The BESIII collaboration reported the first pioneering strong-CP test in hyperon decays based on 10 billion J/ψ and 2.7 billion ψ(3686) data. The results have been recently published in Physical Review Letters on September 4, 2024 (Phys. Rev. Lett. 133 (2024), 101902).
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 CP violations are theoretically and experimentally well-established in weak decays of unstable particles. However, in interactions involving the strong force that forms protons and neutrons, no CP violating effects have been observed. There is no theoretical explanation for this – a conundrum referred to as the strong CP problem. The neutral Σ0 hyperon - a heavy, unstable sibling to the proton, possesses a unique feature that allows for probing strong CP violations through its electromagnetic decay. In the BESIII experiment, quantum entangled pairs of Σ0 hyperons and antihyperons are produced in abundance in electron-positron collisions. We have found that these hyperons are polarized, i.e. their inner magnets are aligned. Hence, their decay chains follow a certain pattern from which both weak and strong CP tests can be simultaneously constructed. This work presents the first study of this kind.
Based on 10 billion J/ψ and 2.7 billion ψ(3686) events collected at BESIII, a detailed study of quantum entangled Σ0-Σbar0 pairs has been performed through the process e+ e-→J/ψ, ψ(3686)→Σ0 Σbar0, Σ0→Λ(→pπ-)γ, Σbar0→Λbar(→pbar π+ )γ. About 1.15 million quantum-entangled Σ0-Σbar0 pairs are reconstructed, and a 7-dimensional likelihood fit is performed to simultaneously determine the decay parameters of Σ0→Λγ and Σbar0→Λbar γ, as well as the decay parameters of Λ→pπ-, Λbar→pbar π+. Using the decay parameters, the first strong CP test in hyperon decays has been performed, and the weak CP symmetry is also tested in the subsequent decays of the Λ. The CP symmetry is found to be conserved in the Σ0 decay with an accuracy of 10-3. Moreover, the polarizations of the Σ0 in J/ψ and ψ(3686) decays are observed with opposite directions for the first time.
Fig. 1. Double-slit interference experiments on the subatomic scale, where the decay of Σ0/Σbar0 is analogized to Young’s double-slit interference experiment, the S-wave and P-wave refer to the double-slit in the Young’s experiment, the angular distribution of the daughter Λ/Λbar is similar to the interference fringes. It reflects the interference effect on the sub-Fermi scale level, the CP violation will be observed if the “interference fringes” are different between the Σ0 and Σbar0 decays.
The PRL publication:https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.133.101902
arXiv: https://arxiv.org/abs/2406.06118