Precise Measurement of Matter-Antimatter Asymmetry with Entangled Hyperon Antihyperon Pairs

Abstract

A search for $CP$ violation with an entangled system of $Ξ^-$-$\barΞ^+$ pairs is performed, using $(10,087\pm44)\times10^{6}$ $J/ψ$ events collected with the BESIII experiment. A nine-dimensional helicity amplitude is used to fit $e^+e^-\to J/ψ\toΞ^-\barΞ^+$ and its subsequent decays. The $Ξ^-$ and $\barΞ^+$ decay parameters are determined with higher precision compared to the best results reported so far. Furthermore, the strong phase difference, $(δ_P-δ_S)=(0.3\pm1.2\pm0.2)\times10^{-2}~\text{rad}$, and the weak phase difference, $(ξ_P-ξ_S)=(-0.2\pm1.2\pm0.1)\times10^{-2}~\text{rad}$, are directly determined. These are the most precise measurements to date. The $CP$ asymmetry observables $A_{CP}^Ξ=(-7.8\pm4.8\pm0.8)\times10^{-3}$ and $Δφ_{CP}^Ξ=(0.6\pm5.1\pm0.2)\times10^{-3}~\text{rad}$ are determined, which are consistent with $CP$ conservation. In addition, independent measurements of the $Λ$ decay parameter and $CP$ asymmetry $A^Λ_{CP}=(-2.9\pm4.3\pm0.7)\times10^{-3}$ are also obtained, which are in agreement with the previous measurements, but with much improved precision.

Figures (3)

• Figure 1: Illustration of the polarization vectors of $\Lambda$ and $\Xi^-$ hyperons in relation to the decay parameters $\alpha$, $\beta$, and $\gamma$ for the $\Xi^-\to\Lambda\pi^-$ with $\Lambda\to p\pi^-$ process, in the $X^-$ rest frame.
• Figure 2: Acceptance distributions of transverse polarization and spin correlations as a function of $\cos\theta_\Xi$. The black points with error bars show the results from local fits in bins of $\cos\theta_\Xi$, where the error bars represent statistical uncertainties. The red curves correspond to the global fit using the production parameters $\alpha_\psi$ and $\Delta \Phi$.
• Figure 3: Strong and weak phase differences in hyperon processes. The red dots with error bars represent the results in this work, which indicate the best results of $(\delta_P-\delta_S)_{\Xi^{-}}$ and $(\xi_P-\xi_S)_{\Xi^{-}}$. The blue dots with error bars denote the previous BESIII results from the processes $J/\psi\to\Xi^{-}\bar{\Xi}^{+}\to\Lambda(p\pi^-)\pi^-\bar{\Lambda}(\bar{n}\pi^0)\pi^+$BESIII:2023jhj, $J/\psi\to\Xi^{-}\bar{\Xi}^{+}\to\Lambda(p\pi^-)\pi^-\bar{\Lambda}(\bar{p}\pi^+)\pi^+$BESIII:2021ypr, and $J/\psi\to\Xi^0\bar{\Xi}^0$Besiii:2023drj, respectively. The purple dot with error bar indicates the result of $(\delta_P-\delta_S)_{\Xi^{-}}$ measured by the HyperCP collaboration HyperCP:2004zvh. The black square with error bar implies the predictions from the SM for weak phase difference Salone:2022lpt. The green squares with error bars denote the theoretical predictions on the strong phase differences using various models Kamal:1998seDatta:1998pvTandean:2000dxBarros:2004pwHuang:2017bmx. The inner, middle, and outer bands represent 68.2%, 95.4%, and 99.7% confidence intervals of variables in this work, respectively.