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

TL;DR

This study measures matter-antimatter asymmetry in hyperon pairs with unprecedented precision, finding results consistent with $CP$ conservation and improving existing decay parameter measurements.

Contribution

It provides the most precise measurements of hyperon decay parameters, phase differences, and $CP$ asymmetries using entangled hyperon-antihyperon pairs at BESIII.

Findings

Decay parameters determined with higher precision than previous results.

Strong and weak phase differences measured with high accuracy.

$CP$ asymmetry observables consistent with $CP$ conservation.

Abstract

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