Precise Measurements of Decay Parameters and $CP$ Asymmetry with Entangled $\Lambda-\bar{\Lambda}$ Pairs

TL;DR

This paper reports the most precise measurements of decay parameters and $CP$ asymmetry in $\Lambda$ baryon decays using 10 billion $J/\psi$ events, providing stringent tests of $CP$ violation in baryons.

Contribution

It presents the first high-precision determination of $\Lambda$ decay parameters and $CP$ asymmetry using entangled $\Lambda-ar{\Lambda}$ pairs from BESIII data.

Findings

Decay parameters $\alpha_{-}$ and $\alpha_{+}$ measured with unprecedented precision.

$CP$ asymmetry $A_{CP}$ consistent with zero within uncertainties.

Results will aid future studies of $CP$ violation in baryons.

Abstract

Based on 10 billion $J/\psi$ events collected at the BESIII experiment, a search for $CP$ violation in $\Lambda$ decay is performed in the difference between $CP$-odd decay parameters $\alpha_{-}$ for $\Lambda \rightarrow p\pi^-$ and $\alpha_{+}$ for $\bar\Lambda \rightarrow \bar{p}\pi^+$ by using the process $e^+e^- \to J/\psi \rightarrow \Lambda \bar\Lambda $. With a five-dimensional fit to the full angular distributions of the daughter baryon, the most precise values for the decay parameters are determined to be $\alpha_{-} = 0.7519 \pm 0.0036 \pm 0.0024$ and $\alpha_{+} = -0.7559 \pm 0.0036 \pm 0.0030$, respectively. The $\Lambda$ and $\bar{\Lambda}$ averaged value of the decay parameter is extracted to be $\alpha_{\rm{avg}} = 0.7542 \pm 0.0010 \pm 0.0024$ with unprecedented accuracy. The $CP$ asymmetry $A_{CP}=(\alpha_{-}+\alpha_{+})/(\alpha_{-}-\alpha_{+})$ is determined to be $-0.0025 \pm 0.0046 \pm 0.0012$, which is one of the most precise measurements in the baryon sector. The reported results for the decay parameter will play an important role in the studies of the polarizations and $CP$ violations for the strange, charmed and beauty baryons.