Precision measurements of decay parameters and $CP$ asymmetry in $\Lambda$ decays

Based on 10 billion $J/\psi$ events collected in the BESIII experiment,the phase angle $\Delta\Phi$ associated with hyperon polarization in $\Lambda/\bar{\Lambda}$ production, and the decay parameters $\alpha_{-}$ for $\Lambda \rightarrow p\pi^-$ and $\alpha_{+}$ for $\bar\Lambda \rightarrow \bar{p}\pi^+$ are studied in the process $J/\psi \rightarrow (\Lambda \rightarrow p\pi^-)(\bar\Lambda \rightarrow \bar{p}\pi^+)$. The free parameters of the joint angular distribution are determined by means of a maximum likelihood fit, and the results are: $\Delta\Phi = 0.7521 \pm 0.0042 \pm 0.0080$, $\alpha_{-} = 0.7519 \pm 0.0036 \pm 0.0019$, and $\alpha_{+} = -0.7559 \pm 0.0036 \pm 0.0029$. The average value of the $\Lambda$ decay parameter is $\alpha_{\rm{avg}} = 0.7542 \pm 0.0010 \pm 0.0020$. The CP asymmetry observable $A_{CP}=(\alpha_{-}+\alpha_{+})/(\alpha_{-}-\alpha_{+})$ is determined to be $-0.0025 \pm 0.0046 \pm 0.0011$, which is the most precise measurement in the strange baryon sector.