Test of $CP$ Symmetry in the Neutral Decays of $\Lambda$ via $J/\psi\to\Lambda\bar{\Lambda}$

TL;DR

This study performs a precise test of $CP$ symmetry in neutral $ ext{ extLambda}$ decays using a large $J/ ext{ extPsi}$ dataset, finding results consistent with $CP$ conservation and evidence for $ ext{ extDelta} I=3/2$ transitions.

Contribution

It provides the most precise measurement of $CP$ symmetry in neutral $ ext{ extLambda}$ decays and confirms the $ ext{ extDelta} I=3/2$ transition presence, improving theoretical understanding.

Findings

$A_{CP}^{0}$ consistent with zero within uncertainties

Ratios $rac{ extalpha_0}{ extalpha_-}$ and $rac{ extalpha_0}{ extalpha_+}$ less than unity by over $5\sigma$

Confirmation of $ ext{ extDelta} I=3/2$ transition in $ ext{ extLambda}$ decays

Abstract

Using $(10087\pm44)\times10^{6}$ $J/\psi$ events collected with the BESIII detector, a full angular distribution analysis is carried out on the process $J/\psi\rightarrow\Lambda\bar{\Lambda}\rightarrow n\pi^{0}\bar{p}\pi^{+}+c.c.$ The decay parameters $\alpha_{0}$ for $\Lambda\rightarrow n\pi^{0}$ and $\bar{\alpha}_{0}$ for $\bar{\Lambda}\rightarrow \bar{n}\pi^{0}$ are measured to be $0.668\pm0.007\pm0.002$ and $-0.677\pm0.007\pm0.003$, respectively, yielding the most precise test for $CP$ symmetry of neutral decays of $\Lambda$, $A_{CP}^{0}=(\alpha_{0}+\bar{\alpha}_{0})/(\alpha_{0}-\bar{\alpha}_{0})$, to be $-0.006\pm0.007\pm0.002$. The ratios $\alpha_{0}/\alpha_{-}$ and $\bar{\alpha}_{0}/\alpha_{+}$ are determined to be $0.884\pm0.013\pm0.006$ and $0.885\pm0.013\pm0.004$, where $\alpha_{-}$ and $\alpha_{+}$ are the decay parameters of $\Lambda\rightarrow p\pi^{-}$ and $\bar{\Lambda}\rightarrow\bar{p}\pi^{+}$, respectively. The ratios, found to be smaller than unity by more than $5\sigma$, confirm the presence of the $\Delta I = 3/2$ transition in the $\Lambda$ and $\bar{\Lambda}$ decays, which is expected to improve the theoretical calculations for strong and weak phases, and $A_{CP}$, in hyperon decays. In all results, the first and second uncertainties are statistical and systematic, respectively.