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.
Categories: hep-ex