Test of CP Symmetry in the Neutral Decays of Λ via J/ψ → ΛΛbar

Using $(10087 \pm 44)\times10^{6}$ $J/\psi$ events collected with the BESIII detector, a full angular analysis is carried out for the process $J/\psi\to\Lambda\bar{\Lambda}\to n\pi^0\bar{p}\pi^+ + \mathrm{c.c.}$. The decay parameters $\alpha_0$ for $\Lambda\to n\pi^0$ and $\bar{\alpha}_0$ for $\bar{\Lambda}\to\bar{n}\pi^0$ are measured to be $$\alpha_0 = 0.668 \pm 0.007\,(\mathrm{stat}) \pm 0.002\,(\mathrm{syst})$$ and $$\bar{\alpha}_0 = -0.677 \pm 0.007\,(\mathrm{stat}) \pm 0.003\,(\mathrm{syst}),$$ yielding the most precise test of $CP$ symmetry in the neutral decays of the $\Lambda$ hyperon, $$A_{CP}^{0}=\frac{\alpha_0+\bar{\alpha}_0}{\alpha_0-\bar{\alpha}_0}= -0.006 \pm 0.007\,(\mathrm{stat}) \pm 0.002\,(\mathrm{syst}).$$ The ratios $$\frac{\alpha_0}{\alpha_-}=0.884 \pm 0.013\,(\mathrm{stat}) \pm 0.006\,(\mathrm{syst})$$ and $$\frac{\bar{\alpha}_0}{\alpha_+}=0.885 \pm 0.013\,(\mathrm{stat}) \pm 0.004\,(\mathrm{syst}),$$ where $\alpha_-$ and $\alpha_+$ are the decay parameters of $\Lambda\to p\pi^-$ and $\bar{\Lambda}\to\bar{p}\pi^+$, respectively, are also determined. These 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. This result is expected to improve theoretical calculations of the isospin amplitudes and the $CP$ asymmetry in hyperon decays. In all results, the first uncertainties are statistical and the second are systematic.