The $J/\psi \to \Xi^0 \bar{ \Xi}^{ 0}$ process and subsequent decays are investigated using $(10087 \pm 44)\times 10^6$ $J/\psi$ events collected at the BESIII experiment. The decay parameters of $\Xi^0$ and $\bar{ \Xi}^0$ are measured with greatly improved precision over previous measurements to be $\alpha_{ \Xi} = -0.3750 \pm 0.0034 \pm 0.0016$, $\bar{ \alpha}_{ \Xi} = 0.3790 \pm 0.0034 \pm 0.0021$, $\phi_{ \Xi} = 0.0051 \pm 0.0096 \pm 0.0018$~rad, $\bar{ \phi}_{ \Xi} = -0.0053 \pm 0.0097 \pm 0.0019$~rad, where the first and the second uncertainties are statistical and systematic, respectively. From these measurements, precise $CP$ symmetry tests in $\Xi^0$ decay are performed, and $A^{ \Xi}_{ CP} = (-5.4 \pm 6.5 \pm 3.1) \times 10^{ -3}$ and $\Delta\phi^{ \Xi}_{ CP} = (-0.1 \pm 6.9 \pm 0.9) \times 10^{ -3}$~rad are consistent with $CP$ conservation. The sequential decay also enables a separation of weak and strong phase differences, which are found for the first time to be $\xi_{ P}-\xi_{ S} = (0.0 \pm 1.7 \pm 0.2) \times 10^{ -2}$~rad and $\delta_{ P}-\delta_{ S} = (-1.3 \pm 1.7 \pm 0.4)\times 10^{ -2}$~rad, respectively. In addition, we measure the $\Lambda$ decay parameters and test $CP$ symmetry in $\Lambda$ decays.
Categories: hep-ex