Measurement of the asymmetry parameter for the decay $\overline{\Lambda }\to \overline{p}{\pi }^{+}$

Based on a sample of $58\times {10}^{6}J/\psi$ decays collected with the BESII detector at the BEPC, the $\overline{\Lambda }$ decay parameter ${\alpha }_{\overline{\Lambda }}$ for $\overline{\Lambda }\to \overline{p}{\pi }^{+}$ is measured using about 9000 $J/\psi \to \Lambda \overline{\Lambda }\to p\overline{p}{\pi }^{+}{\pi }^{-}$ decays. A fit to the joint angular distributions yields ${\alpha }_{\overline{\Lambda }}(\overline{\Lambda }\to \overline{p}{\pi }^{+})=-0.755\pm 0.083\pm 0.063$, where the first error is statistical, and the second systematic.

Figure 1

Distributions for selected $J/\psi \to \Lambda \overline{\Lambda }$ candidates. (a) ${\chi }_{4\mathrm{C}}^2$. Dots with error bars are data; histogram is for MC simulation. (b) Scatter plot of $m_{p{\pi }^{-}}$ versus $m_{\overline{p}{\pi }^{+}}$.

Figure 2

Comparisons of invariant mass distributions between data and MC. Histograms are normalized MC; points with error bars are data. (a) $m_{p{\pi }^{-}}$ and (b) $m_{\overline{p}{\pi }^{+}}$.

Figure 3

Definition of the helicity frame for $J/\psi \to \Lambda \overline{\Lambda }\to p{\pi }^{-}\overline{p}{\pi }^{+}$.

Figure 4

Comparison between data (dots with error bars) and fit results (histograms). (a) Distribution of $\cos \theta$ of $\Lambda$ in $J/\psi$ rest frame. (b) Distribution of $\cos {\theta }_1\cos {\overline{\theta }}_1+\sin {\theta }_1\sin {\overline{\theta }}_1\cos ({\varphi }_1+{\overline{\varphi }}_1)$.

Figure 5

Fitting results of the control sample $J/\psi \to p\overline{p}{\pi }^{+}{\pi }^{-}$ used for determining the uncertainty associated with the MC simulation and detector response. Points with error bars are data, and the histograms are the fit results. (a) Distribution of $\cos \theta$. (b) Distribution of $\cos {\theta }_1\cos {\overline{\theta }}_1+\sin {\theta }_1\sin {\overline{\theta }}_1\cos ({\varphi }_1+{\overline{\varphi }}_1)$.