Hyperon studies at BESIII

Abstract

Recent results on $e^+e^-\to\Lambda\bar\Lambda$ from BESIII Collaboration are reported: ($i$) for the process at $J/\psi$ and ($ii$) for the non-resonant production. ($i$) Using a data sample of $1.31\times 10^9$ $J/\psi$ events we report the first observation of spin polarization of $\Lambda$ and $\bar\Lambda$ hyperons. The phase between the helicity flip and helicity non-flip amplitudes of $\Delta\Phi=(42.4\pm0.6\pm0.5)^\circ$ is determined. The decay asymmetries for $\Lambda\to p\pi^-$ ($\alpha_-$), $\bar\Lambda\to\bar p\pi^+$ ($\alpha_+$) and $\bar\Lambda\to\bar n\pi^0$ ($\bar\alpha_0$) are measured to be $\alpha_-=0.750\pm0.009\pm0.004$, $\alpha_+=-0.758\pm0.010\pm0.007$ and $\bar\alpha_0=-0.692\pm0.016\pm0.006$, respectively. The obtained value of $\alpha_-$ is higher by $(17\pm3)\%$ than the world average of $0.642\pm0.013$ used for all $\Lambda$ polarization measurements since 1978. The $CP$ asymmetry $A_{CP}=(\alpha_-+\alpha_+)/(\alpha_--\alpha_+)$ of $-0.006\pm0.012\pm0.007$ and the ratio $\bar{\alpha}_{0}/\alpha_{+} = 0.913\pm 0.028 \pm 0.012$ are also measured. ($ii$) Using 66.9 pb$^{-1}$ collected at $\sqrt{s} = 2.396$ GeV $\Lambda$ form factors are determined. This is the first complete determination of the time-like elastic form factors $G_M$ and $G_E$ for any baryon: the ratio $R = |G_E/G_M|$ and the relative phase $\Delta\Phi=\arg(G_E/G_M)$. Using the decay asymmetry parameters from analysis ($i$) as input, the obtained values are $R = 0.96\pm 0.14\pm 0.02$ and $\Delta\Phi = (37\pm 12\pm 6)^\circ$, respectively. The normalization of the form factors is set by the determined Born cross section cross section of $\sigma_{Born} = 119.0\pm5.3\pm5.1$ pb. In a separate analysis a surprisingly large Born cross section of $305\pm45^{+66}_{-36}$ pb is measured at $\sqrt{s}$=2.2324 GeV, which is 1.0 MeV above the threshold.