arXiv2019
The BESIII collaboration here reports the first observation of polarized $Λ$ and $\barΛ$ hyperons produced in two different processes: i) the resonant $e^+e^- \to J/ψ\toΛ\barΛ$, using a data sample of 1.31 $\times$ 10$^9$ $J/ψ$ events and ii) the non-resonant $e^+e^-\to γ^* \to Λ\barΛ$, using a 66.9 pb$^{-1}$ data sample collected at $\sqrt{s} =$ 2.396 GeV. In $e^+e^-\to J/ψ\toΛ\barΛ$, the phase between the electric and the magnetic amplitude is measured for the first time to be $42.3^{\mathrm{o}} \pm 0.6^{\mathrm{o}} \pm 0.5^{\mathrm{o}}$. The multi-dimensional analysis enables a model-independent measurement of the decay parameters for $Λ\to pπ^-$ ($α_-$), $\barΛ\to\bar{p}π^+$ ($α_+$) and $\barΛ\to\bar{n}π^0$ ($\barα_0$). The obtained value $α_-=0.750\pm0.009\pm0.004$ differs with ~5$σ$ from the PDG value. This value, together with the measurement $α_+=-0.758\pm0.010\pm0.007$ allow for the most precise test of CP violation in $Λ$ decays so far: $A_{CP} = (α_- + α_+)/(α_- - α_+)$ of $-0.006\pm0.012\pm0.007$. The decay asymmetry $\barα_0 = -0.692\pm0.016\pm0.006$ is measured for the first time. The $e^+e^- \to Λ\barΛ$ reaction at $\sqrt{s} =$ 2.396 GeV enables a first complete measurement of the time-like electric and magnetic form factor of any baryon, of the modulus of the ratio $R=|G_E/G_M|$ and of the relative phase $ΔΦ=Φ_E-Φ_M$. With the decay asymmetry parameters from the $J/ψ$ data, the obtained values are $R=0.96\pm0.14\pm0.02$ and $ΔΦ= 37^{\mathrm{o}} \pm 12^{\mathrm{o}} \pm 6^{\mathrm{o}}$. In addition, the cross section has been measured with unprecedented precision to be $σ= 119.0\pm 5.3\pm5.1$ pb, which corresponds to an effective form factor of $|G|=0.123 \pm 0.003 \pm 0.003$.