Spin polarization of $\Lambda$ hyperons in $e^{+}{e}^{-}\to \Lambda \bar{\Lambda }$ at BESIII

Abstract

The BESIII collaboration here reports the first observation of polarized $\Lambda$ and $\bar{\Lambda }$ hyperons produced in two different processes: i) the resonant $e^{+}{e}^{-}\to J/\psi \to \Lambda \bar{\Lambda }$, using a data sample of 1.31 $\times$ 10${}^9$ $J/\psi$ events and ii) the non-resonant $e^{+}{e}^{-}\to {\gamma }^{\ast }\to \Lambda \bar{\Lambda }$, using a 66.9 pb${}^{-1}$ data sample collected at $\sqrt{s}=$ 2.396 GeV. In $e^{+}{e}^{-}\to J/\psi \to \Lambda \bar{\Lambda }$, 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 $\Lambda \to p{\pi }^{-}$ (${\alpha }_{-}$), $\bar{\Lambda }\to \bar{p}{\pi }^{+}$ (${\alpha }_{+}$) and $\bar{\Lambda }\to \bar{n}{\pi }^0$ (${\bar{\alpha }}_0$). The obtained value ${\alpha }_{-}=0.750\pm 0.009\pm 0.004$ differs with ~5$\sigma$ from the PDG value. This value, together with the measurement ${\alpha }_{+}=-0.758\pm 0.010\pm 0.007$ allow for the most precise test of CP violation in $\Lambda$ decays so far: $A_{CP}=({\alpha }_{-}+{\alpha }_{+})/({\alpha }_{-}-{\alpha }_{+})$ of $-0.006\pm 0.012\pm 0.007$. The decay asymmetry ${\bar{\alpha }}_0=-0.692\pm 0.016\pm 0.006$ is measured for the first time. The $e^{+}{e}^{-}\to \Lambda \bar{\Lambda }$ 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 $\Delta \Phi ={\Phi }_E-{\Phi }_M$. With the decay asymmetry parameters from the $J/\psi$ data, the obtained values are $R=0.96\pm 0.14\pm 0.02$ and $\Delta \Phi =37^{\mathrm{o}}\pm 12^{\mathrm{o}}\pm 6^{\mathrm{o}}$. In addition, the cross section has been measured with unprecedented precision to be $\sigma =119.0\pm 5.3\pm 5.1$ pb, which corresponds to an effective form factor of $|G|=0.123\pm 0.003\pm 0.003$.

1 Talk at BEACH 2018

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• The Proton Radius Puzzle
• Production of the Smallest QED Atom: True Muonium (mu^+ mu^-)