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

The BESIII collaboration here reports the first observation of polarized Λ and ${\bar{\Lambda }}$ hyperons produced in two different processes: i) the resonant ${e}^{+}{e}^{-}\to \psi \to \Lambda \bar{\Lambda }$, using a data sample of 1.31 × 10⁹J/ψ events and ii) the non-resonant ${e}^{+}{e}^{-}\to {{\rm{\gamma }}}^{\ast }\to \Lambda \bar{\Lambda }$, using a 66.9 pb⁻¹ data sample collected at $\sqrt{s}=2.396\,{\rm{GeV}}$. In ${e}^{+}{e}^{-}\to \psi \to \Lambda \bar{\Lambda }$, the phase between the electric and the magnetic amplitude is measured for the first time to be 42.3° ± 0.6° ± 0.5°. The multi-dimensional analysis enables a model-independent measurement of the decay parameters for $\Lambda \to p{\pi }^{-}({\alpha }_{-}),{\rm{}}\bar{\Lambda }\to \bar{p}{\pi }^{+}({\alpha }_{+})$ and $\bar{\Lambda }\to \bar{n}{\pi }^{0}({\bar{\alpha }}_{0})$. The obtained value α₋ = 0.750 ± 0.009 ± 0.004 differs with 5σ from the PDG value. This value, together with the measurement α₊ = −0.758 ± 0.010 ± 0.007 allow for the most precise test of CP violation in decays so far: A_CP = ( α₋ + α₊)/(α₋ − α₊) of −0.006 ± 0.012 ± 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 $\sqrt{s}=2.396\,{\rm{GeV}}$ 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 ± 0.14 ± 0.02 and ΔΦ = 37° ± 12° ± 6°. In addition, the cross section has been measured with unprecedented precision to be σ = 119.0 ± 5.3 ± 5.1 pb, which corresponds to an effective form factor of |G| = 0.123 ± 0.003 ± 0.003.