Λ hyperon spin polarization and CP tests at BESIII

Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden

^* e-mail: patrik.adlarson@physics.uu.se

Published online: 1 February 2019

Abstract

In these proceedings two preliminary results from the BESIII collaboration on polarized Λ and $\overline{\Lambda }$ are reported. Both results are first observations and concern the resonant and non-resonant processes $e^{-}{e}^{+}\to J/\psi \to \Lambda \overline{\Lambda }$ and $e^{-}{e}^{+}\to {\gamma }^{*}\to \Lambda \overline{\Lambda }$ at $\sqrt{s}=2.396\hspace{0.17em}\text{GeV}$, respectively. For $J/\psi \to \Lambda \overline{\Lambda }$ the relative phase between the electric and magnetic form factors, ΔΦ, has for the first time been determined, $\Delta \Phi =42.4{(6)}_{\text{stat}}{(5)}_{{}_{\text{syst}}}^{°}$. That $\Delta \Phi \ne 0$ allows for a simultaneous measurement also of the asymmetry decay parameters $\Lambda \to p{\pi }^{-}\hspace{0.17em}({\alpha }_{-})$, $\overline{\Lambda }\to \overline{p}{\pi }^{+}\hspace{0.17em}({\alpha }_{+})$ and $\overline{\Lambda }\to \overline{n}{\pi }^0\hspace{0.17em}({\overline{\alpha }}_0)$. The measured value of ${\alpha }_{-}=0.750\pm 0.009\pm 0.004$ differs by 17(3)% from the PDG value. The simultaneous measurement for the asymmetry parameters allows for the most precise test of CP violation conducted for Λ decays, A_CP = −0.006(12)_stat(7)_syst. The reaction $e^{-}{e}^{+}\to \Lambda \overline{\Lambda }$ at $\sqrt{s}=2.396\hspace{0.17em}\text{GeV}$ is the first complete measurement of the time-like electric (G_E) and magnetic (G_M) form factor of any baryon as also the ratio R = |(G_E/(G_M)| and ΔΦ have been determined: R = 0.96(14)(12) and $\Delta \Phi =37{(12)}_{\text{stat}}{(6)}_{\text{stat}}^{°}$. The obtained cross-section and effective form factor are σ = 119.0(53)_stat(51)_syst pb and |G| = 0.123(3)_stat(3)_syst, respectively.