Hyperon Structure and CP tests at BESIII

Abstract

Abstract Hyperons are a powerful diagnostic tool that can shed light on some of the most intriguing questions in contemporary physics. Electromagnetic form factors (EMFFs) are currently the best way to study hyperon structure. In the time-like region the EMFFs can be complex with a relative phase. A non-zero phase polarizes the final state even when the initial state is unpolarized. A dedicated data sample collected by the BESIII experiment allowed for a first complete reconstruction of Λ in the time-like region. The ratio, R and relative phase between the electric and magnetic form factor, ΔΦ are measured to be 0.96±0.14(stat)±0.002(syst) and 37°±12°(stat)±6°(syst), respectively. In addition, the BESIII experiment has collected the world’s largest J/ψ data sample. Due to symmetric excellent detector conditions and low hadronic background, the experiment offers a clean environment for CP-violation tests using J / ψ → B B ¯ . That hyperons can be polarized allows for a simultaneous measurement of angular distributions of hyperons and antihyperons and to test CP symmetry directly. This has been done for the process e + e − → J / ψ → Λ Λ ¯ recently measured by the BESIII experiment. The experimental results showed that the asymmetry decay parameter α(Λ → ππ −) = +0.750±0.009±0.004, is nearly twenty percent larger compared to the old PDG tabulated value. The test on CP symmetry, A CP , Λ Λ ¯ = − 0.006 ± 0.012 ± 0.007 is the most sensitive test for Λ CP-violation. Preliminary results are presented for the process e + e − → J / ψ → ∑ + ∑ − → p π 0 p ¯ π 0 . The asymmetry decay parameter α ( ∑ ¯ − → p ¯ π 0 ) = + 0.992 ± 0.037 ± 0.008 and A CP , ∑ ​ + ∑ ​ − − 0.015 ± 0.037 ± 0.008 −0.015 ± 0.037 ± 0.008 are both measured for the first time.