Hyperon structure and decay with BESIII

Abstract

In these proceedings, we report on the Λ hyperon structure and decay, studied through two different processes: i) the resonant e+e− → J/ψ → ΛΛ¯, using a data sample of 1.31×109 J/ψ events and ii) the non-resonant e+e− → γ* → ΛΛ¯, using a 66.9 pb−1 data sample collected at s = 2.396 GeV. In e+e− → J/ψ → ΛΛ¯, the phase between the hadronic form factors is measured for the first time to be 42.3° ± 0.6° ± 0.5°. The large data sample enabled a multi-dimensional analysis and hence a model-independent measurement of the decay parameters for Λ → pπ− (α−), Λ¯ → p̄π+ (α+) and Λ¯ → n̄π° (α¯0). The value obtained, α− = 0.750 ± 0.009 ± 0.004, differs with 5σ from the PDG value. The simultaneous measurement for the Λ¯, i.e. α+ = −0.758 ± 0.010 ± 0.007, allows for the most precise test of CP violation in Λ decays so far: ACP = (α− + α+)/(α− − α+) of −0.006 ± 0.012 ± 0.007. Furthermore, the decay asymmetry α¯0 = −0.692 ± 0.016 ± 0.006 is measured for the first time. The large data sample at √s = 2.396 GeV of the e+e− → ΛΛ¯ reaction enabled the first complete measurement of the time-like electric and magnetic form factors GE and GM of any baryon. This includes the modulus of the ratio R = | GE/GM | and the relative phase, ΔΦ = ΦE − ΦM. Using the decay asymmetry parameters from the J/ψ data as input, the obtained values of R and ΔΦ are R = 0.96 ± 0.14 ± 0.02 and ΔΦ = 37° ± 12° ± 6°, respectively. In addition, the cross section has been measured to unprecedented precision to σ = 119.0 ± 5.3 ± 5.1 pb, corresponding to an effective form factor value of | G | = 0.123 ± 0.003 ± 0.003.