Hyperon and antihyperon physics

Abstract

Abstract Structure, interactions and decays of hyperons can be studied at an electron-positron collider in annihilations to a hyperon-antihyperon pair. This method already has huge impact on hyperon physics since it was used in a first modern measurement of decay asymmetry in the weak Λ → p π − decay and the result calls for reinterpretation of all polarization results for the Λ hyperon. In the experiment by the BESIII Collaboration nearly half million e + e − → ψ → Λ Λ ¯ events were used. The Λ Λ ¯ pair production process involves two (complex) hadronic form factors. The observed large relative phase ΔΦ between the form factors, (42.4±0.6±0.5)°, makes it possible to use both transverse polarization and the spin correlations of the Λ Λ ¯ pair to simultaneously measure decay parameters for Λ and Λ ¯ hyperons. Of particular importance is the result for the decay asymmetry parameter α− in Λ → p π − the most probable Λ decay used to determine Λ polarization. The new α− value from BESIII, 0.750±0.009±0.004, is 17(3)% larger than the world average of 0.642 ± 0.013 adopted by all experiments between 1978 and 2019 to determine Λ polarization from the measured asymmetry in the decay proton distribution. A comparison of the concurrently measured at BESIII α− and α+, the parameter for the charge conjugated process Λ ¯ → p ¯ π + , results in a direct CP symmetry test for the hyperon. The e + e − → Λ Λ ¯ reaction was studied at BESIII also outside the J/ψ resonance using 66.9 pb−1 collected at s = 2.396 GeV. The result is the first complete determination of the time-like elastic form factors GM and GE for any baryon including the relative phase ΔΦ = arg(GE/GM ) of (37 ± 12 ± 6)°. The absolute normalization of the form factors is set by the determined Born cross section of σ Born= 119.0 ± 5.3 ± 5.1 pb. In a separate run at s = 2.2324 GeV, 1.0 MeV above threshold for e + e − → Λ Λ ¯ , a surprisingly large cross section for of 305 ± 45 − 36 + 66 pb was measured.