Hyperon and antihyperon physics

Structure, interactions and decays of hyperons can be studied at an electronpositron 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 Lambda -> p pi(-) decay and the result calls for reinterpretation of all polarization results for the A hyperon. In the experiment by the BESIII Collaboration nearly half million e(+) e(-) -> J/psi -> Lambda(Lambda) over bar events were used. The Lambda(Lambda) over bar pair production process involves two (complex) hadronic form factors. The observed large relative phase Delta Phi between the form factors, (42.4 +/- 0.6 +/- 0.5) degrees , makes it possible to use both transverse polarization and the spin correlations of the Lambda(Lambda) over bar pair to simultaneously measure decay parameters for Lambda and (Lambda) over bar hyperons. Of particular importance is the result for the decay asymmetry parameter alpha(-) in Lambda -> p pi(-) , the most probable A decay used to determine A polarization. The new alpha(-) 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 A polarization from the measured asymmetry in the decay proton distribution. A comparison of the concurrently measured at BESIII alpha(-) and alpha(+) , the parameter for the charge conjugated process Lambda -> (p) over bar pi(+) , results in a direct CP symmetry test for the hyperon. The e(+)e(-) -> Lambda(Lambda) over bar reaction was studied at BESIII also outside the J/psi resonance using 66.9 pb(-1) collected at root s = 2.396 GeV. The result is the first complete determination of the time-like elastic form factors G(M) and G(E) for any baryon including the relative phase Delta Phi = arg(G(E)/G(M)) of (37 +/- 12 +/- 6) degrees . The absolute normalization of the form factors is set by the determined Born cross section of sigma(Born) = 119.0 +/- 5.3 +/- 5.1 pb. In a separate run at root s =2.2324 GeV, 1.0 MeV above threshold for e(+)e(-) -> Lambda(Lambda) over bar, a surprisingly large cross section for of 305 +/- 45(-3)(6)(+66) pb was measured