Seminarium Fizyki Wielkich Energii

Spin degrees of freedom for baryon-antibaryon pairs produced in electron-positron collisions are sensitive probes of baryon properties and discrete symmetries. Precision studies of the spin structure of the processes are possible due to large data samples of baryon-antibaryon pairs from the J/psi and psi' decays. The spin orientations of the produced baryon and antibaryon are correlated and, since the process involves at least two amplitudes, a relative phase can lead to a transverse spin polarization. Spin of a hyperon can be easily analyzed using weak decays. Using 1.3x10^9 J/psi collected at BESIII experiment we observe the polarization signal for the e+e- -> J/psi -> Lambda antiLambda process (*). The polarization allows to determine values of the asymmetry parameters for hyperon decays. In particular the asymmetry value of the for the well-known Lambda hyperon polarimeter process, the Lambda weak decay into proton and negative pion, was found to be wrong by 17(3)%. This calls for a reinterpretation of all polarization results for Lambda. The J/psi -> hyperon antihyperon decays allow a direct comparison of the asymmetry parameters for the charge conjugated decay systems. This CP symmetry test, when applied to the foreseen future data sets, has potential to reach the precision level compatible with the expectations from the CKM mechanism.(*) BESIII Nature Phys. (2019) DOI: 10.1038/s41567-019-0494-8 e-Print:arXiv:1808.08917