Measurement of Spin Transfer at $\Lambda^0$ Production in Polarized Lepton Nucleon Scattering in the HERMES Experiment

Spin transfer to $\Lambda^0$-Hyperons in semi-inclusive deep inelastic scattering of longitudinally polarized positrons from both unpolarized and longitudinally polarized nucleons has been investigated. The data were collected by the HERMESExperiment at DESY in Hamburg during 1996 and 1997 covering a kinematic range of 0.8 < $Q^2$ , 0.02 < x < 0.5 and 0.2 $\leq$ z. To avoid asymmetries caused by acceptance effects, datasets with all possible permutations for polarization directions of beam and target have been combined. The result for the spin transfer, dominated by scattering from u-quarks is compatible with zero and therefore in agreement with the Naive Quark-Parton Model expectation. However limited statistic precision does not allow to exclude other models significantly. Supplementary events with no scattered positron in coincidence with the $\Lambda^0$ - decay products seen in the HERMES-detector have been analysed. They show much better statistic precision, but due to their small averaged $Q^2$ of 0.21 their interpretation in the frame of the Naive Quark-Parton Model is at least questionable. The resulting spin transfer from the lepton beam to the outgoing $\Lambda^0$-Hyperon for this dataset is 0.122 ± 0.047 ± 0.05