Longitudinal Λ0 Polarization Measured at Hermes

This paper presents a new measurement of the longitudinal spin transfer to Λ hyperons produced in the current fragmentation region in positron deep-inelastic scattering. This analysis is based on a sample of 9300 reconstructed Λ hyperons collected at HERMES during the period 1999-2000. This sample is more than 5 times larger than that used in a previous analysis based on the 1996-1997 data, and has enabled the investigation of the dependence of the Λ polarization on the kinematic variables z and x_F. No significant dependence on these variables is observed, which is an apparent contradiction with theoretical models predicting a strong rise in the spin transfer at high z.