Longitudinal Spin Transfer to the $\Lambda$ Hyperon in Semi-Inclusive Deep-Inelastic Scattering

The transfer of polarization from a high-energy positron to a \lam hyperon produced in semi-inclusive deep-inelastic scattering has been measured. The data have been obtained by the HERMES experiment at DESY using the 27.6 GeV longitudinally polarized positron beam of the HERA collider and unpolarized gas targets internal to the positron (electron) storage ring. The longitudinal spin transfer coefficient is found to be $\dll = 0.11 \pm 0.10 \mathrm{(stat)} \pm 0.03 \mathrm{(syst)}$ at an average fractional energy carried by the \lam hyperon $<z >= 0.45$. The dependence of \dll on both the fractional energy $z$ and the fractional longitudinal momentum $x_F$ is presented.