Longitudinal spin transfer to $\Lambda$ and anti-$\Lambda$ hyperons

The polarisations of $\Lambda$ and $\bar{\Lambda}$ hyperons produced in deep inelastic scattering of 160 $GeV$ polarised muons off unpolarised deuterons were studied at the COMPASS experiment at CERN. The longitudinal spin transfer was measured for both hyperons, as a function of the Bjorken $x$ and the Feynman $x_{F}$ variables. The spin transfer to the $\Lambda$ is small and compatible with zero for both $x$ and $x_{F}$ . In contrast, the spin transfer to $\bar{\Lambda}$ is positive for both variables, and shows a steep increase for large $x_{F}$ . Comparison with theoretical predictions indicates that the polarisation of the $\bar{\Lambda}$ hyperon is sensitive to the strange anti-quark distribution in the target nucleon.