Measurement of transverse $\Lambda$ polarisation at COMPASS

Preliminary data on spontaneous transverse Lambda polarisation obtained at COMPASS are presented. While $\Lambda$ polarisation transverse to the production plane is a well-established experimental fact in hadronic high-energy collisions, the underlying polarisation mechanism is not yet settled. COMPASS extends this field to high-energy photo production. We have extracted the polarisation at different values of the Feynman variable $x_{F}$ and transverse momentum $p_{T}$. A bias cancelling method has been used to reduce systematic effects.