Observation of Global Hyperon Polarization in Ultrarelativistic Heavy Ion Collisions

Non-central heavy-ion collisions provide a system with non-zero total angular momentum which can be transferred, in part, to the fireball via baryon stopping. It has been predicted that a net spin of emitted particles aligned with the system angular momentum may emerge through coupling with the bulk material. Due to its parity violating decay the Lambda baryon is self-analyzing, which allows us to associate the daughter proton decay direction with Lambda particle spin. Ultimately this allows us to use them as a probe of net-particle spin. In preliminary STAR measurements of the net Lambda baryon polarization from Au+Au collisions at 7.7, 11.5, 14.5, 19.6, 27, and 39 GeV we find that both Lambda and (Lambda) over bar particles are polarized in the direction of the system angular momentum. Including previously published STAR results we see a polarization of about 2% for mid-central collisions when root s(NN) < 100GeV.