Longitudinal Spin Transfer in Inclusive Λ and Λ-bar Production in Polarized Proton-proton Collisions at √(s) =200 GeV

This contribution reports on a proof-of-principle measurement of the longitudinal spin transfer DLL in inclusive Λ and Λ-bar production in polarized proton-proton collisions at a center of mass energy √(s) = 200 GeV. The data sample consists of about 3 x 106 minimum bias events collected in the year 2005 by the STAR experiment at RHIC with proton beam polarizations of up to 50%. The Λ(Λ-bar) candidates are reconstructed at mid-rapidity (|η| < 1) using the STAR Time Projection Chamber via the dominant decay channel Λ → pπ- (Λ-bar → p-barπ+). Their mean transverse momentum pT is about 1.3 GeV/c and longitudinal momentum fraction xF = 7.5 x 10-3. The longitudinal Λ(Λ-bar) polarization is determined using a method in which the detector acceptance mostly cancels