Longitudinal Spin Transfer in Inclusive $Lambda$ and $bar Lambda$ Production in Polarized Proton-proton Collisions at $sqrt s$ =200 GeV

This contribution reports on a proof-of-principle measurement of the longitudinal spin transfer $D_{LL}$ in inclusive $Lambda$ and $bar Lambda$ production in polarized proton-proton collisions at a center of mass energy $sqrt{s}$ = 200 GeV. The data sample consists of about 3 $times$ $10^6$ minimum bias events collected in the year 2005 by the STAR experiment at RHIC with proton beam polarizations of up to 50%. The $Lambda$($bar Lambda$) candidates are reconstructed at mid-rapidity ($|eta|<1$) using the STAR Time Projection Chamber via the dominant decay channel $Lambdato p pi^-$ ($bar Lambda to bar p pi^+$). Their mean transverse momentum $p_T$ is about 1.3 GeV/c and longitudinal momentum fraction $x_F = 7.5 times 10^{-3}$ . The longitudinal $Lambda$($bar Lambda$) polarization is determined using a method in which the detector acceptance mostly cancels.