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 $\Lambda\to p \pi^-$ ($\bar \Lambda \to \bar p \pi^+$). 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.