The longitudinal spin transfer $D_{LL}$ to $Lambda$ and $bar{Lambda}$ hyperons produced in high-energy polarized proton--proton collisions is expected to be sensitive to the helicity distribution functions of strange quarks and anti-quarks of the proton, and to longitudinally polarized fragmentation functions. We report an improved measurement of $D_{LL}$ from data obtained at a center-of-mass energy of $sqrt{s}$ = 200 GeV with the STAR detector at RHIC. The data have an approximately twelve times larger figure-of-merit than prior results and cover $|eta|<$ 1.2 in pseudo-rapidity with transverse momenta $p_T$ up to 6 GeV/c. In the forward scattering hemisphere at largest $p_T$, the longitudinal spin transfer is found to be $D_{LL}$ = -0.036 $pm$ 0.048 (stat) $pm$ 0.013(sys) for $Lambda$ hyperons and $D_{LL}$ = 0.032 $pm$ 0.043,(stat) $pm$ 0.013,(sys) for $bar{Lambda}$ anti-hyperons. The dependences on $eta$ and $p_T$ are presented and compared with model evaluations.
Download