Improved measurement of the longitudinal spin transfer to $\Lambda$ and $\bar \Lambda$ hyperons in polarized proton-proton collisions at $\sqrt s$ = 200 GeV.

The longitudinal spin transfer D-LL to Lambda and (Lambda) over bar hyperons produced in high-energy polarized proton-proton collisions is expected to be sensitive to the helicity distribution functions of strange quarks and antiquarks 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 root 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 vertical bar eta vertical bar<1.2 in pseudorapidity 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 +/- 0.048 (stat) +/- 0.013 (sys) for Lambda hyperons and D-LL = 0.032 +/- 0.043 (stat) +/- 0.013(sys) for <(Lambda)over bar> antihyperons. The dependences on eta and p(T) are presented and compared with model evaluations.