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

Abstract

The longitudinal spin transfer DLL to Λ and Λ¯ 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 DLL from data obtained at a center-of-mass energy of 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 |η|< 1.2 in pseudo-rapidity with transverse momenta pT up to 6 GeV/c. In the forward scattering hemisphere at largest pT, the longitudinal spin transfer is found to be DLL = -0.036 ± 0.048 (stat) ± 0.013(sys) for Λ hyperons and DLL = 0.032 ± 0.043,(stat) ± 0.013,(sys) for Λ¯ anti-hyperons. The dependences on η and pT are presented and compared with model evaluations.