UCLA

Studies on the spin structure of the proton have been an active area of research; after the EMC experiment and subsequent experiments found that only about 30% of the total proton spin is carried by quark spins. The Relativistic Heavy Ion Collider (RHIC) is the world's first and only polarized proton collider. The Solenoidal Tracker At RHIC (STAR) has full azimuthal acceptance and is ideally suited to advance studies of the proton spin. The longitudinal spin transfer, D_LL, of λ and anti-λ hyperons in longitudinally polarized proton-proton collisions is sensitive to quark and anti-quark polarization in the polarized proton; as well as to polarized fragmentation; and has been proposed as a possible probe of (anti-)strange quark polarization.

The STAR collaboration has previously reported an initial proof-of-concept measurement of D_LL of λ and anti-λ hyperons from a data sample obtained at sqr(s)=200 GeV in 2005. The data correspond to an integrated luminosity of 2 pb^- with 50% beam polarization. Considerably larger data samples corresponding to 6.5 pb^- and 25 pb^- with beam polarizations of 57% at sqr(s)=200 GeV were obtained in 2006 and 2009 using an upgraded instrument. Improvements were made on the analysis procedure to reduce background contribution to the λ + anti-λ measurements. These new measurements of D_LL form the main topic of this dissertation. The sample of hyperons residing within a jet that triggered the experiment are classified as near-side hyperons, and are analyzed separately from an away-side sample that has similar precision. In addition to D_LL, the double longitudinal spin asymmetry, A_LL, for the production of λ and anti-λ hyperons has been extracted. The dependences of D_LL on pseudo-rapidity, p_T, and the fragmentation ratio, z, are studied. The stated D_LL from λ and anti-λ each disfavor one of the published model predictions for D_LL for a combined λ and anti-λ sample, and are consistent with other predictions as well as with the preceding data. The disfavored model prediction is based on the assumption that up, down, and strange (anti-)quark spins contribute equally to polarized fragmentation into λ + anti-λ hyperons. Future data with improved statistical precision, including data at sqr(s)=500 GeV, are needed to discriminate between the other models.