Longitudinal Spin Transfer in Inclusive $\Lambda$ and $\bar{\Lambda }$ Production in Polarized Proton-proton Collisions at $\sqrt{s}$ =200 GeV

Abstract

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.

arXiv comment4 pages, 3 eps figures. Prepared for the 17th International Spin Physics Symposium (SPIN2006), Kyoto, Japan, October 2 to 7, 2006

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