Longitudinal Spin Transfer in Inclusive $Λ$ and $\bar Λ$ 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 $Λ$ and $\bar Λ$ 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 $Λ$($\bar Λ$) candidates are reconstructed at mid-rapidity ($|η|<1$) using the STAR Time Projection Chamber via the dominant decay channel $Λ\to p π^-$ ($\bar Λ\to \bar p π^+$). 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 $Λ$($\bar Λ$) polarization is determined using a method in which the detector acceptance mostly cancels.