Longitudinal Spin Transfer in Inclusive Λ and Λ̄ Production in Polarized Proton‐proton Collisions at $s =200 GeV$

This contribution reports on a proof‐of‐principle measurement of the longitudinal spin transfer D_LL in inclusive Λ and Λ̄ production in polarized proton‐proton collisions at a center of mass energy $s = 200 GeV$⁠. The data sample consists of about 3 × 10⁶ minimum bias events collected in the year 2005 by the STAR experiment at RHIC with proton beam polarizations of up to 50%. The Λ(Λ̄) candidates are reconstructed at mid‐rapidity (|η| < 1) using the STAR Time Projection Chamber via the dominant decay channel Λ → pπ⁻ (Λ̄ → p̄π⁺). Their mean transverse momentum p_T is about 1.3 GeV/c and longitudinal momentum fraction x_F = 7.5 × 10⁻³. The longitudinal Λ(Λ̄) polarization is determined using a method in which the detector acceptance mostly cancels.