Measurements of Λ and Λ polarization in longitudinally polarized proton-proton collisions at s =200 GeV at STAR

Preliminary results for the longitudinal polarization of Λ and Λ̄ hyperons in longitudinally polarized proton‐proton collisions at s = 200 GeV are presented. The Λ(Λ̄) candidates are reconstructed at mid‐rapidity (|η| < 1) with the time projection chamber of the STAR experiment at RHIC, using 0.5 pb−1 collected in 2003 and 2004 with beam polarizations of up to 45%. Their mean longitudinal momentum fraction x F is about 8 ×x 10−3 and their mean transverse momentum p T is about 1.5 GeV. The analysis uses asymmetries of counts for different spin states of the colliding proton beams and does not require detailed knowledge of the detector acceptance. The preliminary Λ(Λ̄) polarization values are consistent with zero within their statistical uncertainties of 0.05.