Measurements of $\Lambda$ and $\bar \Lambda$ polarization in longitudinally polarized proton-proton collisions at $\sqrt s$ =200 GeV at STAR

TL;DR

This paper reports preliminary measurements of the longitudinal polarization of Lambda and anti-Lambda hyperons in polarized proton-proton collisions at 200 GeV, finding results consistent with zero polarization within uncertainties.

Contribution

First measurement of Lambda and anti-Lambda polarization in polarized proton-proton collisions at RHIC using the STAR detector.

Findings

Polarization values are consistent with zero within statistical uncertainties.

Analysis method does not require detailed detector acceptance knowledge.

Data collected at 200 GeV with beam polarizations up to 45%.

Abstract

Preliminary results for the longitudinal polarization of $\Lambda$ and ${\bar \Lambda}$ hyperons in longitudinally polarized proton-proton collisions at $\sqrt s$ = 200 GeV are presented. The $\Lambda$($\bar \Lambda$) candidates are reconstructed at mid-rapidity ($|\eta|<1$) 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 $\times$$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 $\Lambda$(${\bar \Lambda}$) polarization values are consistent with zero within their statistical uncertainties of 0.05.