Global polarization measurement in Au+Au collisions

Abstract

The system created in noncentral relativistic nucleus-nucleus collisions possesses large orbital angular momentum. Because of spin-orbit coupling, particles produced in such a system could become globally polarized along the direction of the system angular momentum. We present the results of \ensuremath{\Lambda} and $\overline{\ensuremath{\Lambda}}$ hyperon global polarization measurements in Au+Au collisions at $\sqrt{{s}_{\mathit{NN}}}=62.4$ and 200 GeV performed with the STAR detector at the BNL Relativistic Heavy Ion Collider (RHIC). The observed global polarization of \ensuremath{\Lambda} and $\overline{\ensuremath{\Lambda}}$ hyperons in the STAR acceptance is consistent with zero within the precision of the measurements. The obtained upper limit, $|{P}_{\ensuremath{\Lambda},\overline{\ensuremath{\Lambda}}}|\ensuremath{\leqslant}0.02$, is compared with the theoretical values discussed recently in the literature.