Effects of finite coverage on global polarization observables in heavy ion collisions

Abstract

In non-central relativistic heavy ion collisions, the created matter possesses a large initial orbital angular momentum. Particles produced in the collisions could be polarized globally in the direction of the orbital angular momentum due to spin–orbit coupling. Recently, the STAR experiment has presented polarization signals for hyperons and possible spin alignment signals for φmesons. Here we discuss the effects of finite coverage on these observables. The results from a multi-phase transport and a toy model both indicate that a pseudorapidity coverage narrower than |η| <∼1will generate a larger value for the extracted φ-meson ρ00parameter; thus a finite coverage can lead to an artificial deviation of ρ00from 1/3. We also show that a finite ηand pTcoverage affect the extracted pHparameter for hyperons when the real pHvalue is non-zero. Therefore proper corrections are necessary to reliably quantify the global polarization with experimental observables.