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

TL;DR

This paper investigates how limited detector coverage in heavy ion collision experiments can artificially influence measurements of global polarization observables, emphasizing the need for proper corrections to ensure accurate interpretation.

Contribution

It demonstrates through models that finite pseudorapidity and transverse momentum coverage can bias polarization measurements, highlighting the importance of correction procedures.

Findings

Finite coverage increases the measured $ ho_{00}$ for $$ mesons.

Finite coverage affects the extracted $p_H$ parameter for $\Lambda$ hyperons.

Proper corrections are essential for accurate polarization quantification.

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 $\Lambda$ hyperons and possible spin alignment signals for $\phi$ mesons. 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 $|\eta|< \sim 1$ will generate a larger value for the extracted $\phi$-meson $\rho_{00}$ parameter; thus a finite coverage can lead to an artificial deviation of $\rho_{00}$ from 1/3. We also show that a finite $\eta$ and $p_T$ coverage affect the extracted $p_H$ parameter for $\Lambda$ hyperons when the real $p_H$ value is non-zero. Therefore proper corrections are necessary to reliably quantify the global polarization with experimental observables.