R_{out}/R_{sid} and Opacity at RHIC

TL;DR

This paper investigates the ratio of radii in pion interferometry at RHIC, showing that surface emission from an opaque source explains the observed decrease in R_{out}/R_{sid} with increasing transverse momentum.

Contribution

It introduces the effect of nuclear opacity on the R_{out}/R_{sid} ratio, providing a new interpretation consistent with experimental data.

Findings

R_{out}/R_{sid} ratio decreases below unity at higher K_T

Opacity of nuclei can explain surface emission effects

Results align with STAR and PHENIX measurements

Abstract

One of the most dramatic results from the first RHIC run are the STAR results for pi+- pi+- interferometry. They showed that the ratio of the so-called R_{out} and R_{sid} radii seem to decrease below unity for increasing transverse momentum of the pair (K_T). This was subsequently confirmed by PHENIX, which also extended the K_T range of the measurements. We consider here the effects of opacity of the nuclei on this ratio, and find that such a small value is consistent with surface emission from an opaque source.