The nonflow issue in connecting anisotropy measurements to hydrodynamics in relativistic heavy-ion collisions

TL;DR

This paper highlights the significant impact of nonflow correlations on anisotropy measurements in heavy-ion collisions and demonstrates that current assessment methods are inadequate, emphasizing the need for better correction techniques.

Contribution

It critically evaluates existing nonflow contamination assessment methods and shows, using HIJING simulations, that they underestimate nonflow effects, affecting physical parameter extraction.

Findings

Nonflow can significantly bias anisotropy measurements.

Current subevent methods are insufficient to assess nonflow contamination.

Proper correction for nonflow is essential for accurate hydrodynamic comparisons.

Abstract

Hydrodynamics can describe majority of the measured azimuthal anisotropies in relativistic heavy-ion collisions. Many of the anisotropy measurements are contaminated by nonflow correlations (i.e., those unrelated to global event-wise correlations). Those nonflow contamination can cause incorrectness or compromise the accuracy of the physics extracted from data-hydrodynamics comparison, particularly when one relies on subtle difference in the measurements. In the recent preprint by STAR (arXiv:2401.06625) extracting the Uranium nucleus deformation parameter, nonflow contamination is assessed by subevents in the limited STAR acceptance. In this note, we demonstrate that such assessment is inadequate and illustrate how large an effect nonflow can cause by using the HIJING model, in which all correlations are nonflow and non-hydrodynamic. We thereby conclude that the extracted Uranium deformation parameter is premature and emphasize the importance of an earnest assessment of or correction for nonflow contamination, not only for this STAR analysis but more generally for studies relying on comparing anisotropy measurements to hydrodynamic calculations.