Importance of non-flow in mixed-harmonic multi-particle correlations in small collision systems

TL;DR

This paper demonstrates that non-flow effects significantly influence mixed-harmonic multi-particle correlations in small collision systems and advocates for subevent methods to accurately identify genuine collectivity signals.

Contribution

It shows that standard cumulant methods may overestimate collectivity signals due to non-flow effects and highlights the importance of using subevent methods for reliable analysis.

Findings

Non-flow effects dominate in standard cumulant measurements.

Subevent methods effectively suppress non-flow contributions.

Reanalysis with subevent methods is crucial for accurate collectivity assessment.

Abstract

Recently CMS Collaboration measured mixed-harmonic four-particle azimuthal correlations, known as symmetric cumulants SC(n,m), in pp and pPb collisions, and interpreted the non-zero SC(n,m) as evidence for long-range collectivity in these small collision systems. Using the PYTHIA and HIJING models which do not have genuine long-range collectivity, we show that the CMS results, obtained with standard cumulant method, could be dominated by non-flow effects associated with jet and dijets, especially in $pp$ collisions. We show that the non-flow effects are largely suppressed using the recently proposed subevent cumulant methods by requiring azimuthal correlation between two or more pseudorapidity ranges. We argue that the reanalysis of SC(n,m) using the subevent method in experiments is necessary before they can used to provide further evidences for a long-range multi-particle collectivity and constraints on theoretical models in small collision systems.