Azimuth multipoles vs minimum-bias jets in 2D angular correlations on $\eta$ and $\phi$

TL;DR

This paper analyzes 2D angular correlations in proton-proton and heavy ion collisions, distinguishing jet-related peaks from flow-like multipoles, and challenges the interpretation of certain correlation features as flow phenomena.

Contribution

It demonstrates that 2D angular correlation structures are distinct from 1D Fourier multipoles, emphasizing the jet origin of the same-side peak across collision systems.

Findings

2D correlation structures can be clearly separated from 1D multipoles.

The 2D peak properties are consistent with jet fragmentation in all collision types.

1D Fourier amplitudes cannot fully describe the 2D angular correlations.

Abstract

Angular correlations measured in \pp and heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC) include a same-side (SS) 2D peak. In peripheral \aa and p-p collisions the SS peak properties are consistent with predicted minimum-bias jet correlations. However, in more-central \auau collisions the SS peak becomes elongated on pseudorapidity $\eta$. Arguments have been proposed to explain the SS peak $\eta$ elongation in terms of possibly-fluctuating initial-state geometry multipoles coupled with radial flow to produce final-state momentum-space multipoles. Such arguments are based on Fourier decomposition of 2D angular correlations projected onto 1D azimuth. In this analysis we show that measured correlation structure on $\eta$ (large curvatures) establishes a clear distinction between the SS 2D (jet) peak and 1D multipoles. Measured 2D peak systematics can predict inferred 1D Fourier amplitudes interpreted as "higher harmonic flows." But 1D Fourier amplitudes alone cannot describe 2D angular correlations. The SS 2D peak remains a unique structure which can be interpreted in terms of parton scattering and fragmentation in all cases.