Hierarchy of azimuthal anisotropy harmonics in collisions of small systems from the Color Glass Condensate

TL;DR

This paper shows that the Color Glass Condensate (CGC) effective field theory can reproduce the observed azimuthal anisotropy harmonics in small-system collisions, challenging previous claims that initial state effects are insufficient.

Contribution

The study demonstrates that the CGC framework explains the systematics of azimuthal harmonics in small-system collisions, contradicting prior assertions that data rules out initial state explanations.

Findings

CGC reproduces $v_2$ and $v_3$ systematics in small systems.

Contradicts previous claims ruling out initial state effects.

Suggests further tests in light-heavy ion collisions.

Abstract

We demonstrate that the striking systematics of two-particle azimuthal Fourier harmonics $v_2$ and $v_3$ in ultrarelativistic collisions of protons, deuterons and helium-3 ions off gold nuclei measured by the PHENIX Collaboration [arXiv:1805.02973] at the Relativistic Heavy Ion Collider (RHIC) is reproduced in the Color Glass Condensate (CGC) effective field theory. This contradicts the claim in [arXiv:1805.02973] that their data rules out initial state based explanations. The underlying systematics of the effect, as discussed previously in [arXiv:1705.00745,arXiv:1706.06260,arXiv:1801.09704], arise from the differing structure of strong color correlations between gluon domains of size $1/Q_S$ at fine ($p_\perp \gtrapprox Q_S$) or coarser ($p_\perp \lessapprox Q_S$) transverse momentum resolution. Further tests of the limits of validity of this framework can be carried out in light-heavy ion collisions at both RHIC and the Large Hadron Collider. Such measurements also offer novel opportunities for further exploration of the role of the surprisingly large short-range nuclear correlations measured at Jefferson Lab.