Measurement of long-range angular correlations and azimuthal anisotropies in high-multiplicity $p$$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV

TL;DR

This paper reports the first measurements of long-range angular correlations and elliptic flow in high-multiplicity proton-gold collisions at 200 GeV, supporting a collective origin linked to initial geometry.

Contribution

It provides new experimental data on azimuthal anisotropies in small systems and compares these with hydrodynamic models, highlighting the role of initial geometry.

Findings

v2 correlates with initial eccentricity ε2

Hydrodynamic models agree with measured v2

Results suggest collective behavior in small systems

Abstract

We present the first measurements of long-range angular correlations and the transverse momentum dependence of elliptic flow $v_2$ in high-multiplicity $p$$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV. A comparison of these results with previous measurements in high-multiplicity $d$$+$Au and $^3{\rm He}$$+$Au collisions demonstrates a relation between $v_2$ and the initial collision eccentricity $\varepsilon_2$, suggesting that the observed momentum-space azimuthal anisotropies in these small systems have a collective origin and reflect the initial geometry. Good agreement is observed between the measured $v_2$ and hydrodynamic calculations for all systems, and an argument disfavoring theoretical explanations based on momentum-space domain correlations is presented. The set of measurements presented here allows us to leverage the distinct intrinsic geometry of each of these systems to distinguish between different theoretical descriptions of the long-range correlations observed in small collision systems.