Azimuthal asymmetries and the emergence of "collectivity" from multi-particle correlations in high-energy pA collisions

TL;DR

This paper investigates how azimuthal angular asymmetries in high-energy proton-nucleus collisions can emerge from anisotropic fluctuations in the target's saturation momentum, providing insights into collective phenomena observed in experiments.

Contribution

It introduces a model incorporating anisotropic fluctuations with finite correlation length to explain azimuthal asymmetries and computes multi-particle cumulants, highlighting the role of genuine correlations.

Findings

Only the fully factorizable contribution to c2{4} is negative.

Genuine multi-particle correlations contribute positively to azimuthal cumulants.

The model offers qualitative understanding of azimuthal asymmetries in p+Pb collisions.

Abstract

We show how angular asymmetries $\sim\cos 2\phi$ can arise in dipole scattering at high energies. We illustrate the effects due to anisotropic fluctuations of the saturation momentum of the target with a finite correlation length in the transverse impact parameter plane, i.e.\ from a domain-like structure. We compute the two-particle azimuthal cumulant in this model including both one-particle factorizable as well as genuine two-particle non-factorizable contributions to the two-particle cross section. We also compute the full BBGKY hierarchy for the four-particle azimuthal cumulant and find that only the fully factorizable contribution to $c_2\{4\}$ is negative while all contributions from genuine two, three and four-particle correlations are positive. Our results may provide some qualitative insight into the origin of azimuthal asymmetries in p+Pb collisions at the LHC which reveal a change of sign of $c_2\{4\}$ in high-multiplicity events.