# Parton model description of multiparticle azimuthal correlations in $pA$ collisions

## TL;DR

This paper extends a parton model to analyze multiparticle azimuthal correlations in $pA$ collisions, challenging hydrodynamic interpretations and exploring the effects of multiple scatterings, saturation, and transverse momentum dependence.

## Contribution

It introduces a detailed analysis of azimuthal correlations in the parton model, including $p_ot$ dependence, $N_c$ effects, and an algorithm for Wilson line correlators, expanding prior work and providing new insights.

## Key findings

- Coherent multiple scattering is crucial for positive $v_2 ext{ }\{4\}$.
- Azimuthal correlations depend on the number of color domains and $p_ot$ window.
- Extended the analysis to symmetric cumulants and $N_c$ dependence.

## Abstract

In arXiv:1705.00745, an initial state "parton model" of quarks scattering off a dense nuclear target was shown to qualitatively reproduce the systematics of multiparticle azimuthal anisotropy cumulants measured in proton/deuteron-nucleus ($pA$) collisions at RHIC and the LHC. The systematics included i) the behavior of the four-particle cumulant $c_2\{4\}$, which generates a real four-particle second Fourier harmonic $v_2\{4\}$, ii) the ordering $v_2\{2\}>v_2\{4\}\approx v_2\{6\}\approx v_2\{8\}$ for two-, four-, six-, and eight-particle Fourier harmonics, iii) the behavior of so-called symmetric cumulants $\text{SC}(2,3)$ and $\text{SC}(2,4)$. These features of azimuthal multiparticle cumulants were previously interpreted as a signature of hydrodynamic flow; our results challenge this interpretation. We expand here upon our previous study and present further details and novel results on the saturation scale and transverse momentum ($p_\perp$) dependence of multiparticle azimuthal correlations. We find that the dependence of $v_2\{2\}$ and $v_2\{4\}$ on the number of color domains in the target varies with the $p_\perp$ window explored. We extend our prior discussion of symmetric cumulants and compute as yet unmeasured symmetric cumulants. We investigate the $N_c$ dependence of $v_2\{2\}$ and $v_2\{4\}$. We contrast our results, which include multiple scatterings of each quark off the target, to the Glasma graph approximation, where each quark suffers at most two gluon exchanges with the target. We find that coherent multiple scattering is essential to obtain a positive definite $v_2\{4\}$. We provide an algorithm to compute expectation values of arbitrary products of the "dipole" lightlike Wilson line correlators.