# Elliptic flow from color-dipole orientation in pp and pA collisions

## TL;DR

This paper explores how the orientation of color dipoles in high-energy proton collisions contributes to elliptic flow, revealing non-perturbative effects and matching observed data at the LHC.

## Contribution

It introduces a semi-analytic model linking dipole orientation to elliptic flow, emphasizing the role of multiple scattering and non-perturbative gluon exchanges.

## Key findings

- Angular dependence driven by soft gluon exchanges
- Multiple scattering alters the sign of v2
- v2 peaks at a transverse momentum related to saturation momentum

## Abstract

For ultrarelativistic proton-proton and proton-nucleus collisions, we perform an exploratory study of the contribution to the elliptic flow $v_2$ coming from the orientation of the momentum of the produced particles with respect to the reaction plane. Via the CGC factorization valid at high energies, this contribution is related to the orientation of a color dipole with respect to its impact parameter, which in turn probes the transverse inhomogeneity in the target. Using the McLerran-Venugopalan model (with impact-parameter dependence) as an effective description for the soft gluon distribution in the (proton or nuclear) target, we present a semi-analytic calculation of the dipole scattering amplitude, including its angular dependence. We find that the angular dependence is controlled by soft gluon exchanges and hence is genuinely non-perturbative. The effects of multiple scattering turn out to be essential (in particular, they change the sign of $v_2$). We find that sizable values for $v_2$, comparable to those observed in the LHC data and having a similar dependence upon the transverse momenta of the produced particles, can be easily generated via peripheral collisions. In particular, $v_2$ develops a peak at a transverse momentum which scales with the saturation momentum in the target.