Azimuthal Asymmetry of pions in pp and pA collisions

TL;DR

This paper studies the azimuthal asymmetry v2 of pions in pp and pA collisions using pQCD and QCD-dipole models, revealing model sensitivity and different patterns for quarks and gluons.

Contribution

It introduces an improved formalism for azimuthal asymmetry based on color-dipole orientation and compares saturation and Born approximation models.

Findings

v2 of pions is positive but small in pp and pA collisions.

Gluon v2 can be negative at low transverse momentum and positive at high.

Quark v2 remains positive across all transverse momenta.

Abstract

We investigate the azimuthal asymmetry v2 of produced pions in pp and pA collisions at both RHIC and SPS energies. In our approach, based on the pQCD parton model and the light-cone QCD-dipole formalism, the azimuthal asymmetry results from a correlation between the color-dipole orientation and the impact parameter of the collision. We introduce the color-dipole orientation within an improved Born approximation and the saturation model which satisfies available DIS data, showing that the azimuthal asymmetry of partons and pions is very sensitive to the choice of the model, and that it is reduced in the saturation model. We find that v2 of quarks and gluons in parton-nucleus collisions have very different patterns. The azimuthal asymmetry of gluons in gluon-nucleus collisions can be negative at small transverse momentum, changes the sign and becomes positive at high transverse momentum. The azimuthal asymmetry of quarks in quark-nucleus collisions is positive at all values of transverse momentum. We find that the azimuthal anisotropy v2 of produced pions in both pp and pA collisions is positive, albeit rather small.