Azimuthal asymmetry of pions in pp and pA collisions

We investigate the azimuthal asymmetry v(2) of produced pions in pp and pA collisions at both the Relativistic Heavy Ion Collider and CERN SPS energies. In our approach, based on the perturbative QCD 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 deep inelastic scattering 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 v(2) 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, and can change sign and become 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 v(2) of produced pions in both pp and pA collisions is positive, albeit rather small.