Handedness of direct photons

TL;DR

This paper investigates the azimuthal asymmetry of direct photons in nuclear collisions, introducing a dipole orientation within the saturation model to explain the observed elliptic anisotropy and its dependence on collision centrality.

Contribution

The study incorporates dipole orientation into the saturation model to analyze direct photon azimuthal asymmetry, revealing a sign change in elliptic anisotropy for peripheral collisions.

Findings

Elliptic anisotropy v2 becomes negative in peripheral collisions.

The effect is small for nuclear collisions at RHIC energy.

Dipole orientation influences photon azimuthal asymmetry.

Abstract

The azimuthal asymmetry of direct photons originating from primary hard scatterings between partons is calculated. This can be accounted for by the inclusion of the color dipole orientation, which is sensitive to the rapid variation of the nuclear profile. To this end we introduce the dipole orientation within the saturation model of Golec-Biernat and W\"usthoff, while preserving all its features at the cross-section level. We show that the direct photon elliptic anisotropy v2 coming from this mechanism changes sign and becomes negative for peripheral collisions, albeit it is quite small for nuclear collisions at the RHIC energy.