Initial conditions for dipole evolution beyond the McLerran-Venugopalan model

TL;DR

This paper derives an improved scattering amplitude for QCD dipoles on dense targets, incorporating subleading corrections and predicting A-dependence, which refines understanding of dipole interactions beyond the McLerran-Venugopalan model.

Contribution

It introduces the first subleading correction to the dipole scattering amplitude in the semi-classical approximation, extending the McLerran-Venugopalan model.

Findings

N(r) matches phenomenological fits over broad dipole sizes

Predicts A-dependence for heavy-ion targets

Shows suppression of N(r) for finite A at small dipole sizes

Abstract

We derive the scattering amplitude N(r) for a QCD dipole on a dense target in the semi-classical approximation. We include the first subleading correction in the target thickness arising from ~\rho^4 operators in the effective action for the large-x valence charges. Our result for N(r) can be matched to a phenomenological proton fit by Albacete et al over a broad range of dipole sizes r and provides a definite prediction for the A-dependence for heavy-ion targets. We find a suppression of N(r) for finite A for dipole sizes a few times smaller than the inverse saturation scale, corresponding to a suppression of the classical bremsstrahlung tail.