b-CGC versus IP-Sat and high precision combined HERA data

TL;DR

This paper compares the b-CGC and IP-Sat dipole models of small-x physics against high-precision HERA data, highlighting their similarities, differences, and predictions for future high-energy regimes.

Contribution

It provides a detailed confrontation of the impact-parameter dependent b-CGC and IP-Sat models with recent HERA data, analyzing their predictions and differences at low x and high Q^2.

Findings

Both models reproduce most HERA DIS and diffractive data.

The impact-parameter probed is about 2-3 GeV^{-1}.

Predictions diverge at very low x and high Q^2.

Abstract

The Impact-Parameter dependent Color Glass Condensate (b-CGC) and Saturation (IP-Sat) dipole models incorporate key features of small-x physics properties and match smoothly to the perturbative QCD regime at large $Q^2$ for a given $x$. Although both models include saturation effects and depend on impact-parameter, the former is based on the non-linear BK equation, while the latter is based on DGLAP evolution. After confronting the models to the recently released high precision combined HERA data, we show that in both models, the typical impact-parameter probed in the total $\gamma^{*}p$ cross-section is about $b\approx 2\div 3\,\text{GeV}^{-1}$ and the proton saturation scale is $Q_S<1$ GeV in HERA kinematics. We show that most features of inclusive DIS and exclusive diffractive data at HERA are correctly reproduced in both models. Nevertheless, the b-CGC and the IP-Sat models give significantly different predictions beyond the current HERA kinematics for the structure functions at very low x and high virtualities $Q^2$, and for the exclusive diffractive vector meson and DVCS production at high $t$.