Signatures of gluon saturation from structure-function measurements

TL;DR

This paper investigates signals of gluon saturation in deep inelastic scattering by comparing linear and nonlinear QCD evolution models, identifying measurable deviations in structure functions at future collider energies.

Contribution

It provides a quantitative method to distinguish nonlinear gluon saturation effects from linear evolution in structure function measurements.

Findings

Deviations in $F_2$ are a few percent for protons and up to 10% for nuclei.

Deviations in $F_L$ can reach 40% at LHeC and 10% at EIC for protons.

Differences are larger for heavy nuclei, especially in $F_L$, indicating stronger saturation effects.

Abstract

We study experimentally observable signals for nonlinear QCD dynamics in deep inelastic scattering (DIS) at small Bjorken variable $x$ and moderate virtuality $Q^2$, by quantifying differences between the linear Dokshitzer-Gribov-Lipatov-Altarelli-Parisi (DGLAP) evolution and nonlinear evolution with the Balitsky-Kovchegov (BK) equation. To remove the effect of the parametrization freedom in the initial conditions of both equations, we first match the predictions for the DIS structure functions $F_2$ and $F_{\rm L}$ from both frameworks in a region in $x,Q^2$ where both frameworks should provide an accurate description of the relevant physics. The differences in the dynamics are then quantified by the deviations when one moves away from this matching region. For free protons we find that the differences in $F_2$ remain at a few-percent level, while in $F_{\rm L}$ the deviations are larger, up to $10\,\%$ at the EIC and $40\,\%$ at the LHeC kinematics. With a heavy nucleus the differences are up to $10\,\%$ in $F_2$, and can reach $20\,\%$ and $60\,\%$ in $F_{\rm L}$ for the EIC and the LHeC, respectively.