Future Small x Physics with ep and eA Colliders

TL;DR

This paper explores small dipole interactions with nucleons and nuclei at high energies, emphasizing the potential of future ep and eA colliders to uncover nonlinear QCD phenomena, gluon shadowing, and diffraction effects.

Contribution

It provides model-independent predictions for nuclear shadowing and highlights the capabilities of eA collisions at HERA to probe nonlinear QCD and discover new phenomena.

Findings

Dipole interactions approach unitarity limits at high energies.

Gluon shadowing in nuclei is significantly larger than sea quark shadowing.

Future eA colliders can reveal nonlinear QCD effects and large gluon shadowing.

Abstract

The interaction of spatially small dipoles with nucleons, nuclei is calculated in the DGLAP approximation at the top of HERA energies and found to be close to the $S$-channel unitarity limit in the case of the color octet dipoles. The DGLAP analyses of the current diffractive data appear to support this conclusion as they indicate a $\sim 30-40%$ probability of the gluon induced diffraction for $Q^2\sim 4 $GeV$^2$. The need for the high-precision measurements of the $t$-dependence of inclusive and exclusive diffraction for pinpointing higher twist effects in the gluon sector is emphasized.The $eA$ collisions at HERA would provide a strong amplification of the gluon densities allowing to reach deep into the regime of nonlinear QCD evolution. Connection between the leading twist nuclear shadowing and leading twist diffraction in $ep$ scattering is explained. The presented model independent results for the nuclear shadowing for light nuclei indicate much larger shadowing for the gluon sector than for the sea quark sector.It is argued that HERA in $eA$ mode would be able to discover a number of new phenomena including large gluon shadowing, large nonlinearities in parton evolution, small $x$ color transparency in the vector meson production followed by color opacity at $x\le .005$, large probability of inclusive diffraction. Implications for the nucleus-nucleus collisions at LHC are discussed as well.