Unitarity bound for gluon shadowing

TL;DR

This paper establishes a model-independent lower bound on gluon density in nuclei at small x, compares it with predictions from the Balitsky-Kovchegov equation, and discusses implications of recent experimental data suggesting unexpectedly strong gluon shadowing.

Contribution

It introduces a unitarity bound for gluon shadowing in nuclei and highlights discrepancies with recent experimental analyses, challenging current interpretations.

Findings

Gluon density cannot fall below the unitarity bound at small x.

Balitsky-Kovchegov equation predictions are above the unitarity bound.

Recent data suggests gluon shadowing is stronger than the unitarity limit.

Abstract

Although at small Bjorken x gluons originated from different nucleons in a nucleus overlap in the longitudinal direction, most of them are still well separated in the transverse plane, therefore cannot fuse. For this reason the gluon density in nuclei cannot drop at small x below a certain bottom bound, which we evaluated in a model independent manner assuming the maximal strength of gluon fusion. We also calculated gluon shadowing in the saturated regime using on the Balitsky-Kovchegov equation, and found the nuclear ratio to be well above the unitarity bound. The recently updated analysis of parton distributions in nuclei \cite{eps08} including RHIC data on high-pT hadron production at forward rapidities, led to astonishingly strong gluon shadowing, which is far beyond the unitarity bound. This indicates a misconception in the interpretation of the nuclear suppression observed at HRIC.