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Title: Does gluon density saturate at small momentum fraction?
Plain: The gluon density inside protons and nuclei rises steeply as the momentum fraction x shrinks, and theory says gluon recombination must eventually slow it. No measurement has yet shown this slowing in a way that ordinary, linear evolution of the gluon distribution cannot also explain.
Precise: Establish whether the growth of the gluon distribution $x\,g(x, Q^2)$ in protons and heavy nuclei at $x < 1e-3$ and $Q^2$ of a few $\mathrm{GeV}^2$ is modified by nonlinear recombination, as described by the Balitsky-Kovchegov and JIMWLK evolution equations, with a saturation scale $Q_s^2(x) \sim A^{1/3} x^{-\lambda}, \lambda \sim 0.3$. A yes requires an observable whose measured A and x dependence is reproduced by nonlinear evolution and excluded, at high significance, by linear DGLAP evolution with fitted nuclear parton distributions.
Settled by: Electron-nucleus data on diffractive-to-inclusive ratios, the longitudinal structure function and dihadron correlations, versus A and x, that linear evolution with nuclear parton distributions cannot fit.
Kind: well-posed
Answer type: yes-no
References: (empty)
Literature status: contested
Status note: Forward dihadron and coherent $J/\psi$ data from RHIC and the LHC (2022-2023) are consistent with saturation, but linear-evolution and cold-nuclear explanations have not been excluded.
Posed since: (empty)