{"schema":"pubphys.bundle/1","record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"677443d44b087a2cee3119007e0d5ddf7823318145dde78157ff046a5cea4174","created":"2026-10-15T23:43:22Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["4a61b784b6c63b0c31f85a77b65c0981a68c63a042a677b7b7e4068e86bbe785","ce59fb6cd812fc05906d0c60a71c75ea22bc686bcf38e7c3ef7010834d98ee7b"],"salt":"5d42b2b142885b55362d151997208612f7e999064bedd8be61f79c873037f327","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"yes-no","assisted_by":[],"external_id":"nuc.gluon-saturation.saturation-existence","kind":"well-posed","literature_status":"contested","n":"2","parents":[],"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.","posed_since":"","precise":"Establish whether the growth of the gluon distribution $x\\,g(x, Q^2)$ in protons and heavy nuclei at $x < 10^{-3}$ 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.","problem_ref":"4a61b784b6c63b0c31f85a77b65c0981a68c63a042a677b7b7e4068e86bbe785","references":"","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.","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.","title":"Does gluon density saturate at small momentum 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