{"schema":"pubphys.bundle/1","record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"3cf0d06e5eb278f15c4ddf44ccfb302fe59244ccb03cbf6bad65da0359868929","created":"2026-10-03T07:18:07Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["5475cf5dc5b1a72283d26dc95dc6e00d7ea4e00096a7c6492e409a1816eeb011"],"salt":"7d2969ab5eb585257dfb7300b947b08f1b6cb5bc7d2000ad5b99137f2c9a2c1b","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":"1","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 $x < 1e-3$ 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.","problem_ref":null,"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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