{"schema":"pubphys.bundle/1","record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"83e35c6c284e637cba7151466b364fd3bf1283f508e79a00a43b62d35ca97765","created":"2026-10-03T07:18:07Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["4a61b784b6c63b0c31f85a77b65c0981a68c63a042a677b7b7e4068e86bbe785","5475cf5dc5b1a72283d26dc95dc6e00d7ea4e00096a7c6492e409a1816eeb011"],"salt":"9f86b1260a2c41d5476fd9c493872d29fdef7d5f6e1ee67a55338e3815c68e54","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"mechanism","assisted_by":[],"external_id":"nuc.gluon-saturation.coherent-vector-meson","kind":"phenomenon","literature_status":"open","n":"1","parents":[{"note":"","parent_revision":"4a61b784b6c63b0c31f85a77b65c0981a68c63a042a677b7b7e4068e86bbe785","relation":"special_case"}],"plain":"In near-miss collisions of lead nuclei, a photon from one nucleus can knock a $J/\\psi$ particle out of the other while leaving it intact, and the rate measures the gluon density. The measured rate is strongly reduced at very small momentum fractions, and whether this is saturation or a milder linear effect is unclear.","posed_since":"","precise":"In ultraperipheral $\\mathrm{Pb}+\\mathrm{Pb}$ collisions at $\\sqrt{s_{NN}} = 5.02\\,\\mathrm{TeV}$, CMS and ALICE extracted the coherent $J/\\psi$ photonuclear cross section down to $x \\sim 1e-5$ using forward-neutron tagging and found a suppression relative to the impulse approximation that persists or flattens at the smallest $x$. Determine whether leading-twist nuclear shadowing, saturation in a dipole picture with nonlinear evolution, or another mechanism quantitatively accounts for the W_gammaN dependence and the $\\mid t \\mid$ spectrum.","problem_ref":null,"references":"","settled_by":"A model that reproduces the energy dependence of coherent and incoherent $J/\\psi$ and $\\psi(2\\mathrm{S})$ cross sections on Pb and their $\\mid t\\mid$ distributions, with predictions confirmed at the Electron-Ion Collider.","status_note":"The 2023 CMS and ALICE neutron-tagged measurements removed the ambiguity over which nucleus emitted the photon, and no model describes the full $x$ dependence down to $x \\sim 1e-5$.","title":"What suppresses coherent $J/\\psi$ photoproduction on lead at small 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