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Its actual value for gold or lead is uncertain by a large factor.","posed_since":"","precise":"Determine $Q_s^{2}(x)$ for Au and Pb at $x=10^{-3}\\ \\text{and}\\ 10^{-4}$ with uncertainty below 30 percent, including the actual A dependence (versus the naive $A^{1/3}$ scaling) and the impact-parameter profile. Estimates of order $1\\ \\text{to}\\ 2\\ \\mathrm{GeV}^{2}$ at $x\\sim 10^{-3}$ depend on the dipole model and the definition of $Q_s$; the answer must state the definition and the data used.","problem_ref":"8b3ddc17469d58250b5e83400f6f58f243d2fa0883803f18a3f20d96cd0bad2a","references":"","settled_by":"A global fit of small-$x$ electron-nucleus and proton-nucleus data with nonlinear evolution, including running coupling and impact-parameter dependence, that returns $Q_s^2$ with quantified uncertainty.","status_note":"","title":"Size of the saturation scale in heavy 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