Size of the saturation scale in heavy nuclei
In plain words
The saturation scale is the momentum below which gluons in a nucleus are expected to be packed densely, and heavier nuclei should reach it sooner. Its actual value for gold or lead is uncertain by a large factor.
Precise statement
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.
What would settle it
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.
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