We argue that the process $\gamma +A\to J/\psi +“\mathrm{\text{gap}}”+X$ at large momentum transfer $q^2$ provides a quick and effective way to test the onset of a novel perturbative QCD regime of strong absorption for the interaction of small dipoles at the collider energies. We find that already the first heavy-ion run at the LHC will allow one to study this reaction with sufficient statistics via ultraperipheral collisions, hence probing the interaction of $q\overline{q}$ dipoles of sizes $\sim 0.2\mathrm{fm}$ with nuclear media down to $x\sim {10}^{-5}$.

• Received 22 November 2008

DOI:https://doi.org/10.1103/PhysRevLett.102.232001