Tracking fast small color dipoles through strong gluon fields at the LHC

Understand

We argue that the process gamma +A \to J/psi + gap + X at large momentum transfer provides a quick and effective way to test 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 to study this reaction with sufficient statistics via ultraperipheral collisions hence probing the interaction of q\bar q dipoles of sizes ~ 0.2 fm with nuclear media down to x ~ 10^{-5}.