Physicists at CERN have found a new way to peer deep inside atomic nuclei and distinguish between two competing explanations for how gluons behave. Using the ALICE experiment at the Large Hadron Collider, researchers measured particle production at unprecedented spatial resolution, revealing structures as small as about one-quarter the size of a proton. At the smallest scales, they saw a surprising drop in J/ψ production that conventional “nuclear shadowing” struggles to explain.