Only 3 weeks into its first LHC heavy-ion run, ATLAS observed an unexpectedly large imbalance of energy in pairs of jets created in lead-ion collisions at the LHC. This striking effect, which is not seen in proton–proton collisions, could be a sign of strong interactions between jets and a hot, dense medium (quark-gluon plasma) formed by the colliding ions. Details studies of the effect followed this first observation.