Figure 2: (Left) Example of a jet without a visible partner. (Right) Asymmetric jets (where one jet loses most of its energy) are rare in proton-proton collisions, but the ATLAS measurements showed such events occur with a high probability in lead-lead collisions. The asymmetry $A_j$ for two jets with energy $E_1$ and $E_2$ is defined as $A_j=(E_1-E_2)/(E_1+E_2)$.