Credit: G. Aad et al., [2]

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 $Aj$ for two jets with energy $E1$ and $E2$ is defined as $Aj=(E1-E2)/(E1+E2)$.