When two lead nuclei collide at tremendous speed, a cloud of scorching matter is born for an instant — denser and hotter than the center of the Sun. Through this cloud fly jets of particles: narrow, laser-like beams knocked out by the impact. The CMS detector captures their tracks and shows how the jets change as they fight through the hot 'fog'.
Scientists measured how widely the jet particles spread in two directions: along the beam axis and across it. They found that in more head-on collisions, when the nuclei overlap more, the jet spreads wider, with the broadening along the beam axis stronger than across it. A flashlight beam passing through thick smoke smears into a long patch, just like here.
This behavior of jets is direct evidence that they lose energy as they shove through the hot matter. Comparison with proton collisions, where there is no hot medium, confirms that the broadening arises from interaction with the cloud, not from the jets' own properties.
The temperature of the matter created in lead-nucleus collisions is 100,000 times higher than the temperature of the solar core — about 5 trillion degrees Celsius.
In Vernor Vinge's novel 'A Fire Upon the Deep,' a hot plasma cloud hinders ships passing through it, just like jets in the leaden haze.