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.
as written by the authors arXiv:2603.14385
The longitudinal invariance of jet-induced peaks in two-particle correlation functions from relativistic lead-lead collisions is experimentally explored. The data were collected at a center-of-mass energy per nucleon pair of 5.02 TeV in 2018 using the CMS detector. The dataset corresponds to an integrated luminosity of 0.607 nb$$^{-1}$$. Long- and short-range correlations are studied through two-dimensional distributions of the separations in pseudorapidity and azimuth between particles in an event. Jets manifest as a well-defined peak at small angular separations, and the shape of this peak provides insight into jet medium interactions. This Letter examines the evolution of the jet peak shape, focusing on the dependence of its width and longitudinal asymmetry on the transverse momentum, collision centrality, and pseudorapidity of the associated charged particles. The jet-peak distributions of lower transverse momentum particles broaden in both pseudorapidity and azimuth with increasing collision overlap, with the broadening in pseudorapidity being more pronounced. The longitudinal asymmetry of the peaks is also found to increase as the average pseudorapidity increases. These results are compared to proton-proton collision data that were obtained at the same nucleon-nucleon collision center-of-mass energy with an integrated luminosity of 252 nb$$^{-1}$$.
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.