When two protons collide at enormous speed, a narrow beam of particles (a jet, like the spray from a stone thrown into water) shoots out from the point of impact. In collisions of heavy lead nuclei, things are more complicated: a hot cloud forms inside, which interferes with the jet's straight flight.
The measurement showed: if the jet is weak, its path barely changes. But if the jet is powerful, it noticeably deflects in the dense cloud. It's like a fire hose aimed through thick fog: the stronger the pressure, the more the stream shudders and veers.
Scientists compared the data with three different models. All of them describe how particles lose energy in the hot cloud. The best match is the one where the cloud behaves like a viscous liquid.
Such a jet deflection is a new tool. It allows peering inside the cloud without destroying it and understanding what it is made of.
The temperature of the hot cloud in such collisions is thousands of times higher than at the center of the Sun.