The Large Hadron Collider doesn't just smash protons: it also collides entire atomic nuclei. When two nuclei smash together at tremendous speed, they create a tiny blob of matter so hot that protons and neutrons melt into a 'soup' of quarks and gluons. Fast particles have to plow through this soup, like bullets through water.

Each particle loses energy along the way, just like a bullet flying through water: the longer the path, the stronger the drag. Physicists from the CMS collaboration at the LHC have for the first time measured how this drag depends on the size of the colliding nuclei. They compared collisions of neon, oxygen, xenon, and lead nuclei.

The energy loss grows smoothly with the size of the nucleus, as if the particle were pushing through a longer and longer stretch of thick 'soup.' Simple models that only consider the initial state of the nuclei cannot explain these losses. But models where the particle loses energy along the way describe the data beautifully at high particle energies.

This result is another step toward understanding the properties of the hottest matter that can be created on Earth. It resembles the medium that filled the Universe in the first microseconds after the Big Bang.