Deep inside the Large Hadron Collider, lead nuclei collide with such force that, for an instant, matter hotter than the Sun is born. This is the quark-gluon plasma — a soup of the tiniest building blocks of matter that existed only in the first microseconds after the Big Bang. Physicists from the ALICE experiment made this plasma reveal itself: they tracked messenger particles flying out of the fireball.

The main messenger was the neutral pion — a light particle that immediately decays into two photons. These photons are caught by a special calorimeter, like a net for light. Alongside the pion, physicists looked for other charged particles that flew out in the same direction or in the opposite one. It turned out that if the pion flies along the collision plane, its companions are clearly visible. But if it flies across, there are noticeably fewer of them near the pion. The plasma, like thick jelly, slows down particles flying through its bulk more strongly.

But the most interesting part came next. When the pion's companions had high energy, above 3 gigaelectronvolts, the difference between directions disappeared. This means that the fastest particles pierce the plasma almost without loss, like a bullet through fog. Theorists compared the data with the JEWEL model, which describes energy loss in an expanding medium. The model predicted that there should be no difference between directions at all. But the experiment saw one. So, there must be some additional quenching mechanism in the plasma that we don't yet know about.

Thus, the plasma born for an instant in the accelerator tells us how matter cooled after the Big Bang. Each caught pion is a step toward understanding why our world consists of ordinary matter, not a quark soup.