When two atomic nuclei smash together, a tiny fireball is born. It lives for an instant, but inside, entire epochs pass: compression, heating, expansion. Physicists have learned to eavesdrop on this process, and their "microphone" is the pairs of electrons and positrons that fly out of the ball at different stages.
In the new work, two scenarios were compared. In the first, the matter inside the ball rearranges smoothly, like butter softening in a pan. In the second, it changes abruptly, as if ice turns into water. The sharp transition adds an early burst of radiation, while the smooth one enhances the low-energy part later: the ball lives longer and heats itself.
The difference is clearly visible after averaging over the entire evolution. At low beam energies, the two scenarios diverge like branches of a tree. So by measuring the flow of such pairs at accelerators, one can precisely tell what happened to the matter in the first moments after the impact.
A positron is the twin of an electron, but with a positive charge. When it meets an ordinary electron, both vanish, turning into pure energy.