In collisions of heavy nuclei, a fireball of thousands of particles is born. Among them are pions and kaons, two kinds of light particles that look very much alike. To understand what happens in this fireball, physicists need to know exactly how many of each particle flew out. But at high energies, old ways of telling them apart start to get confused: signals from pions and kaons overlap like two similar shades on a palette.
The new method works like rotating a sheet of paper with two axes. For each particle, two characteristics are measured: time of flight and energy loss due to ionization. Previously, these were considered separately, and at high speeds they merged. Now scientists take both quantities together and rotate them in space to find the best separation. It's like finding the right viewing angle for two nearly identical coins.
This trick allowed reliable separation of pions and kaons up to a momentum of 3 GeV/c — three times higher than with conventional methods. The physicists also verified that the new approach does not distort an important characteristic — the elliptic flow, which reveals the shape and dynamics of the fireball. Everything matched.
Now researchers have a tool to peer into the very heart of collisions and measure more precisely the properties of matter that existed in the first moments after the Big Bang.
The pion is the lightest particle among those that carry the nuclear force. Its mass is about 270 times that of the electron, yet it is still several times lighter than the proton.