Nu Xu

Ten microseconds after the Big Bang, the Universe resembled a pot of boiling broth. Floating in it were quarks and gluons — the particles that would later assemble into protons and neutrons. As the broth cooled, at some point the quarks clumped together into lumps — the very protons and neutrons that make up all visible matter.

When two heavy nuclei smash into each other at enormous speeds, they don't collide as whole objects like billiard balls. Only part of their matter comes into contact. How many nucleons (protons and neutrons) actually took part in the collision, and how many flew past, is not known in advance. All subsequent measurements depend on this number, but it cannot be observed directly.