Quark-Gluon Plasma From Small Nuclei: CERN Made It With Oxygen and Neon

Oxygen-16 carries 16 nucleons. Lead-208 carries 208. Since the Large Hadron Collider started producing quark-gluon plasma in 2010, the working assumption was that you needed the heavier of those two. CERN has now made the same plasma from small nuclei: oxygen-16 and neon-20.

Quark-gluon plasma from small nuclei lowers the entry price

Quark-gluon plasma is what matter becomes when protons and neutrons stop holding together. Push the energy density high enough and the quarks and gluons inside come loose and flow as a fluid with almost no viscosity. It is the state the universe was in for its first few microseconds, and until this summer the size of the colliding nuclei was thought to be the reason it forms at all.

The light-ion runs say otherwise. Oxygen and neon collisions produced the same collective flow signatures physicists use to identify the plasma in lead-ion data. Fewer nucleons, same soup.

The fireball remembers the shape it came from

This is the part worth sitting with. An oxygen-16 nucleus is close to spherical, so the debris from an oxygen collision sprays out in a rounded pattern. Neon-20 is shaped more like a bowling pin, with clustered alpha particles sitting in a line, and its debris comes out elongated to match.

The plasma lasts something in the region of ten to the minus twenty-three seconds before it cools and freezes back into ordinary hadrons. In that window it still encodes the geometry of the two nuclei that made it. In plain terms, physicists are reading the shape of an atomic nucleus off the pattern of its own annihilation.

Nuclear shape is normally inferred from low-energy spectroscopy, so this adds a second and completely independent way to measure it. The same trick shows up in reading vacuum structure off a magnetar and in pulling entanglement out of ordinary sunlight: let the extreme system do the measuring.

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The number nobody has yet is the lower bound. It is not clear how few nucleons you can collide before the plasma stops forming, or whether the cutoff is sharp or gradual. Proton-proton collisions have shown plasma-like flow for years without anyone being willing to call it plasma, and oxygen and neon sit in the gap between those two cases. That is why they were run.

The consequence is beam time. Light-ion runs are shorter and cheaper than lead-ion runs, so more plasma physics fits into an LHC schedule without pushing other work off it. A state of matter that needed the biggest hammer in the building now needs a smaller one, and groups that were never going to get lead beams can start asking for oxygen instead.