At accelerators, by smashing heavy nuclei, physicists briefly create a quark-gluon plasma — a state of matter where quarks and gluons are not confined inside particles but are free. For a long time, it was imagined as a gas, but experiments showed it's an almost perfect liquid. Particles in it move in concert, like molecules in honey.
Surprisingly, inside an ordinary proton, the same quarks and gluons, the energy density and pressure are comparable to the plasma. That means the proton isn't fluffy inside but dense and strongly coupled. This sparks an idea: if in atomic nuclei, neutrons and protons often form close pairs (short-range correlations), then quarks in a proton might also stick in pairs. The pair of up and down quarks should be especially tight: their attraction in quantum chromodynamics is stronger.
To test this, plans are to use the Electron-Ion Collider (EIC)—a future accelerator where electrons will probe protons. The idea is simple: smash a proton with an electron and watch the debris. If quarks inside are paired, then upon impact, one quark will fly out as a particle jet, and its partner as a pion. Such events would allow us to see quark pair bonds for the first time, and understand how the liquid emerges in the plasma.
This approach complements the usual methods where the proton is described via single parton distributions. Searching for pairs will open a new perspective on strong interactions and might explain why the hottest liquid in the universe flows so smoothly.
Quarks never appear alone: they are always confined within particles. Even in a plasma, they stay close, like dancers in a crowded hall.
In Bruce Sterling's novel Schismatrix, a sentient civilization lives inside the quark-gluon plasma of a neutron star — where our protons would be mere ripples.