Physicists smashing gold nuclei together have spotted an unexpected dip in particle momentum fluctuations. The result, reported on September 22 in Physical Review Letters, may mark a long-sought turning point in how extreme nuclear matter changes form.

The signal comes from the STAR experiment at Brookhaven National Laboratory’s Relativistic Heavy Ion Collider in New York. Researchers measured a clear dip at lower collision energies, with a statistical significance of about 5 sigma.

Why does this matter? Protons and neutrons are built from quarks bound by gluons. Heat or squeeze them enough and they melt into a quark-gluon plasma, the hot soup thought to have filled the universe in its first milliseconds after the Big Bang.

The team calls it a strong hint, not proof, of a critical point where the transition between nuclear matter and plasma changes character. Like water’s boiling point on a phase diagram, it would be a basic rule of matter under extreme conditions.

Study co-author Rutik Manikandhan called the collisions a mini version of the Big Bang. Mapping this matter also helps physicists understand the insides of neutron stars.