At CERN's Large Hadron Collider in Geneva, physicists have accomplished something once thought impossible: recreating the primordial state of matter that filled the universe just microseconds after the Big Bang — using atomic nuclei far smaller than anyone believed would work.
An international team led by researchers from the Niels Bohr Institute at the University of Copenhagen smashed oxygen-16 and neon-20 nuclei together at nearly the speed of light. These nuclei, containing just 16 and 20 nucleons respectively, are a fraction the size of the heavy lead nuclei that scientists traditionally use for such experiments. Despite their small size, the collisions produced unmistakable signatures of quark-gluon plasma: an ultra-hot, ultra-dense state of matter in which quarks and gluons — the fundamental building blocks of protons and neutrons — move freely rather than being bound together.
"We have pushed the boundary for how small the atomic nuclei can be while still recreating this primordial matter — what you could call a Little Big Bang," said Associate Professor You Zhou, who led the experiment.
The research, published in Physical Review Letters, revealed something unexpected. The particle flow patterns emerging from the collisions preserve information about the geometric shape of the nuclei that created them. Collisions between two spherical oxygen nuclei produce a relatively round pattern, while neon collisions — whose nuclei are shaped somewhat like bowling pins — generate a distinctive asymmetric flow.
"The particles from the primordial matter are directly governed by the geometric shape of the atomic nucleus," explained co-author Emil Gorm Dahlbæk Nielsen. "It is a bit like shining light on an object and seeing its shadow. You cannot see the object directly, but its shadow reveals its shape."
This technique could represent a paradigm shift in nuclear physics. For over 70 years, physicists have probed the structure of atomic nuclei using low-energy experiments — measuring how nuclei rotate and vibrate. The new approach reverses this strategy entirely: instead of gently probing nuclei, researchers smash them at the highest available energies and reconstruct their shapes from the debris patterns left behind.
The team plans to push further, conducting collisions with even lighter nuclei — including helium-4 — to determine exactly how small a collision system can be while still producing quark-gluon plasma. Answering that question could illuminate both the strong nuclear force and the conditions that prevailed during the universe's first millionth of a second.
