CERN collisions reveal primordial matter and neon’s unusual nuclear shape

Researchers at CERN have shown that collisions between relatively light atomic nuclei can create tiny droplets of quark-gluon plasma, while also revealing clues about the nuclei’s hidden geometry. The peer-reviewed findings extend the study of this primordial state of matter beyond the heavy nuclei, such as lead, once thought necessary to produce it.

The team accelerated oxygen-16 and neon-20 nuclei to nearly the speed of light, then compared separate oxygen-oxygen and neon-neon collisions. Quark-gluon plasma, an extremely hot state in which quarks and gluons move freely, is believed to have filled the Universe during its first millionth of a second.

Because each droplet exists for only a tiny fraction of a second, researchers studied the particles produced as it expanded and cooled. Their movement retained an imprint of the original collision geometry. Neon collisions generated patterns consistent with an elongated, bowling-pin-like nucleus, distinct from the more rounded pattern produced by oxygen collisions.

The result is an inference rather than a direct image, and measured triangular-flow changes did not quantitatively match model predictions, so further work is needed. Even so, the findings demonstrate that high-energy particle flow is sensitive to nuclear structure. The researchers now plan experiments with still lighter nuclei, including helium-4, to explore how small a system can be while still forming quark-gluon plasma.