Physicists Recreate Early-Universe Matter Using Smaller Atomic Nuclei Than Ever Before

Researchers at the University of Copenhagen's Niels Bohr Institute, working with scientists at CERN in Switzerland, have recreated the primordial state of matter believed to have filled the universe shortly after the Big Bang, using collisions between atomic nuclei far smaller than scientists once thought possible. At CERN, atomic nuclei are accelerated to nearly the speed of light and smashed together, creating tiny droplets of quark-gluon plasma, the extraordinary state of matter believed to have filled the universe during its first millionth of a second.

Scientists had long thought producing this plasma required collisions between very heavy nuclei, such as lead. The new experiments show that much smaller nuclei can also generate it: researchers created the plasma by colliding oxygen-16 and neon-20 nuclei, published in the journal Physical Review Letters. "We have pushed the boundary for how small the atomic nuclei can be while still recreating this primordial matter," said Associate Professor You Zhou, who led the experiment.

The droplet of plasma survives only a fraction of a second before expanding into other particles, so scientists cannot observe it directly and instead measure how the resulting particles move. Those movement patterns preserve information about the shape of the colliding nuclei: collisions between two round oxygen nuclei generate a rounded pattern, while collisions involving the bowling-pin-shaped neon nucleus create a distinctive bowling-pin pattern in the particles produced. "It is a bit like shining light on an object and seeing its shadow," said postdoctoral researcher Emil Gorm Dahlbæk Nielsen, a co-author of the study. "The movement of the particles reveals the geometric shape of the atomic nuclei that was present at the beginning of the collision."

The technique reverses the traditional approach to studying nuclear structure, which relied on relatively low-energy measurements of how nuclei rotate and vibrate; instead, researchers collide nuclei at the highest energies available and reconstruct their shapes from the patterns left behind. The team next plans to test even lighter nuclei, including helium-4, to determine how small a collision system can become while still producing the plasma.