In the first few instants of the universe, all matter was in a peculiar state known as a quark-gluon plasma. It is a weird, extremely high-energy liquid state and so far it has only been recreated in the most powerful particle accelerator on Earth by smashing heavy nuclei together.
Researchers have now created the smallest samples of this ultra-hot matter yet, and discovered that these tiny droplets can form three distinct geometric shapes as they expand: circles, ellipses, and triangles. The findings are reported in the journal Nature Physics.
Quarks are the fundamental components of protons and neutrons, which are kept together by a strong nuclear force. This force is mediated by particles known as gluons, so named because they act as glue and keep atoms together.
The quark-gluon plasma is often seen as a perfect fluid. If you could observe it flowing without complex machinery, you’d be surprised to discover that it flows around objects with almost no friction. This peculiarity pushed researchers to ask how small a droplet of this fluid can be.
Using the PHENIX experiment at the massive collider at the Brookhaven National Laboratory in Upton, New York, researchers smashed gold atoms with protons, deuterium nuclei, which contain one proton and one neutron, and helium-3 nuclei, which contain one neutron and two protons. The resulting droplets of quark-gluon plasma had circular, elliptical, and triangular configurations respectively.
“Imagine that you have two droplets that are expanding into a vacuum. If the two droplets are really close together, then as they’re expanding out, they run into each other and push against each other, and that’s what creates this pattern,” Professor Jamie Nagle, from the University of Colorado Boulder, said in a statement.
The patterns are created by the interaction between the fundamental components of protons and neutrons. Like ripples on the surface of a lake, the different shapes depend on how many “pebbles” have been thrown in.
“Our experimental result has brought us much closer to answering the question of what is the smallest amount of early universe matter that can exist,” added Nagle, who proposed this set of experiments in 2014 along with his colleagues.
A next-generation experiment, known as PHENIX, is now being built and the team hopes to find out whether it's possible to make even smaller droplets of this bizarre state of matter.