The matter, known as a quark-gluon plasma (or QGP), is predicted to exist when temperatures and densities are so extreme that regular matter cannot exist. Instead, a “perfect liquid” exists for a short time before it cools and condenses into the regular stuff that forms the building blocks of matter.
Although physicists have announced the detection of this exotic state of matter before, new results from the Relativistic Heavy Ion Collider (RHIC) at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory, in Upton, New York, appear to show the tiniest droplets of quark-gluon plasma appear, in a specific pattern, after colliding helium-3 nuclei with gold ions.
“These tiny droplets of quark-gluon plasma were at first an intriguing surprise,” said Berndt Mueller, Associate Laboratory Director for Nuclear and Particle Physics at Brookhaven, in a statement. “Physicists initially thought that only the nuclei of large atoms such as gold would have enough matter and energy to set free the quark and gluon building blocks that make up protons and neutrons. But the flow patterns detected by RHIC’s PHENIX (Pioneering High Energy Nuclear Interaction eXperiment) collaboration in collisions of helium-3 nuclei with gold ions now confirm that these smaller particles are creating tiny samples of perfect liquid QGP.”
Experiments at RHIC and the Large Hadron Collider (LHC), near Geneva, Switzerland, have been chasing the formation of this primordial state of matter for some time. In 2013, LHC physicists also announced the discovery of these quark-gluon plasma droplets after slamming protons into lead ions.
But this is the first time that helium-3, a light ion, has been collided with heavy ions (gold), producing the signature of quark-gluon plasma. This indicates that the stuff can be produced at lower energies, opening a fascinating opportunity to study this quantum ‘goo’ that last existed in nature in the first moments of the birth of our universe, some 13.8 billion years ago.
And the initial results seem to show these tiny droplets act as predicted — like a perfect, frictionless liquid.
“The idea that collisions of small particles with larger nuclei might create minute droplets of primordial quark-gluon plasma has guided a series of experiments to test this idea and alternative explanations, and stimulated a rich debate about the implications of these findings,” added physicist Jamie Nagle, of the University of Colorado and co-spokesperson of the PHENIX collaboration at RHIC. “These experiments are revealing the key elements required for creating quark-gluon plasma and could also offer insight into the initial state characteristics of the colliding particles.”
The discovery of a “perfect liquid” stemming from the collision of heavy ions in RHIC was first announced in 2005. Post-collision analysis seemed to show a collective “flow” of matter erupt from the intense flash of energy. This finding was inconsistent with the uniform expansion of a gaseous state of matter, so high-energy physicists realized that they were looking at a new state of matter, composed of quarks (the subatomic building blocks of protons and neutrons) and gluons (a particle, or “boson”, that carries the strong nuclear force) that acts as a perfect liquid. Since these initial discoveries, physicists have refined their accelerator experiments, colliding different ions together, producing different configurations of the quark-gluon plasma.
In this helium-3 experiment, the helium-3 ion (containing 2 protons and 1 neutron) collided with a gold ion. The PHENIX detector picked up a triangular pattern emerge from the collision, each point of the triangle representing 3 tiny hotspots, each one believed to be the scrambled remains of the helium-3′s 2 protons and 1 neutron. And these hotspots behaved just as a quark-gluon should — like a perfect liquid.
These results back up findings from 2008 RHIC experiments into deuterium (containing 1 proton and 1 neutron) collisions with gold ions. In this case, 2 hotspots were formed during the collision, inspiring the helium-3 experiment to see if the shape of the quark-gluon plasma droplets is influenced by the colliding ions. This is a conclusion that appears to be valid.
“This is a pretty definitive measurement,” Nagle said. “The paper has a plot of elliptical and triangular flow that pretty much matches the hydrodynamic flow calculations we’d expect for QGP. We are really engineering different shapes of the QGP to manipulate it and see how it behaves.”
Once again, the power and precision of RHIC (and its sister ion-colliding accelerator, the LHC) has proven itself not only to understand this extreme state of matter, but it’s also acting as a time machine, peeling back the nature of our universe to the first moments of its existence after the Big Seed.