Researchers at Brookhaven National Laboratory's RHIC particle accelerator have determined that an exotic form of matter produced in their collisions is the most rapidly spinning material ever detected. The material is called a quark-gluon plasma, and it provides us an opportunity to study the state that all matter was in immediately after the Big Bang.

The fact that the quark-gluon plasma spins provides us with an opportunity to study some theoretical ideas about the behavior of the strong force, one of the fundamental forces of nature that's responsible for holding together the matter that we see around us.

The force is strong in these collisions

Brookhaven's Relativistic Heavy Ion Collider (RHIC) and CERN's Large Hadron Collider are the only facilities that can reach energies high enough to produce a quark-gluon plasma. Quarks are the building blocks of the heavier components of atoms; both protons and neutrons consist of three quarks bundled together. Gluons are the particles that hold them together in that bundle. Their interactions are governed by the strong force, and the rules of those interactions are described by a theory called quantum chromodynamics (often simply called QCD).

Those rules dictate that the only stable combination involves three quarks. We've generated other particles with two, four, and even five quarks, but they all decay rapidly.

Collisions with heavy ions—typically gold or lead—put lots of protons and neutrons in a small volume with lots of energy. Under these conditions, the neat boundaries of those particles break down. For a brief instant, quarks and gluons mingle freely, creating a quark-gluon plasma. This state of matter has not been seen since an instant after the Big Bang, and it has plenty of unusual properties. "It has all sorts of superlatives," Ohio State physicist Mike Lisa told Ars. "It is the most easily flowing fluid in nature. It's highly explosive, much more than a supernova. It's hotter than any fluid that's known in nature."

While confirming that the quark-gluon plasma existed was a major accomplishment, studying it can provide fundamental insights about the basic properties of matter. "The quark-gluon plasma, fascinating though it is," Lisa said, "is a tool to try to understand quantum chromodynamics, which is the theory of the strong force."

Put a spin on it

We can now add another superlative to the quark-gluon plasma's list of "mosts:" it can be the most rapidly spinning fluid we know of. Much of the study of the material has focused on the results of two heavy ions smacking each other head-on, since that puts the most energy into the resulting debris, and these collisions spit the most particles out. But in many collisions, the two ions don't hit each other head-on—they strike a more glancing blow.

Because the two ions are traveling in opposite directions at nearly the speed of light, the resulting quark-gluon plasma should inherit a great deal of off-axis force. And that, in turn, should send the plasma spinning.

But a logical "should" doesn't always equal a "does," so it's important to confirm that the resulting material is actually spinning. And that's a rather large technical challenge when you're talking about a glob of material roughly the same size as an atomic nucleus.

To make their measurements, the Brookhaven researchers relied on what happens as the quark-gluon plasma expands and cools. As it does, its quarks and gluons condense into familiar particles, including protons, neutrons, and various unstable quark combinations. These particles all have spin, and that spin will be inherited from any spin possessed by the quark-gluon plasma that's producing them.

Of course, those particles come out of the quark-gluon plasma as a chaotic, energetic spray, so measuring their spins would be a serious challenge. Conveniently, there's a type of particle called a "Λ hyperon," which is essentially a proton with one of its up quarks replaced by a strange quark. The strange quark can decay in a way that converts this into a proton, which can be spotted using the detector hardware at the Relativistic Heavy Ion Collider. Conveniently, any protons produced by this process tend to be emitted along the same direction as the spin axis of the Λ hyperon. So, by tracking these protons, the team behind Brookhaven's STAR detector was able to get some measure of the quark-gluon plasma's rotation.

This measure, called the vorticity, provides a measure of how rapidly a fluid is rotating. And, for the quark-gluon plasma, the answer is "very." The answer the researchers came up with is 1021 per second. For comparison, the vorticity of tiny droplets of a superfluid tops out at about 107 per second. Something like a supercell tornado only reaches 0.1 per second. So the quark-gluon plasma is a staggeringly large step up in terms of vorticity.

Probing QCD

Not only is that measurement an achievement in its own right, it provides us with a new chance to study quantum chromodynamics, the rules that hold together most of the matter around us. One of the ways to study this is to look at how the rules change when matter undergoes a transition, like the one from protons and neutrons to a quark-gluon plasma.

The spin of the quark gluon plasma can provide another opportunity to study a transition between different sets of rules of quantum chromodynamics. Under quantum chromodynamics, there would be a fundamental symmetry between left- and right-handed quarks (the handedness relates to the combination of their spin and momentum). But the symmetry is broken if the quarks have mass—which they do.

Since we can't make a massless quark, there's only one other way to restore the symmetry: raise the quarks to a higher energy. "If the temperature is high enough, regions of space can transform from one configuration of gluon fields to another," Lisa told Ars. He said many scientists think that the energies reached in the Relativistic Heavy Ion Collider are high enough to explore what are called quantum chromodynamics' chiral symmetries.

There are two symmetries. The chiral magnetic symmetry will be accessible if the quark-gluon plasma has a magnetic field—something that it's thought to likely have, but hasn't been measured yet. The other route is the chiral vortical symmetry, which requires that the plasma be rotating. We now know that, provided the collision isn't fully head-on, it is. "The transition from one [set of rules] to another is highly exotic and exciting—it would tell us something fundamental about QCD," Lisa said.

And the implications are large. Not only would it help us understand quantum chromodynamics better, but for a brief moment of time, the entire Universe was at high enough energies for this other set of rules to apply. In effect, we are repairing a symmetry that the Universe broke as it emerged from the Big Bang.

Nature, 2017. DOI: 10.1038/nature23004 (About DOIs).