Physicists Create Quark-Gluon Plasma Droplets

Physicists from the PHENIX Collaboration have created droplets of a liquid-like state of matter called quark-gluon plasma, forming three distinct shapes and sizes — circles, ellipses and triangles.

Visualization of an expanding drop of quark-gluon plasma. Image credit: Javier Orjuela Koop, University of Colorado, Boulder.

Scientists believe that quark-gluon plasma filled the entire Universe during the first few microseconds after the Big Bang when the Universe was still too hot for particles to come together to make atoms.

The PHENIX team used the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory to recreate that matter.

In a series of tests, the physicists smashed packets of small projectiles in different combinations (single protons, two-particle deuterons, and three-particle helium-3 nuclei) into much bigger gold nuclei.

“RHIC is the only accelerator in the world where we can perform such a tightly controlled experiment, colliding particles made of one, two, and three components with the same larger nucleus, gold, all at the same energy,” said PHENIX team member Professor Jamie Nagle, a researcher at the University of Colorado, Boulder.

The scientists discovered that, by carefully controlling conditions, they could generate droplets of quark-gluon plasma that expanded to form three different geometric patterns.

If collisions between small projectiles — protons (p), deuterons (d), and helium-3 nuclei (³He) — and gold nuclei (Au) create tiny hot spots of quark-gluon plasma, the pattern of particles picked up by the detector should retain some ‘memory’ of each projectile’s initial shape. Measurements from the PHENIX experiment match these predictions with very strong correlations between the initial geometry and the final flow patterns. Image credit: Javier Orjuela Koop, University of Colorado, Boulder.

“Imagine that you have two droplets that are expanding into a vacuum,” Professor Nagle said.

“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.”

“In other words, if you toss two stones into a pond close together, the ripples from those impacts will flow into each other, forming a pattern that resembles an ellipse.”

“The same could be true if you smashed a proton-neutron pair, called a deuteron, into something bigger.”

“Likewise, a proton-proton-neutron trio, also known as a helium-3 atom, might expand out into something akin to a triangle.”

And that’s exactly what the PHENIX researchers found: collisions of deuterons formed short-lasting ellipses, helium-3 atoms formed triangles and a single proton exploded in the shape of a circle.

The results, published in the journal Nature Physics, could help theorists better understand how the Universe’s original quark-gluon plasma cooled over milliseconds, giving birth to the first atoms in existence.

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C. Aidala et al (PHENIX Collaboration). Creation of quark–gluon plasma droplets with three distinct geometries. Nature Physics, published online December 10, 2018; doi: 10.1038/s41567-018-0360-0