For a few millionths of a second shortly after the Big Bang, cosmologists believe that the Universe was filled with a hot dense ‘soup’ of quarks and gluons. Now researchers believe they may have used the collision of minuscule projectiles and gold nuclei to recreate tiny specks of this perfect primordial fluid.

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. (Javier Orjuela Koop, University of Colorado, Boulder)

The study of this fluid is expected to shed light on the force governing the binding of quarks and gluons, the fundamental particles which make up protons and neutrons and thus all the visible matter around us. But what researchers did not expect, is to be able to recreate this fluid of fundamental particles.

Nuclear physicists discovered the strange product whilst analyzing data from Brookhaven Lab’s PHENIX detector at the Relativistic Heavy Ion Collider (RHIC)publishing their findings this week in the journal Nature Physics.

Jamie Nagle, a PHENIX collaborator, helped devise the experimental plan as well as the theoretical simulations the team would use to test their results: “This work is the culmination of a series of experiments designed to engineer the shape of the quark-gluon plasma droplets.”

The discovery was made when the team examined the trajectories of particles created by the impact of small projectiles such as single protons, deuterium atoms and helium-3 nuclei on gold-nucleus “targets”. The flow-patterns of these particles were found to match the geometry of the original projectiles, exactly as would be expected if they were creating a perfect-fluid of quark-gluon plasma.

Nagle said: “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.”

Previous sightings of perfect liquids

The PHENIX detector at the Relativistic Heavy Ion Collider (Brookhaven National Lab)

Use of the RHIC, the world’s largest particle accelerator before the LHC was activated, has previously allowed physicists to observe the flow of perfect liquids before and their existence is well established. When the nuclei of gold particles collide at near-light speeds, for example, the extreme energy of hundreds of colliding protons and neutrons melt the boundaries of interacting particles allowing the constituent gluons and quarks to interact freely.

A computer visualization of 7,200,000,000,000⁰ quark-gluon plasma created in the RHIC collider in 2010 (Brookhaven National Lab)

This resultant fluid flows like a liquid with an extremely low-viscosity allowing pressure gradients created early in the collision to persist and influence how other particles strike the detector.

This means that the particles that strike the detector retain a ‘memory’ of each projectile’s initial shape — spherical in the case of protons, elliptical for deuterons, and triangular for helium-3 nuclei.

PHENIX analyzed measurements of two different types of particle flow (elliptical and triangular) from all three collision systems and compared them with predictions for what should be expected based on the initial geometry.

Julia Velkovska, a deputy spokesperson for PHENIX, who led a team involved in the analysis at Vanderbilt University, said: “The measurements match the predictions based on the initial geometric shape. We are seeing very strong correlations between initial geometry and final flow patterns, and the best way to explain that is that quark-gluon plasma was created in these small collision systems.”

The teams compared the resultant geometric flow patterns in this latest experiment with the theory of hydrodynamics which enabled them to rule out correlations suggested by other theories of physics such as quantum mechanics for the previously conducted gold-gold collisions.

“With everything else being equal, we still see greater elliptic flow for deuteron-gold than for proton-gold, which matches more closely with the theory for hydrodynamic flow and shows that the measurements do depend on the initial geometry,” said Velkovska. “ But based on what we are seeing and our statistical analysis of the agreement between the theory and the data, those interactions are not the dominant source of the final flow patterns.”

PHNIX will now examine data from these experiments to determine the temperature achieved in the small-scale collisions, which if hot enough will also support the creation of quark-gluon plasma.

Original research: https://www.nature.com/articles/s41567-018-0360-0