Smashing large atomic nuclei, containing protons and neutrons, together at close to the speed of light re-creates the conditions of the very early universe. It was thought that only the nuclei of large atoms such as gold would have enough matter and energy to produce a primordial soup of matter’s most basic “quark” and “gluon” building blocks — a quark-gluon plasma (QGP). But surprisingly, smaller particles colliding with large nuclei appear to produce tiny droplets of QGP. Recent results show that the tiny droplets behave like a liquid rather than the expected gas. The results support the case that these small particles produce tiny drops of the primordial soup.
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| Initial hot spots created by collisions of one, two, and three-particle ions with much larger gold ions (top row). Expected patterns if the collisions are creating tiny hot spots of the primordial soup, or quark-gluon plasma [Credit: Brookhaven National Laboratory] |
The idea that collisions of small particles with the larger positive center of atoms might create minute droplets of primordial QGP has guided experiments to test the idea and alternative explanations. The research has stimulated a rich debate about the implications of the findings. These experiments are revealing the key elements required for creating QGP and could offer insight into the initial characteristics of the colliding particles.
The discovery of elliptic-shaped flow in particles streaming out of the Relativistic Heavy Ion Collider (RHIC), a particle collider for nuclear physics research at Brookhaven National Laboratory, when gold nuclei collided showed that that the matter created in these collisions behaved like a liquid rather than the expected gas. Additional experiments confirmed that this liquid is indeed composed of visible matter’s most fundamental building blocks, quarks and gluons, and that the flow occurs with minimal resistance — making it a nearly “perfect” liquid plasma.
The RHIC scientists caused helium-3 nuclei (each made of two protons and one neutron) to collide with gold and single protons with gold to discover a triangular pattern of flow that is consistent with the creation of tiny droplets of QGP. The data also indicate that these small particle collisions could be producing the extreme temperatures required to free quarks and gluons — albeit at a much smaller, more localized scale than in the relatively big domains of QGP created in collisions of two heavy ions.
Because not all of the key signatures of QGP formation exist, scientists at the RHIC are continuing to study colliding protons with gold ions — to explore whether there are other interesting phenomena occurring in these collisions in addition to particle flow. There’s widespread agreement that the measurements at RHIC are going to advance this scientifically robust debate and our understanding of the fundamental structure and interactions of visible matter’s most fundamental building blocks.
Source: Department of Energy, Office of Science [August 31, 2015]
