| Carnegie Mellon Scientists Step Closer to Establishing Quark-Gluon Plasma and Understanding the Immediate Aftermath of the Big Bang
Work conducted as part of a multi-institutional collaboration including Carnegie Mellon University puts scientists one step closer to identifying a quark-gluon plasma, or QGP, according to Morton Kaplan, Ph.D., Professor of Chemistry and Physics and a member of the Solenoidal Tracker At RHIC (STAR) team, which reported its latest findings at a colloquium June 18 at Brookhaven National Laboratory. Brookhaven houses the Relativistic Heavy Ion Collider (RHIC) used to conduct the STAR experiments. The findings were summarized in recent articles by the New York Times and Newsday.
A quark-gluon plasma represents the hottest, densest type of matter, and its discovery, if confirmed through additional experiments, would mark an impressive milestone in understanding the creation of the universe. This matter is thought to have formed briefly in the instant after the Big Bang.
"We have demonstrated the creation in the laboratory of a here-to-for unseen form of matter with many times the normal nuclear density, and this is a necessary condition for forming a quark-gluon plasma, as envisioned at the beginning of the Universe," said Dr. Kaplan.
Postulated for more than 20 years, a quark-gluon plasma, or QGP, represents matter before the formation of atomic nuclei, which are composed of protons and neutrons. In the standard model of physics, protons and neutrons are themselves composed of smaller particles, called quarks. Gluons represent forces that hold quarks of different types, or flavors, together. But matter, as we know it today, exists as discrete particles (quarks or combinations of quarks) that are tightly confined within a small volume. The discovery of a QGP represents the de-confinement of quarks and gluons into a super-dense form of matter. Studies of a QGP would allow scientists to peer into the origins of the universe and establish an exciting link between particle physics and cosmology.
To search for a QGP, the STAR team has been accelerating and colliding atomic particles many times and then using sophisticated detectors and computational techniques to analyze each collision. The STAR experiments began in 2000. By last year, STAR reported findings from their collisions of heavy gold particles that suggested the formation of a QGP. But the group needed to conduct additional research that involved colliding gold particles, which are heavy, with deuterium particles, which are light. In this way, they validated that the findings observed from their gold-gold particle collisions were not due to poorly understood experimental effects. The recent colloquium and the published news stories addressed this latest work.
The next step, according to Dr. Kaplan, is to conduct corroborating studies with gold-gold particle collisions. These would involve longer experimental runs and variations in the energy of the collisions. What needs to be teased out, according to Kaplan, is exactly when the super-dense matter forms after the collision.
"If the postulated QGP forms early after a particle collision, then it would be consistent with matter forming immediately after the Big Bang," Kaplan states. If the matter forms later in the collision, then it could be something important, but essentially different, he points out.
Kaplan also says that experiments planned over the next year will attempt to elucidate whether this new matter is in equilibrium with its local surroundings; that is, can it be characterized by a well-defined temperature and density. If the new matter satisfies these conditions, however briefly, then the case for its identification as a QGP becomes much stronger, particularly from the viewpoint of the scientific community at large.
On the other hand, if the STAR team cannot confirm that their high density matter is in equilibrium, then the interpretation is less clear and more experiments will be needed, according to Kaplan.
The STAR team also will measure whether the new form of high density matter is produced at lower colliding beam energies. "Basically, the formation of a QGP should depend upon the energy available in the particle collision beams," says Kaplan. "This matter should not exist at all available energies, so it would be very interesting and significant to observe a transition from “no QGP” to “QGP formation” as the beam energy is changed.” Such a result may be forthcoming in the next year or two.
Computer reconstruction of charged-particle tracks (curved lines) as recorded in the STAR Detector. On the left is an image of proton-proton (p - p) collisions, in which two jets of particles are emitted in back-to-back pairs from the collision zone (center of image). On the right is an image of a head-on collision between two energetic gold (Au) nuclei, resulting in thousands of emitted particles - -hence the many tracks. Analysis of the Au – Au collision data has demonstrated the significant absence of one of the jets from most of the back-to-back pairs. The “disappearance” of this backward-moving jet likely happened because an ultra-dense medium created by the Au-Au collisions has absorbed it. The result, along with other data from this study, strongly indicates that the experiment has indeed produced a new form of very-high-density matter, which might be the theoretically predicted quark-gluon plasma (QGP).
July 2003
For more information, see:
 Morton Kaplan
Solenoidal Tracker At RHIC (STAR)
Brookhaven National Laboratory
Relativistic Heavy Ion Collider (RHIC)
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