Latest RHIC results make news headlines at Quark Matter 2004
A computer image of RHIC particle events generated by the STAR experiment.
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Dominating the agenda of Quark Matter 2004, presentations of analyses from recent runs of the four experiments at the Relativistic Heavy Ion Collider (RHIC) created quite a media buzz during the week-long, 17th international conference on ultra-relativistic nucleus-nucleus collisions. For this meeting, more than 600 nuclear physicists from around the world gathered this January in Oakland, California.
Located at Brookhaven Lab and funded primarily by the U.S. Department of Energy's Office of Science, RHIC was built to collide gold ions at nearly the speed of light, to recreate hot, dense conditions that existed at the dawn of the universe. Under those conditions, quarks and gluons are expected to be free of the strong force which holds them confined within protons and neutrons within the atomic nucleus. Freed, they are expected to form a state of matter called quark-gluon plasma, which scientists think last existed a few microseconds after the Big Bang.
Studying the freed quarks and gluons within the plasma will help scientists develop an understanding of how matter evolved over time since the Big Bang. It will also provide more insight into the strong force, which is one of the four forces in nature and the short-range attraction responsible for holding the atomic nucleus together.
At the Quark Matter conference, new evidence was presented that gold-ion collisions at RHIC are producing an extremely dense form of matter -- which may, quite possibly, be the long-sought quark-gluon plasma. There was also animated discussion about other intriguing RHIC results, including the likelihood that RHIC experiments have detected the existence of an exotic type of particle containing five quarks, and may have uncovered signs of another dense form of matter called color glass condensate. "We have intriguing results, but it may take some time to sort out their significance in relation to the search for quark-gluon plasma or other new discoveries," says Sam Aronson, Chair of Brookhaven's Physics Department and a collaborator on PHENIX, one of the two larger RHIC experiments.
As reporters covering Quark Matter discovered, however, not all those at the conference were quite so circumspect. After the first day of the conference, for instance, stories in the Oakland Tribune and The New York Times, respectively, quoted scientists who all but declared that discoveries of quark gluon plasma and color glass condensate have already been made.
Gold, Deuterons, Protons
The RHIC data presented at Quark Matter are from January through March of 2003, when beams of heavy gold nuclei were collided with beams of deuterons, which are much smaller and lighter nuclei, each consisting of one proton plus one neutron. These deuteron-gold runs, along with other data from colliding two beams of protons, serve as a basis for comparison for the gold-gold collisions at RHIC
When two gold nuclei collide head-on, the temperatures reached are so extreme -- more than 300 million times the surface temperature of the sun -- that the protons and neutrons inside the merged gold nuclei are expected to melt, releasing their quarks and gluons to form quark-gluon plasma. In contrast, when a much smaller deuteron strikes a large gold nucleus, only a small part of the gold nucleus is heated up, and so the matter within the gold nucleus is thought to remain close to its normal state, with distinct protons and neutrons.
In either type of collision, a pair of quarks can be knocked loose from a proton or neutron, with each of these loose quarks producing a spray, or "jet," of ordinary particles. The two jets will emerge back to back from the collision region -- unless they are stopped within a dense medium. In the 2003 deuteron-gold experiments, back-to-back jets were seen; but, in head-on collisions during the earlier gold-gold runs, one of the two jets was missing. In addition, fewer highly energetic individual particles were observed coming from gold-gold than from deuteron-gold collisions.
One explanation of the missing jets is that a quark traveling through this environment would interact strongly, losing most of its energy. Thus, if a quark pair is produced near the surface of the nuclear fireball resulting from a head-on gold-gold collision, then the outward-bound quark is likely to escape, while the inward-bound quark is absorbed. As a result, only one jet is detected. This phenomenon, called "jet quenching," is predicted to occur within quark-gluon plasma. The same calculations also predicted the observed suppression of individual high-energy particles.
RHIC physicists are intrigued by these distinctions, which clearly show that head-on gold-gold collisions are producing a nuclear environment quite different from that of deuteron-gold collisions. These phenomena are new at RHIC, as they have not been observed in previous experiments at lower energies.
The Color Glass Condensate Debate
As postulated by some nuclear physics theorists, color glass condensate (CGC) may be another extreme, universal form of matter thought to be an intrinsic property of strongly interacting particles that can only be observed under high-energy conditions such as those at RHIC. If CGC exists, then it may explain many unsolved problems, such as how particles are produced in high-energy collisions and the distribution of matter itself inside of these particles. However, there is considerable controversy among nuclear physicists about the existence of CGC and the interpretation of RHIC data regarding its existence. The debate may not be settled until after RHIC is upgraded to become eRHIC.
According to Einstein's theory of relativity, a high-energy particle appears to be Lorentz contracted, or compressed, along its direction of motion. As a result, the gluons inside one gold ion appear to the other ion as a "gluonic wall" traveling near the speed of light. At very high energies, the density of the gluons in this wall is seen to increase greatly. Unlike the quark-gluon plasma produced in the collision of such walls, CGC describes the walls themselves.
"Color" in the name color glass condensate refers to a type of charge that quarks and gluons carry as a result of the strong force. The word "glass" is borrowed from the term for silica and other disordered materials that act as solids over short time scales but as liquids over long time scales; in gluonic walls, the gluons themselves are disordered and do not change their positions rapidly because of Lorentz time dilation. "Condensate" means that the gluons have a very high density.
The Continuing Search for Quark-Gluon Plasma
"When we are convinced that we have found quark-gluon plasma, it will be a tremendous return in terms of knowledge on the nation's investment in RHIC science, as well as very satisfying for the people involved," concludes Thomas Kirk, Brookhaven's Associate Laboratory Director for High Energy & Nuclear Physics. "Although we are optimistic about being able to report that discovery soon, we are exercising due caution about interpreting the results that RHIC has produced to date." And so the search for quark-gluon plasma and other physics continues at RHIC.
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