In QCD, it is the vacuum that imprisons the quarks. While it may sound like a barren place, the vacuum of QCD is a complex, dynamic arena. It writhes with virtual particles that appear in pairs, then annihilate and disappear again. It is haunted by strange creatures of various kinds, too, topologically complex knots and twists that are relatives of wormholes, places where space turns in on itself and seems treacherous. These knots and twists carve out paths for the gluons to travel along, thereby keeping the quarks together. These strange ideas have credence because of the success of QCD in predicting the reactions of fundamental particles. The only way to unglue quarks is to "melt" the vacuum between them. But the vacuum doesn't give in easily. To raze its jagged terrain requires enormous amounts of concentrated energy, found only in powerful nuclear collisions, or the fireball at the earliest moments of time. Melting the vacuum is like returning to the state of the universe at the time it first existed. The RHIC at Brookhaven was built to do just that, and its experiments were designed to allow physicists to study what happens when the vacuum is heated so much that quarks and gluons are freed and matter reverts to a fundamental state. Beginning in 2000, RHIC has repeatedly sent two beams of gold nuclei, each containing hundreds of protons and neutrons, speeding in opposite directions around a 4-kilometre track. Steered by superconducting magnets and achieving energies of 100 billion electronvolts, they collide, producing a fireball 300 million times hotter than the surface of the sun. Inside the fireball over a thousand quarks are unleashed. When, by chance, two quarks hit each other head on, the extreme energy of their collision is turned into matter.
A pair of virtual particles from the vacuum are given enough energy that they become real, and fly apart in opposite directions. Each of them goes on to drag further pairs of particles out of the vacuum, and the process repeats again and again, creating streams of particles called jets. The jets rush out of the collision site and are eventually captured by detectors. These jets reveal the presence of the quark-gluon plasma. The plasma's lifetime is a mere fraction of an instant, roughly 10-23 seconds. But that is long enough for it to block particles from the jets as they stream out. Jets are always produced in pairs and stream off in opposite directions. But they rarely originate in the exact centre of the collision site, so one jet usually has further to go before it hits the detector than the other. If the quark-gluon plasma has formed, it gets in the way of particles from one jet more than from the other. So when researchers on all four of the RHIC's detectors saw uneven jets emerging, they knew immediately that the quark-gluon plasma could be causing this effect. A further experiment provided more evidence for this idea, yet RHIC as a whole has held off making an official announcement. "Many theoretical physicists, and some number of the experimenters, say it's pretty clear to us that this is the discovery," says Thomas Kirk, the associate laboratory director for high energy and nuclear physics at Brookhaven. But some experimenters at the laboratory remain wary of claiming it for certain. This is partly because of a controversy that erupted in 2000 when researchers at CERN, the European centre for particle physics in Geneva, claimed (many experts said wrongly) to have seen a hint of the quark-gluon plasma there. But Brookhaven's case is far stronger, and most researchers tend to refer to the RHIC quark-gluon plasma as fact.
"There's no doubt. It is quite evident," says University of Arizona and CERN physicist Johann Rafelski. Much of the reason for Brookhaven's hesitancy is to be found in the bizarre and perplexing effects they have seen in their quark-gluon plasma. "It is different than we thought it was going to be," says Kirk. "It's brand new." Prior to the RHIC experiments, researchers had assumed that the vacuum structure would melt away easily once the energy exceeded 170 million electronvolts- the energy at which the plasma forms. At this energy, calculations suggested the plasma would be like a weakly interacting gas, with quarks and gluons floating haphazardly about, barely bothering each other. The researchers had figured that the jets streaming through the fireball would encounter only mild resistance on their way out. But measurements confirmed last year shocked experimenters. Not only were the jets uneven, but their absorption by the plasma was 10 times as high as anyone had expected. "Because the quark-gluon plasma is so opaque, these quarks simply can't get through to make a jet," explains Kirk. The jet particles appear to be getting stuck in the plasma like flies trapped in honey. This means the quark-gluon plasma is extremely dense - 30 to 50 times as dense as predicted - which suggests the quarks in the plasma are exhibiting incredibly synchronised group behaviour and interacting strongly with each other and the surrounding gluons.
This makes the plasma more similar to a liquid than a gas. "Instead of flying past each other, as in a gas, the whole liquid moves more coherently," says physicist Edward Shuryak, director of the Center for Nuclear Theory at Stony Brook University, New York. In fact, the strength of interactions in the quark-gluon plasma make it the most ideal liquid ever observed- 10 to 20 times as liquid-like as water. "That was surprising," says Shuryak. "If you could take a few thousand water molecules, make a tiny drop and let it explode, they would not flow, they would kind of move individually. But these thousands of particles actually move coherently." It seems that from the time of the big bang until 10 microseconds later, the universe was liquid.
This is part of a Feature article that appears in New Scientist issue: 16 October 2004. For the full story please contact Claire Bowles.
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