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Background on findings relevant to the search for quark-gluon plasma

The latest RHIC findings come from experiments conducted from January through March of 2003, in which a beam of heavy gold nuclei collides head-on with a beam of deuterons (much smaller and lighter nuclei, each consisting of one proton plus one neutron). These deuteron-gold experiments, along with other experiments using two colliding beams of protons, serve as a basis for comparison with collisions of two gold beams at RHIC.

The gold-gold collisions, which bring nearly 400 protons and neutrons into collision at once, are designed to recreate, for a fleeting instant in the laboratory, the extremely hot, dense conditions of the early universe. 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 individual protons and neutrons inside the merged gold nuclei are expected to melt, releasing the quarks and gluons normally confined within them to form a tiny sample of particle “soup” called quark-gluon plasma. In contrast, when the much smaller deuteron strikes the large gold nucleus, it heats up only a small part of it. The matter in the gold nucleus is believed to remain close to its normal state, with distinct protons and neutrons.

In either type of collision, a pair of incoming quarks can collide with each other (be knocked loose) from within a proton or neutron. Each of these loose quarks will end up producing a “jet” of ordinary particles and the two jets will emerge back-to-back from the collision region — unless they are stopped by the dense quark-gluon plasma described above. Scientists can use these scattered quark collisions to probe nuclear environments.

In the deuteron-gold experiments conducted in 2003, back-to-back jets were seen to emerge, but in head-on collisions from the earlier gold-gold experiments, 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. Scientists 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; they have not been observed in previous experiments at lower energies.

One possible explanation of the missing jets is that a quark traveling through this new environment would interact strongly and lose most of its energy. Thus, if a quark pair is produced near the surface of the nuclear fireball resulting from a head-on collision of gold nuclei, the outward-bound quark is likely to escape, while the inward-bound quark is absorbed. The physicists detect only one jet. This phenomenon is called “jet quenching” and was predicted to occur in quark-gluon plasma. The same calculations also predicted the observed suppression of high-energy individual particles.

Adapted from: http://www.bnl.gov/bnlweb/pubaf/pr/2003/bnlpr061103.htm

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