| Subject: quark-gluon plasma |
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Date Posted: 20:00:53 06/19/03 Thu
Scientists are on the verge of creating a quark-gluon plasma, a state of matter that has not existed since the Universe was less than a millionth of a second old.
The optimism stems from the latest results to come out of the Relativistic Heavy Ion Collider (RHIC), the world's most powerful facility for nuclear physics research, sited in the US.
The properties of the quark-gluon plasma will hopefully answer many questions about the laws that govern nature's fundamental particles.
The scientists are not yet ready to claim a definite discovery of this ancient state of matter, however. They say that must await corroborating experiments, also under way at the RHIC.
The latest RHIC findings come from experiments conducted from January through to March 2003, in which a beam of heavy gold nuclei collided 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 the RHIC.
The gold-gold collisions, which bring nearly 400 protons and neutrons into collision at once, are designed to recreate, for a fleeting instant, the extremely hot, dense conditions of the early Universe.
When two gold nuclei collide head-on, the temperatures reached are more than 300 million times the surface temperature of the Sun. It is thought the individual protons and neutrons inside the merged gold nuclei "melt" to form a tiny sample of quark-gluon plasma.
When this happens a pair of energetic quarks can be knocked loose from within a proton or neutron. Each of these loose quarks produces a stream of particles emerging in opposite directions from the site of the collision.
In the deuteron-gold experiments, back-to-back jets were seen to emerge but, puzzlingly, 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 seen 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.
One possible explanation is that if a quark pair is produced near the surface of the quark-gluon plasma, the outward-bound quark is able to escape, while the inward-bound quark is absorbed.
This phenomenon is called "jet quenching" and was predicted to occur in quark-gluon plasmas.
If further work proves that a quark-gluon plasma has been made then scientists hope to learn more about the strong nuclear force - the force that holds quarks together in protons and neutrons.
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