Quark-Gluon Plasma Press Conference Set

The scientists are not yet ready to claim the discovery of
the quark-gluon plasma, however. That must await corroborating
experiments, now under way at RHIC, that seek other signatures
of quark-gluon plasma and explore alternative ideas for the kind
of matter produced in these violent collisions.

“This
is a very exciting result that clearly indicates we are on the
right track to an important scientific discovery,” said Thomas
Kirk, Brookhaven’s Associate Laboratory Director for High Energy
and Nuclear Physics
. “But the case for having created
quark-gluon plasma is not yet closed. We have
four
experiments looking for a number of different `signatures’
of this elusive form of extremely hot, dense nuclear matter.”

“These results from RHIC are profoundly important,” said

Raymond L. Orbach, Director of the Department of Energy’s
Office of Science,
the primary funding agency for research at RHIC. “They go to a
fundamental question in science: how did the universe look at
the beginning of time? People have always been fascinated by the
question of how our world began. And every time something
fundamental is learned, society eventually benefits, either
directly from that knowledge or from the technology developed to
obtain it.”

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, the small deuteron
passes through the large gold nucleus like a bullet, without
heating or compressing it very much. The gold nucleus remains in
its usual state, composed of distinct protons and neutrons.

In either type of collision, a pair of energetic quarks can
be knocked loose from within a proton or neutron. Each of these
loose quarks will produce a “jet” of ordinary particles, and the
two jets will emerge back-to-back from the collision region.
Scientists can use these jets to probe nuclear environments.

In the deuteron-gold experiments conducted this spring,
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 are 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.

One possible explanation of the missing jets is that a quark
traveling through this new environment would interact strongly
and lose a substantial amount 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 able to escape, while the inward-bound
quark is absorbed. Only one jet is detected by the physicists.
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.

If further scientific research proves that a quark-gluon
plasma has been made, the physics story has just begun. By
studying the behavior of free quarks and gluons in the plasma,
RHIC scientists hope to learn more about the strong nuclear
force — the force that holds quarks together in protons and
neutrons.

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