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Brookhaven National Laboratory

Contact:
Karen McNulty Walsh, 631-344-8350, kmcnulty{at}bnl.gov
or
Mona S. Rowe, 631-344-5056, mrowe{at}bnl.gov

Released: June 11, 2003

Number: 03-49

Exciting First Results from Deuteron-Gold Collisions at Brookhaven

Findings intensify search for new form of matter

UPTON, NY -- The latest results from the Relativistic Heavy Ion
Collider (RHIC), the world's most powerful facility for nuclear
physics research, strengthen scientists' confidence that RHIC
collisions of gold ions have created unusual conditions and that they
are on the right path to discover a form of matter called the
quark-gluon plasma, believed to have existed in the first
microseconds after the birth of the universe. The results will be
presented at a special colloquium at the U.S. Department of Energy's
Brookhaven National Laboratory on June 18 at 11 a.m., to coincide
with the submission of scientific papers on the results to Physical
Review Letters by three of RHIC's international collaborations.

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 Results

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.

This research was funded primarily by the U.S. Department of Energy,
Office of Science, Nuclear Physics Division, with additional funding
from the National Science Foundation and a large number of
international agencies (see a full list of funding sources,
http://www.bnl.gov/rhic/funding.htm).

The U.S. Department of Energy's Brookhaven National Laboratory
conducts research in the physical, biomedical, and environmental
sciences, as well as in energy technologies. Brookhaven also builds
and operates major facilities available to university, industrial,
and government scientists. The Laboratory is managed by Brookhaven
Science Associates, a limited liability company founded by Stony
Brook University and Battelle, a nonprofit applied science and
technology organization.

Other background information:

* On the Trail of Quark-Gluon Plasma
   http://www.bnl.gov/bnlweb/pubaf/pr/2003/RHIC-background-1.htm
* Cooking Up Quark Soup
   http://www.bnl.gov/bnlweb/pubaf/pr/2003/RHIC-background-2.htm
* Getting Additional Evidence
   http://www.bnl.gov/bnlweb/pubaf/pr/2003/RHIC-background-3.htm
* Interesting findings from RHIC so far (as of January, 2003)
   http://www.bnl.gov/bnlweb/pubaf/pr/2003/RHIC-background-4.htm
* RHIC Data Basics
   http://www.bnl.gov/bnlweb/pubaf/pr/2003/RHIC-background-5.htm
* Why Does Quark Matter Matter?
   http://www.bnl.gov/bnlweb/pubaf/pr/2003/RHIC-background-6.htm

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