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

IMAGE CAPTION:
[http://www.bnl.gov/bnlweb/pubaf/pr/photos/2003/RHIC-event-front-hr.gif (37KB)]
An end view of collision between deuterons and gold ions captured by the STAR
detector at Brookhaven. (Brookhaven National Lab/RHIC-STAR)