New Collider Sees Hints of Quark-Gluon Plasma

-- Charles Seife

SCIENCE:  Volume 291, Number 5504, Issue of 26 Jan 2001, p. 573. 
Copyright  2001 by The American Association for the Advancement of

STONY BROOK, NEW YORK--Ever since the Relativistic Heavy Ion Collider
(RHIC) was turned on last June, physicists have been eagerly awaiting news
from the newest, biggest particle accelerator on the block. The wait is
over. The first results, presented at an international particle physics
conference here last week,* hinted that scientists have finally managed to
coax atomic nuclei to melt--creating a state of matter that hasn't existed
since the big bang.

Inside RHIC's tunnels at Brookhaven National Laboratory in Upton, New
York, gold nuclei accelerate to more than 99.99% of the speed of light and
smash into each other head on. By analyzing the showers of particles that
fly off the colliding nuclei, physicists are attempting to figure out how
matter behaves when so much energy is poured into so small a space. Last
year, scientists at CERN in Geneva implied that their collider had slammed
nuclei together so hard that the individual particles that make up the
atom melted into a liquid melange of the particles' components (Science,
11 February 2000, p. 949). When RHIC started up, physicists hoped that its
data would show evidence of such a quark-gluon plasma. So far, the most
tantalizing hints have come from what scientists don't see.

At low energies, a nucleus behaves something like a clump of hard wax
pellets. Slam two into each other, and particles shoot in all directions,
caroming off one another like hard billiard balls. By studying jets of
particles spraying from the sides of these collisions, physicists can
figure out what took place during the collision. At RHIC's higher
energies, something different is happening. "The distribution of fast-
moving particles is lower than one would predict," says Yale physicist
John Harris, spokesperson for STAR, one of RHIC's four experiments. There
seem to be fewer high-energy particles coming off the sides of the
collisions than expected.

Just as someone counting wax pellets might explain such a deficit by
saying that the wax had melted at high energies, particle physicists
suspect that the particles in the nuclei might be melting into a sticky
quark- gluon plasma that slows down particles shooting out the
sides--quenching the jets. "It's a very exciting observation. It hasn't
been seen before," says Tim Hallman, a physicist at Brookhaven working on
STAR. "It's early enough that people are guarded, but it matches
predictions pretty well of when you make a transition to the quark-gluon

Another line of evidence for a quark- gluon plasma has to do with how the
wreckage of the collisions sprays away. Most often, the two colliding gold
nuclei don't slam directly head on. Instead, the nuclei--flattened to
pancakes by the extreme relativistic speeds at which they are
traveling--strike each other off center, colliding only in an
almond-shaped region where the disks overlap. To scientists' surprise,
particles scattered off in an almond-shaped distribution, rather than
evenly. Calculations showed that it would be very hard to preserve the
almond shape if the subatomic particles were intact, but easier if the
particles had broken down into a soup of components. "It seems to imply
that something weird is happening," says Jim Thomas, a physicist at
Lawrence Berkeley National Laboratory in California who is working on the
STAR experiment at RHIC. "But more than that wouldn't be prudent to say."

Although the evidence is suggestive, nobody is willing to claim that RHIC
has actually spotted a quark-gluon plasma. "It's a consistent picture if
the quark-gluon plasma is being formed," says CERN physicist Carlos
Lourenco. But Lourenco warns that the RHIC measurements don't show a
sharp, well-defined transition between ordinary matter and a quark-gluon
plasma: "What we're looking for is big--to see a phase transition."

That might happen during RHIC's next run, scheduled to begin in May, which
will last longer, reach higher energies, and employ more sophisticated
detectors. In the meantime, particle physicists are simply saying that
interesting things are happening in RHIC's tunnels--not bad for a first
run. "Something is going on that we don't understand," says Columbia
University's Bill Zajc, spokesperson for RHIC's PHENIX experiment. "We
expected to open up a new frontier, but this is too easy," he adds--and
that "has some people a little concerned."