February 10,
2000
Particle Physicists Getting Closer to the
Bang That Started It All
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By JAMES
GLANZ
cientists in Geneva have re-created a primordial form of matter
that
physicists believe last existed in
abundance when the universe was
an
exploding fireball only a fraction of a
second old.
The new
material is a highly compressed gas of the particles called
quarks and
gluons, the building
blocks of ordinary particles like the
protons and
neutrons within all the
atoms in the universe today. In scientific
importance, the long-sought
achievement might be compared to
the first
splitting of the atom to reveal its individual parts.
The
achievement will be announced today at CERN, the European particle
physics laboratory
where the work was carried out. The
finding moves
experimental physics
closer than it has ever been to the
presumed
moment at which the universe came into being and could help
cosmologists
better understand the
driving forces behind the primordial
explosion
itself. The matter's existence confirms one of the most abstruse of all
predictions by theoretical particle physicists.
Quarks, and the
gluons that powerfully bind them, are normally joined
to form protons
and neutrons and
cannot be shaken loose individually
no matter how hard
pairs of the
ordinary particles are smashed together. To create the new
material,
the scientists have, in effect, compressed and heated a ball
of protons
and neutrons so that they melted into
their constituent
quarks and gluons,
which then floated freely in a laboratory for the
first time.
The compression was achieved by
smashing together
entire lead nuclei
containing hundreds of protons and
neutrons each,
rather than mere
pairs of them.
"It does indicate that a new state
of matter is created," said Dr. Johanna Stachel, a physicist at the
University of Heidelberg in Germany
who is the spokeswoman for one of
the multinational collaborations that
operates a large particle
detector,
called NA45, at the Geneva laboratory. "This new state we
think the
universe was in until about 10 microseconds after the Big
Bang, and then
crystallized into the particles as we
know them now."
The Big Bang is the colossal explosion in which most cosmologists
believe the universe was born, some 15
billion years ago. A microsecond
is a
millionth of a second.
The laboratory's experiment, said
Dr.
Michael Turner, a cosmologist at
the University of Chicago, "helps
take
us back to when the universe
was a soup of the most fundamental
particles we know."
Because of the tight connection to
cosmology,
said Dr. Edward Shuryak, a physicist at the State University of New York
in Stony Brook, the
Geneva laboratory's achievement is
being called the
"Little Bang."
Known more technically as
"quark-gluon matter," the
material
is also a boon for theoretical physicists, since their theory
of strong
particle interactions, called quantum chromodynamics, had
predicted
that the bizarre state should exist.
Physicists know of
six different
types of quarks, which go by the
somewhat whimsical
designations
up, down, charm, strange, top and
bottom. Pairs and
threesomes of
quarks bind together to make up
ordinary particles of
matter. Protons, for example, consist primarily
of two up quarks and one
down
quark, while the less common particles called kaons consist of a
strange
quark and either an up or a down.
Oddly, the strength with
which
gluons bind quarks turns out to be
weak when the quarks are close
together and grows powerful when
they are distant from each other, as
if they were connected by elastic.
But Dr. Shuryak and others came
to the conclusion that quarks could
roam free if enough protons and
neutrons could be heated to a temperature about 100,000 times higher than
the center of the sun and compressed
to a density roughly 10 times that
of
an ordinary atomic nucleus. Small
clouds of quarks effectively
screen
one another from the gluon force of
more distant quarks, cutting
the
elastic connections.
The equations of quantum chromodynamics
are so complex that the
theoretical properties of quark-gluon
matter
can be explored only on the
world's largest computers.
To test
whether this state of matter can exist in reality, the scientists
in
Geneva used their Super Proton
Synchrotron to accelerate lead nuclei to
an energy of 33 trillion electron volts. Traveling at nearly the
speed
of light, those nuclei were
smashed into a lead foil, producing
hot,
dense matter in the collisions.
After a fleeting existence, the
quark-gluon matter should then cool
and condense into ordinary matter
and explode in a hail of thousands of
ordinary particles. Seven
different
particle detectors examined the residue of millions of lead
collisions for
evidence that the quark-gluon matter had been created.
"It is sort of a criminal court procedure, where you have proof by
circumstantial evidence," said Dr.
Reinhard Stock, a physicist at the
University of Frankfurt who is the
spokesman for a collaboration
centered on a detector called NA49.
The fingerprints of the deed are
clear, said Dr. Stock. They include
detection of many more particles
that contain strange quarks than an
ordinary smash-up would produce,
and fewer particles containing
charm quarks -- the indications seen
by
the Geneva lab's detectors.
"We have opened the door," said
Dr.
Claude Détraz, the laboratory's
director of research, calling the
new
results "compelling evidence that we
have created a new state of
matter in
which quarks are 'deconfined.' "
Cosmologists believe
that much of
the character of the universe, and
perhaps the fury of the
Big Bang
explosion itself, was determined by a
series of so-called
phase transitions
like the coalescence of ordinary matter from the
quark-gluon plasma.
"This is really a concrete illustration of how
cosmologists can benefit
from accelerators, which can recreate the
conditions that existed
during the earliest moments of the
universe,"
said Dr. Turner of the
University of Chicago.
Though quark-gluon
matter is
rare, physicists theorize that small
amounts of it may be
generated
when energetic particles from space
called cosmic rays crash
into planetary bodies like Earth.
The announcement sets the stage
for much more powerful experiments, expected to begin this spring,
using
the Relativistic Heavy Ion Collider at the federal Brookhaven National
Laboratory in Upton, N.Y.
Those experiments will initially
collide
gold nuclei with 10 times higher energy than CERN has mustered,
said Dr.
Thomas Ludlam, a physicist
who is an administrator of the program at
Brookhaven. The experiments should produce an even more
exotic entity
called a quark-gluon
plasma and allow for much more
intensive study of
the substances.
"It's as if you're trying to discover
steam and you
can make a few little
puffs of steam," he added. "But with
a better
pressure cooker you have
enough time to stick a thermometer
in and
discover the thermodynamic
properties of steam."
