accelerator, gold ions zoom toward each other at almost the speed
of light. They crash together with enough force to melt the ions
into a quarkgluon plasma. This hot, primordial quark soup
is thought to have existed in the first millionth of a second after
the big bang that created our universe. The entire universe, small
though it was then, is thought to have been a quarkgluon plasma.
As the universe began to expand and cool, the quarks and gluons
bound together and have remained virtually inseparable ever since.
the alchemists of old tried to turn all sorts of materials into
gold, modern-day physicists, including several from Livermore, are
attempting reverse alchemyturning gold into a different state
of nuclear matter. By smashing gold ions together in the Relativistic
Heavy-Ion Collider (RHIC) at Brookhaven National Laboratory in Upton,
New York, they are working to free quarks and gluons and re-create
a quarkgluon plasma on Earth.
quark is the most elementary building block of matter. (See the
below.) By exchanging gluonsmassless particles that make quarks
stick togethergroups of quarks constitute particles such as
protons and neutrons. The binding force carried by both gluons and
quarks is known as the strong force, and for good reason. Although
theory says that at extremely high energy densities, protons and
neutrons should dissolve into a quarkgluon plasma, no particle
accelerator had been powerful enough to create the necessary conditions
with high certainty.
possibility of creating hot, dense nuclear matter by colliding large
nuclei was first proposed in 1973 by several Livermore physicists,
including George Chapline and Edward Teller (Physical Review
D 8, 43024308). They predicted that experiments using
Lawrence Berkeley National Laboratorys Bevelac, then the most
powerful particle accelerator for heavy nuclei, would probably result
in the production of matter in a new regime of temperature
recognized that since the experiments explore regions very
far from our experience, it is reasonable to expect surprises.
In fact, they surmised that the main result of the experiment would
be the unexpected phenomena. That is what great science
is often all about.
|Aerial photograph of the 3.8-kilometer-circumference
Relativistic Heavy-Ion Collider and associated particle accelerators
at Brookhaven National Laboratory.
The Experiment Today
More than 25 years later, in 1999, physicists
from institutions all over the world believed they might have finally
established the laboratory conditions required to create not only
the hot and dense region described in 1973 but also a new phase
of matter. (See the box near the end of this article.)
Beams of gold ions or nucleiatoms
that have been stripped of their electronsare propelled around
RHICs loops in opposite directions at 99.9 percent of light
speed. When any two nuclei collide, the collision acts as a pressure
cooker, liberating more than a trillion electronvolts of energy
in a volume the size of an atomic nucleus. Some of the energy each
nucleus had before the collision is transformed into intense heat
and new particles such that new matter is created at a temperature
ten thousand times that of the Sun. The collisions are highly explosive,
and if a quarkgluon plasma is created, it decays into particles
(bound quarks) almost as quickly as the plasma is formed.
To determine whether a quarkgluon
plasma existed during an experiment, scientists look for signatures
in the distribution and composition of the particles that reach
the Pioneering High-Energy Nuclear Interaction Experiment (PHENIX)
detector, which Livermore helped to design and build during the
If the experiment continues
according to plan, we will have made a quarkgluon plasma,
says Livermore physicist Ron Soltz, a principal investigator for
Livermores work at RHIC. What were essentially
trying to do is find the boiling point of nuclear matter.
|(a) A schematic of the Pioneering
High-Energy Nuclear Interaction Experiment (PHENIX) detector.
(b) Inside the PHENIX detector.
|(a) Inside Brookhavens
Relativistic Heavy-Ion Collider, two gold nuclei approach one
another at almost the speed of light. Traveling at relativistic
speeds causes them to look flat rather than spherical. (b) As
the two nuclei collide and pass through each other, some of
the energy they had before the collision is transformed into
intense heat and new particles. (c) If conditions are right,
the collision liberates the quarks and gluons in the nuclei
to form a quarkgluon plasma. (d) As the area cools off,
thousands more particles form. Many of these new particles will
travel to a detector where their distinctive signatures give
physicists clues about what occurred inside the collision zone.
Soltzs team, including
physicists Stephen Johnson and Ed Hartouni and postdoctoral fellows
Mike Heffner and Jane Burward-Hoy, are measuring the volume, lifetime,
and violence of the collision zone (or source). A large volume and
long lifetime are one of the purported signatures of a quarkgluon
plasma. To take the measurements, the team examines the production
of pions, a two-quark particle that is the most prevalent product
of these collisions. The team is exploiting a simple property of
quantum mechanics, which is that the more highly correlated the
pions are in a given direction, the larger the emission volume is
along that axis. A long-lived source should appear as an apparent
elongation of the fireball in the direction of the detector relative
to the geometric radius of the fireball. The Livermore team found
almost no elongation, in contradiction to most recent theoretical
However, the story does not
end there. Even before the Livermore team had finished its analysis,
other collaborators were finding signs of the plasma in another
If no plasma is formed,
particles with high momentum escape the collision unscathed,
notes Johnson. But if a quarkgluon plasma has been created,
the interaction between high-momentum particles and the medium increases
dramatically, significantly lowering the velocity of the particles.
Quantum chromodynamics theory
(see the box below) predicts that in the presence of a quarkgluon
plasma, substantially fewer high-momentum particles will make their
way to the detector. That relatively low number is, in fact, what
PHENIX found by counting high-energy pions.
So right now, the data are
inconclusive. Obviously, we need more information, says
Soltz. Were considering two options at this point. One
is to study rarer particle signatures, which would require a lot
of data that we dont have. The other option is to go to a
simpler experiment whose results will be easier to interpret.
|A paucity of high-momentum
particles during the collision of gold nuclei, which is the
result of an opaque source, is consistent with theory and indicates
the existence of a quarkgluon plasma.
|Results from the STAR and
PHENIX detectors show that the elongation of the collision zone
(indicated by the ratio of two radii) was considerably less
than theory predicted. In fact, there was no elongation at all,
with ratios of about 1.
two gold nuclei collide, many interactions occur between all of
the nucleons (protons and neutrons). Although these numerous interactions
are responsible for creating the conditions necessary to form a
quarkgluon plasma, researchers have difficulty differentiating
between signals resulting from the plasma and those that may be
caused by other interactions of the nucleons.
Simpler to study than the
collision between two nuclei is a collision between a single proton
and a nucleus. While one proton may have several interactions within
a nucleus, scientists do not expect that these interactions will
create a plasma. But exactly how many such interactions are there?
Finding the answer to this question for each protonnucleus
collision will allow scientists to make proper comparisons with
the results from nucleusnucleus collisions. If scientists
can measure the number of interactions, they should be able to verify
the underlying signatures of a quarkgluon plasma.
Under Johnsons leadership,
the Livermore team has begun adding a detector to PHENIX that will
make these measurements in protonnucleus collisions. The new
detector is a calorimeter that measures the pieces of the fragmenting
nucleus after a proton has blasted through it. To provide this entirely
new capability in short order at a minimal cost, the Livermore group
adopted detectors and equipment from previous Brookhaven experiments.
The result is what they jokingly refer to as the Scrounge-a-Cal.
The calorimeter is being instrumented in PHENIX now, and the first
results will be analyzed this spring.
Parts and Particles
All things, living and inanimate, are made of atoms. Almost
all of an atoms mass is in its nucleus, where protons
and neutrons reside. Protons and neutrons consist of various
combinations of quarks. At the moment, scientists believe
that quarks are the smallest particles in our universe
and that they form the basis for all matter. However,
just as scientists until the late 19th century believed
that the atom was the smallest particle, they may someday
discover particles smaller than quarks.
In the meantime,
theory holds that quarks, with the help of gluons to hold
the quarks together, make up everything in the nuclei
of atoms. Up, down, charm, strange, top, and bottomthese
are the six flavors of quarks. Up and down
quarks are the least massive and are more prevalent than
other types. Protons always have two up quarks and one
down quark, whereas neutrons have two down quarks and
an up quark. Other more exotic and more massive particles
are composed of other quark combinations. A lambda particle,
for example, has an up, a down, and a strange quark, while
a kaon has a strange and an up quark. Gluons carry the
strong force that glues quarks together to form protons,
neutrons, and other particles and keeps them together
in an atoms nucleus.
In contrast, the
electron is not made of quarks and is not subject to the
strong force. Instead, the electromagnetic force keeps
an electron in its orbit spinning around an atoms
that the atom was not the most elementary particle, scientists
realized that subatomic
particles behave differently from larger, bulk quantities
of matter. The field of quantum mechanics was developed
to explain this apparently eccentric behavior.
Then they discovered
quarks and gluons, whose existence was first inferred
from the spectra of elementary particles and from electron-scattering
experiments in particle accelerators. Quarks and gluons
possess a type of charge that has been whimsically termed
color. Color is the source of the powerful forces that
first cluster the quarks and gluons to make protons and
neutrons and, in turn, grip these nucleons to one another
to form atomic nuclei. A new theory, quantum chromodynamics,
was developed late in the 1960s and early 1970s to describe
which explains the strong force, bears many similarities
to quantum electrodynamics, which explains electrical
charges and light. Atoms can be ionized and the fundamental
electrical charges of quantum electrodynamics can appear
in isolation, but in quantum chromodynamics, the fundamental
quark and gluon constituents of protons and neutrons can
only be liberated in conditions identical to those of
the big bang. This property of quarkgluon confinement
gives stability to all matter as we know it.
theory predicts that deconfinement will occur at sufficiently
high temperatures, nuclear densities, or both. Quarks
and gluons will break free of their bondage in atomic
nucleons, re-creating the earliest moments of our universe.
at Brookhaven and elsewhere is beginning to answer some new and
old questions about quarks. Researchers at the National Aeronautics
and Space Administrations Chandra X-Ray Observatory recently
discovered what appears to be a collapsed star with a quark core.
If accurate, this discovery complements the current search for the
quarkgluon plasma at RHIC. It also confirms a prediction made
25 years ago by Livermore physicist Chapline about extremely dense
stars with a quarkgluon plasma at their core rather than the
bound quarks usually found in neutron stars.
and RHIC Rise
Only massive experimental equipment such as the Relativistic
Heavy-Ion Collider (RHIC) and the Pioneering High-Energy
Nuclear Interaction Experiment (PHENIX) detector at
Brookhaven National Laboratory make it possible to study
the almost infinitesimally small dot known as the quark.
An inverse relationship
exists between the size of the object being studied
and the size and expense of the equipment needed to
examine it: the smaller the object, the greater the
energy needed to probe it, and thus, the larger the
equipment required. Particle and nuclear physicists,
who want to examine the fundamental building blocks
of matter up close, need hugely expensive machines often
measuring a kilometer or more in diameter.
The layout of the detectors around
the RHIC tunnel.
Given their expense and size, few such machines can be
built. Many nuclear and particle physicists must concentrate
their efforts on the few particle accelerators and colliders
available around the world.
Lawrence Livermore is just 1 of 55 institutions from 11
countries involved in the quarkgluon plasma experiments
being performed on the PHENIX detector at Brookhaven.
There are 450 scientists participating, each responsible
for a different task in this challenging endeavor.
RHIC, which measures
3.8 kilometers in circumference, was commissioned in October
1999. It was the first colliding-beam facility specifically
designed to accommodate the requirements of heavy-ion
physics at relativistic, or speed-of-light, energies.
RHIC is actually
the newest link in a chain of accelerators that make up
the RHIC accelerator complex. Heavy ions destined for
RHIC originate in a Tandem Van de Graaff Accelerator,
proceed into the Booster Accelerator, and then into the
Alternating Gradient Synchrotron, which injects heavy
ions into RHIC for experiments. Before RHIC was completed,
Livermore physicists contributed to major discoveries
about the properties of nuclear matter using the Alternating
When RHIC is operating,
bunches of heavy ions can be injected into each of its
two rings, which are in a tunnel. Then, with both rings
filled, the ions are accelerated in minutes to the top
energy where the ion beams coast for hours in stable orbits
around the rings. The tunnel is configured so that the
circulating ion beams can cross and collide in six places.
Four of the six collision spaces now hold detectors that
electronically record the results of the interactions
between particles. During experiments, particles are made
to collide head-on at the rate of tens of thousands of
collisions per second at the position of each detector,
shown in the right-hand figure on p. 8. The two largest
detectors are PHENIX and STAR. Two smaller detectors are
known as PHOBOS and BRAHMS. Each detector uses different
technologies to determine whether a quarkgluon plasma
is present in RHIC.
A team of scientists
from Lawrence Livermore and Brookhaven national laboratories
developed the three powerful magnets in the PHENIX detector.
Livermore was responsible for the design and supervised
fabrication and testing of the magnets.
3.6 million kilograms and has a dozen detector subsystems.
The three magnets produce high magnetic fields to bend
charged particles along curved paths. Tracking chambers
record hits along the flight path to measure the curvature
and thus determine each particles momentum. Other
subsystems identify the particle type and measure the
particles energy with calorimeters. Still others
record where the collisions occurred and determine whether
a collision was head-on or peripheral.
Each type of particle
has a distinctive mass and charge associated with it,
which allows accurate identification. Each particle will
bend and move differently in the magnetic field, allowing
scientists to resolve the type of particle it is and its
energy. By combining information from all detectors, scientists
attempt to reconstruct what happened during the collision.
team celebrates completion of testing of the PHENIX detector.
importantfor basic science and a better understanding of how
our universe got startedthe experiments at Brookhavens
RHIC hold the key to answering one of the questions posed recently
by the National Research Council Committee on Physics of the Universe.
In its report, Connecting Quarks with the Cosmos: 11 Science Questions
for the New Century, number 7 on the councils list was Are
there new states of matter at ultrahigh temperatures and densities?
Livermore researchers and their collaborators hope to answer that
Key Words: Brookhaven
National Laboratory, particle physics, Pioneering High-Energy Nuclear
Interaction Experiment (PHENIX) detector, quarkgluon plasma,
Relativistic Heavy-Ion Collider (RHIC).
For further information contact Ron Soltz (925) 423-2647 (firstname.lastname@example.org).
Additional information on the Relativistic Heavy-Ion Collider is available