What is especially intriguing about QCD is
that--contrary to what happens with such familiar forces
as gravity and electromagnetism--the coupling strength
grows
weaker as quarks approach one another.
Physicists have called this curious counterintuitive
behavior asymptotic freedom. It means that when two
quarks are substantially closer than a proton diameter
(about 10-13 centimeter), they feel a reduced force,
which physicists can calculate with great precision by
means of standard techniques. Only when a quark begins
to stray from its partner does the force become truly
strong, yanking the particle back like a dog on a leash.
In quantum physics, short distances between particles
are associated with high-energy collisions. Thus,
asymptotic freedom becomes important at high
temperatures when particles are closely packed and
constantly undergo high-energy collisions with one
another.
More than any other single factor, the asymptotic
freedom of QCD is what allows physicists to lift
Weinberg's veil and evaluate what happened during those
first few microseconds. As long as the temperature
exceeded about 10 trillion degrees Celsius, the quarks
and gluons acted essentially independently. Even at
lower temperatures, down to two trillion degrees, the
quarks would have roamed individually--although by then
they would have begun to feel the confining QCD force
tugging at their heels.
To simulate such extreme conditions here on earth,
physicists must re-create the enormous temperatures,
pressures and densities of those first few microseconds.
Temperature is essentially the average kinetic energy of
a particle in a swarm of similar particles, whereas
pressure increases with the swarm's energy density.
Hence, by squeezing the highest possible energies into
the smallest possible volume we have the best chance of
simulating conditions that occurred in the big bang.
Fortunately, nature provides ready-made, extremely
dense nuggets of matter in the form of atomic nuclei. If
you could somehow gather together a thimbleful of this
nuclear matter, it would weigh 300 million tons. Three
decades of experience colliding heavy nuclei such as
lead and gold at high energies have shown that the
densities occurring during these collisions far surpass
that of normal nuclear matter. And the temperatures
produced may have exceeded five trillion degrees.
Colliding heavy nuclei that each contain a total of
about 200 protons and neutrons produces a much larger
inferno than occurs in collisions of individual protons
(as commonly used in other high-energy physics
experiments). Instead of a tiny explosion with dozens of
particles flying out, such heavy-ion collisions create a
seething fireball consisting of thousands of particles.
Enough particles are involved for the collective
properties of the fireball--its temperature, density,
pressure and viscosity (its thickness or resistance to
flowing)--to become useful, significant parameters. The
distinction is important--like the difference between
the behavior of a few isolated water molecules and that
of an entire droplet.
The RHIC Experiments
Funded by the U.S. Department of Energy and operated by
Brookhaven, RHIC is the latest facility for generating
and studying heavy-ion collisions. Earlier nuclear
accelerators fired beams of heavy nuclei at stationary
metal targets. RHIC, in contrast, is a particle collider
that crashes together two beams of heavy nuclei. The
resulting head-on collisions generate far greater
energies for the same velocity of particle because all
the available energy goes into creating mayhem. This is
much like what happens when two speeding cars smash
head-on. Their energy of motion is converted into the
random, thermal energy of parts and debris flying in
almost every direction.
At the highly relativistic energies generated at RHIC,
nuclei travel at more than 99.99 percent of the speed of
light, reaching energies as high as 100 giga-electron
volts (GeV) for every proton or neutron inside. (One GeV
is about equivalent to the mass of a stationary proton.)
Two strings of 870 superconducting magnets cooled by
tons of liquid helium steer the beams around two
interlaced 3.8-kilometer rings. The beams clash at four
points where these rings cross. Four sophisticated
particle detectors known as BRAHMS, PHENIX, PHOBOS and
STAR record the subatomic debris spewing out from the
violent smashups at these collision points.
When two gold nuclei collide head-on at RHIC's
highest attainable energy, they dump a total of more
than 20,000 GeV into a microscopic fireball just a
trillionth of a centimeter across. The nuclei and their
constituent protons and neutrons literally melt, and
many more quarks, antiquarks (antimatter opposites of
the quarks) and gluons are created from all the energy
available. More than 5,000 elementary particles are
briefly liberated in typical encounters. The pressure
generated at the moment of collision is truly immense, a
whopping 1030 times atmospheric pressure, and the
temperature inside the fireball soars into the trillions
of degrees.
But about 50 trillionths of a trillionth (5 10-23) of
a second later, all the quarks, antiquarks and gluons
recombine into hadrons that explode outward into the
surrounding detectors. Aided by powerful computers,
these experiments attempt to record as much information
as possible about the thousands of particles reaching
them. Two of these experiments, BRAHMS and PHOBOS, are
relatively small and concentrate on observing specific
characteristics of the debris. The other two, PHENIX and
STAR, are built around huge, general-purpose devices
that fill their three-story experimental halls with
thousands of tons of magnets, detectors, absorbers and
shielding.
The four RHIC experiments have been designed,
constructed and operated by separate international teams
ranging from 60 to more than 500 scientists. Each group
has employed a different strategy to address the
daunting challenge presented by the enormous complexity
of RHIC events. The BRAHMS collaboration elected to
focus on remnants of the original protons and neutrons
that speed along close to the direction of the colliding
gold nuclei. In contrast, PHOBOS observes particles over
the widest possible angular range and studies
correlations among them. STAR was built around the
world's largest "digital camera," a huge cylinder of gas
that provides three-dimensional pictures of all the
charged particles emitted in a large aperture
surrounding the beam axis. And PHENIX searches for
specific particles produced very early in the collisions
that can emerge unscathed from the boiling cauldron of
quarks and gluons. It thus provides a kind of x-ray
portrait of the inner depths of the fireball.
A Perfect Surprise
The physical picture emerging from the four experiments
is consistent and surprising. The quarks and gluons
indeed break out of confinement and behave collectively,
if only fleetingly. But this hot mélange acts like a
liquid, not the ideal gas theorists had anticipated.
The energy densities achieved in head-on collisions
between two gold nuclei are stupendous, about 100 times
those of the nuclei themselves--largely because of
relativity. As viewed from the laboratory, both nuclei
are relativistically flattened into ultrathin disks of
protons and neutrons just before they meet. So all their
energy is crammed into a very tiny volume at the moment
of impact. Physicists estimate that the resulting energy
density is at least 15 times what is needed to set the
quarks and gluons free. These particles immediately
begin darting in every direction, bashing into one
another repeatedly and thereby reshuffling their
energies into a more thermal distribution.
Evidence for the rapid formation of such a hot, dense
medium comes from a phenomenon called jet quenching.
When two protons collide at high energy, some of their
quarks and gluons can meet nearly head-on and rebound,
resulting in narrow, back-to-back sprays of hadrons
(called jets) blasting out in opposite directions. But
the PHENIX and STAR detectors witness only one half of
such a pair in collisions between gold nuclei. The lone
jets indicate that individual quarks and gluons are
indeed colliding at high energy. But where is the other
jet? The rebounding quark or gluon must have plowed into
the hot, dense medium just formed; its high energy would
then have been dissipated by many close encounters with
low-energy quarks and gluons. It is like firing a bullet
into a body of water; almost all the bullet's energy is
absorbed by slow-moving water molecules, and it cannot
punch through to the other side.
Indications of liquidlike behavior of the quark-gluon
medium came early in the RHIC experiments, in the form
of a phenomenon called elliptic flow. In collisions that
occur slightly off-center--which is often the case--the
hadrons that emerge reach the detector in an elliptical
distribution. More energetic hadrons squirt out within
the plane of the interaction than at right angles to it.
The elliptical pattern indicates that substantial
pressure gradients must be at work in the quark-gluon
medium and that the quarks and gluons from which these
hadrons formed were behaving collectively, before
reverting back into hadrons. They were acting like a
liquid--that is, not a gas. From a gas, the hadrons
would emerge uniformly in all directions.
This liquid behavior of the quark-gluon medium must
mean that these particles interact with one another
rather strongly during their heady moments of liberation
right after formation. The decrease in the strength of
their interactions (caused by the asymptotic freedom of
QCD) is apparently overwhelmed by a dramatic increase in
the number of newly liberated particles. It is as
though our poor prisoners have broken out of their
cells, only to find themselves haplessly caught up in a
jail-yard crush, jostling with all the other escapees.
The resulting tightly coupled dance is exactly what
happens in a liquid. This situation conflicts with the
naive theoretical picture originally painted of this
medium as an almost ideal, weakly interacting gas. And
the detailed features of the elliptical asymmetry
suggest that this surprising liquid flows with almost no
viscosity. It is probably the most perfect liquid ever
observed.
The Emerging Theoretical Picture
Calculating the strong interactions occurring in a
liquid of quarks and gluons that are squeezed to almost
unimaginable densities and exploding outward at nearly
the speed of light is an immense challenge. One approach
is to perform brute-force solutions of QCD using huge
arrays of microprocessors specially designed for this
problem. In this so-called lattice-QCD approach, space
is approximated by a discrete lattice of points (imagine
a Tinkertoy structure). The QCD equations are solved by
successive approximations on the lattice.
Using this technique, theorists have calculated such
properties as pressure and energy density as a function
of temperature; each of these dramatically increases
when hadrons are transformed into a quark-gluon medium.
But this method is best suited for static problems in
which the medium is in thermodynamic equilibrium, unlike
the rapidly changing conditions in RHIC's mini bangs.
Even the most sophisticated lattice-QCD calculations
have been unable to determine such dynamic features as
jet quenching and viscosity. Although the viscosity of a
system of strongly interacting particles is expected to
be small, it cannot be exactly zero because of quantum
mechanics. But answering the question "How low can it
go?" has proved notoriously difficult.
Remarkably, help has arrived from an unexpected
quarter: string theories of quantum gravity. An
extraordinary conjecture by theorist Juan Maldacena of
the Institute for Advanced Study in Princeton, N.J., has
forged a surprising connection between a theory of
strings in a warped five-dimensional space and a QCD-like
theory of particles that exist on the four-dimensional
boundary of that space [see "The Illusion of Gravity,"
by Juan Maldacena; Scientific American, November
2005]. The two theories are mathematically equivalent
even though they appear to describe radically different
realms of physics. When the QCD-like forces get strong,
the corresponding string theory becomes weak and hence
easier to evaluate. Quantities such as viscosity that
are hard to calculate in QCD have counterparts in string
theory (in this case, the absorption of gravity waves by
a black hole) that are much more tractable. A very small
but nonzero lower limit on what is called the specific
viscosity emerges from this approach--only about a tenth
of that of superfluid helium. Quite possibly, string
theory may help us understand how quarks and gluons
behaved during the earliest microseconds of the big
bang.
Future Challenges
Astonishingly, the hottest, densest matter ever
encountered far exceeds all other known fluids in its
approach to perfection. How and why this happens is the
great experimental challenge now facing physicists at
RHIC. The wealth of data from these experiments is
already forcing theorists to reconsider some cherished
ideas about matter in the early universe. In the past,
most calculations treated the freed quarks and gluons as
an ideal gas instead of a liquid. The theory of QCD and
asymptotic freedom are not in any danger--no evidence
exists to dispute the fundamental equations. What is up
for debate are the techniques and simplifying
assumptions used by theorists to draw conclusions from
the equations.
To address these questions, experimenters are
studying the different kinds of quarks emerging from the
mini bangs, especially the heavier varieties. When
quarks were originally predicted in 1964, they were
thought to occur in three versions: up, down and
strange. With masses below 0.15 GeV, these three species
of quarks and their antiquarks are created copiously and
in roughly equal numbers in RHIC collisions. Two
additional quarks, dubbed charm and bottom, turned up in
the 1970s, sporting much greater masses of about 1.6 and
5 GeV, respectively. Because much more energy is
required to create these heavy quarks (according to E
= mc2), they appear earlier in the mini bangs (when
energy densities are higher) and much less often. This
rarity makes them valuable tracers of the flow patterns
and other properties that develop early in the evolution
of a mini bang.
The PHENIX and STAR experiments are well suited for
such detailed studies because they can detect
high-energy electrons and other particles called muons
that often emerge from decays of these heavy quarks.
Physicists then trace these and other decay particles
back to their points of origin, providing crucial
information about the heavy quarks that spawned them.
With their greater masses, heavy quarks can have
different flow patterns and behavior than their far more
abundant cousins. Measuring these differences should
help tease out precise values for the tiny residual
viscosity anticipated.
Charm quarks have another characteristic useful for
probing the quark-gluon medium. Usually about 1 percent
of them are produced in a tight embrace with a charm
antiquark, forming a neutral particle called the J/psi.
The separation between the two partners is only about a
third the radius of a proton, so the rate of J/psi
production should be sensitive to the force between
quarks at short distances. Theorists expect this force
to fall off because the surrounding swarm of light
quarks and gluons will tend to screen the charm quark
and antiquark from each other, leading to less J/psi
production. Recent PHENIX results indicate that J/psi
particles do indeed dissolve in the fluid, similar to
what was observed earlier at CERN, the European
laboratory for particle physics near Geneva [see
"Fireballs of Free Quarks," by Graham P. Collins, News
and Analysis; Scientific American, April 2000].
Even greater J/psi suppression was expected to occur at
RHIC because of the higher densities involved, but early
results suggest some competing mechanism, such as
reformation of J/psi particles, may occur at these
densities. Further measurements will focus on this
mystery by searching for other pairs of heavy quarks and
observing whether and how their production is
suppressed.
Another approach being pursued is to try to view the
quark-gluon fluid by its own light. A hot broth of these
particles should shine briefly, like the flash of a
lightning bolt, because it emits high-energy photons
that escape the medium unscathed. Just as astronomers
measure the temperature of a distant star from its
spectrum of light emission, physicists are trying to
employ these energetic photons to determine the
temperature of the quark-gluon fluid. But measuring this
spectrum has thus far proved enormously challenging
because many other photons are generated by the decay of
hadrons called neutral pions. Although those photons are
produced long after the quark-gluon fluid has reverted
to hadrons, they all look the same when they arrive at
the detectors.
Many physicists are now preparing for the next energy
frontier at the Large Hadron Collider (LHC) at CERN.
Starting in 2008, experiments there will observe
collisions of lead nuclei at combined energies exceeding
one million GeV. An international team of more than
1,000 physicists is building the mammoth ALICE detector,
which will combine the capabilities of the PHENIX and
STAR detectors in a single experiment. The mini bangs
produced by the LHC will briefly reach several times the
energy density that occurs in RHIC collisions, and the
temperatures reached therein should easily surpass 10
trillion degrees. Physicists will then be able to
simulate and study conditions that occurred during the
very first microsecond of the big bang.
The overriding question is whether the liquidlike
behavior witnessed at RHIC will persist at the higher
temperatures and densities encountered at the LHC. Some
theorists project that the force between quarks will
become weak once their average energy exceeds 1 GeV,
which will occur at the LHC, and that the quark-gluon
plasma will finally start behaving properly--like a gas,
as originally expected. Others are less sanguine. They
maintain that the QCD force cannot fall off fast enough
at these higher energies, so the quarks and gluons
should remain tightly coupled in their liquid embrace.
On this issue, we must await the verdict of experiment,
which may well bring other surprises.