Argonne-designed instruments vital in RHIC discovery
ARGONNE, Ill. (May 27, 2005) — Argonne researchers played a significant
role in research that led to the surprising finding of a possible ideal liquid
instead of the expected quark-gluon plasma at Brookhaven National Laboratory's
Relativistic Heavy-Ion Collider (RHIC).
On April 18, each of the four major experiments at RHIC released white papers
summarizing the first four years of RHIC operation and their findings from
high-energy collisions of gold nuclei. At the incredibly high temperatures
and pressures created in the collisions, physicists expected to create “quark-gluon
plasma” — a gaseous state of matter thought to have existed in the first few
microseconds after the Big Bang. Instead, the matter created inside the detectors
behaved like a liquid — a completely unexpected result.
“It's being debated,” said Argonne physicist Birger Back, a member of the
Phobos Detector team at RHIC. “Did we actually find what we set out to find,
which is the quark-gluon plasma? In some sense, we haven't. We found something
different. We found that nature behaves different than what we — perhaps naively — expected.”
RHIC as a racetrack
RHIC accelerates two beams of ions — atoms stripped of their electrons — giving
them a strong positive electric charge to take full advantage of the machine's
accelerating and bending capabilities. RHIC primarily uses ions of gold, one
of the heaviest common elements, because its nucleus is densely packed with
particles. The beams travel in opposite directions around a 2.4-mile, two-lane “racetrack” at
99.995 percent of the speed of light. Objects nearing the speed of light gain
mass via relativistic effects (hence the machine's name), so when the beams
intersect, the colliding ions achieve fantastic temperatures and densit ies:
more than a trillion Kelvins and 10 times the density of normal nuclear matter.
These extreme conditions were expected to have liberated quarks and gluons from their quantum prisons inside the gold protons and neutrons, producing
a primordial particle soup — quark-gluon plasma.
Argonne's STARring role
Scientists from Argonne's Physics and High
Energy Physics divisions
were involved in two of the four RHIC experiments: STAR and Phobos.
The STAR detector is a large volume, gas-filled detector to record charged
particle tracks in a solenoidal magnetic field. This is surrounded by electromagnetic
calorimeters, which were partially designed and constructed at Argonne . The
calorimeters can also detect high energy photons and neutral particles.
High-energy physicists Bob Cadman, Dave Underwood and Hal Spinka analyzed
proton-proton collisions and gold-on-gold collisions at STAR, which contributed
to the observation of qualitative differences in particle production at RHIC
The Argonne high-energy physicists are primarily interested in the ways quarks
and gluons contribute to the total “spin” — or intrinsic angular momentum — of
protons. They study collisions of beams of polarized protons, which are produced
in such a way that their spins are aligned.
The Phobos detector measures the temperature, size and density of the fireball
produced in the heavy-ion collisions. It also reveals the ratios of the particles
produced.
Phobos consists of silicon detectors surrounding the interaction region, which
were designed, tested and assembled in a joint effort by scientists and engineers
at Argonne and the University of Illinois at Chicago . FermiLab's Silicon Detector
Laboratory performed microwire bonding to connect the detectors to the microchip
pre-amplifiers. Massachusetts Institute of Technology developed two high-quality
magnetic spectrometers that study 1 percent of the produced particles in detail.
Surprising interactions
“The original expectation was that quarks and gluons would move freely inside
the plasma with very little interaction,” Back said. “The surprising fact is
that there is very strong interaction among the particles. We've seen several
signals to verify that.”
A key signal that lets physicists know when two quarks collide is the production
of a pair of pions, which leave the scene of the collision in exactly opposite
directions perpendicular to the beam.
In the high-energy gold-gold experiment, in which the colliding particles
each contained 79 protons and 118 neutrons, some of those pions went missing — a
key clue.
“When gold protons collided near the edge of the interaction, one pion could
escape in the usual way,” Cadman said. “The other disappeared, which
indicated that the matter produced in the collision had abnormally high density.”
Rather than behaving like a gas, as was expected, the collisions produced
a state of matter more like a liquid. In fact, it behaved like an “ideal liquid,” with
extremely low viscosity.
“From the emission pattern of the particles coming from the collisions, we
see what we call a ‘collective flow.' That was a surprise,” Back said. “That
was one thing that our experiment, Phobos, contributed to.”
In a gas, particles can be emitted in all directions; in a denser medium,
such as liquid, the particles are emitted preferentially. The matter produced
in the RHIC collisions occurred in an almond-shaped region; more particles
were emitted along the thin axis than the long axis, suggesting that particles
emitted in that direction were being scattered or reabsorbed.
“I think everyone agrees that we have created matter with an energy density
that far exceeds what is needed for a quark-gluon plasma,” Back said. “However,
it's circumstantial evidence: all the conditions are right, but it doesn't
exactly prove that you have plasma. It didn't behave as it was supposed to.
There were a number of signals predicted, but they have not been found. There's
no smoking gun.”
At higher energies, the liquid may become non-interacting — more like the
quark-gluon plasma that was expected, Back said. RHIC researchers are now analyzing
the results of copper-copper collisions at various energies. Results may be
released in August.
“A lot is up to the theorists to interpret what we've seen,” Back said. “There
are years of analysis and paper-writing ahead.”
The experiments have allowed physicists to probe the theories of quark behavior.
The theory, called quantum
chromodynamics, is too difficult to calculate even
on today's most powerful supercomputers. Data from these experiments will allow
physicists to improve their models and their ability to calculate the interactions
of quarks and gluons.
“It's nice when you understand what's going on, but it's more exciting when
you find something you don't understand,” Back said. — Dave Jacqué
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