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slam! Crumble, Create Scientists will smash gold ions together to achieve a new state of matter Monday, June 14, 1999 By Byron Spice, Science Editor, Post-Gazette
Scientists are about to step into a realm where solid, liquid and gas have no meaning. Bereft of familiar landmarks in a formless soup, they nevertheless hope to make one of the most important discoveries of the century.
And they won't even need to leave Long Island to do it.
At Brookhaven National Laboratory in Upton, N.Y., the journey may begin yet this week, as researchers crank up their new, $616 million atom smasher, the Relativistic Heavy Ion Collider, or RHIC.
The machine will take atoms of gold, strip them of their electrons and then fling their naked nuclei into a 2.4-mile underground ring along one of two parallel guideways -- one running clockwise, the other counterclockwise. Moving in tight bunches, the gold ions will accelerate to speeds approaching that of light.
At these "relativistic" speeds, the normally spherical nuclei will flatten into pancakes. But that deformation is nothing compared with what will happen at a handful of experimental sites along the ring, where the two beams of gold ions, travelling in opposite directions, will cross over each other. Each time two nuclei slam into each other head-on, they will disintegrate into thousands of weird particles.
"The reason for doing all of this is to create a new state of matter," said Thomas Ludlam, associate project director for RHIC.
This new state would not be solid, liquid or gas; matter as we know it in the form of molecules, atoms, atomic nuclei, or even subatomic particles such as protons and neutrons would not exist. Instead, it's expected to be what's called a quark-gluon plasma -- a trillion-degree hot soup of the elementary particles, called quarks, that normally comprise protons and neutrons inside atomic nuclei and the particles called gluons that hold quarks together.
These conditions, many physicists believe, have not existed since the early moments of the universe. Within the first second following the Big Bang thought to have begun the universe's relentless expansion, all matter would have existed in the form of quarks and gluons.
The only place that such a plasma might exist today is in the center of neutron stars -- dense, tiny stars created when an old star explodes.
Morton Kaplan, a Carnegie Mellon University chemist involved in one of RHIC's major experiments, plans to be on hand when the first collisions take place within a few weeks. "To actually be on the scene, to feel the excitement, see the signals coming in, . . . It's hard to describe unless you see it," he said. "There's a tingling in the air.
"We're going to see so many new phenomena that we've never seen before."
Exploring this quark-gluon plasma, Ludlam agreed, "should be one of the most important discoveries of the century."
The question may be which century. Construction of the accelerator, just now reaching completion, has stretched on for almost nine years. Though Ludlam expects that the first ion collisions may occur early next month, the initial accelerator run through the end of July is intended only as an engineering shakedown. The formal science program won't begin until the accelerator is switched back on in November.
The researchers using the Brookhaven atom smasher aren't the only ones trying to create a quark-gluon plasma, Kaplan acknowledged, but because they'll be using heavy ions and only heavy ions, they may enjoy advantages over other physicists.
Some of the highest energy accelerators in the world, such as Fermilab near Chicago and the European Laboratory for Particle Physics, known as CERN, in Switzerland, he explained, primarily produce beams of individual protons or other "light" ions. When protons slam into a target or collide with each other, they release tremendous heat. Gold, by contrast, is a heavy element, with 79 protons and 118 neutrons in its massive nucleus, so when two gold ions slam together, the result is not only high temperatures, but also high nuclear density, he explained.
The result of a heavy ion collision also is messy -- between 5,000 and 10,000 particles might be generated in a head-on collision.
The willingness of the RHIC experimenters to cope with that mess reflects a cultural divide within the physics community often unappreciated by the general public. Particle physicists using high-energy machines such as Fermilab tend to design elegant experiments that seek a specific answer to a specific question -- looking for a particular type of particle, for instance, whose existence is predicted by a theory.
RHIC, by contrast, is a project of nuclear physicists, who are more interested in studying the behavior of nuclear matter than they are in tidying up theories. "I think we are less intimidated by complexity," said Kaplan, a chemist whose interest in nuclei drew him to nuclear physics experiments. "It's somewhat more a case of, 'Let's do it and see what we find.' "
Which is not to say that people don't have a lot of ideas about what they expect. "You need to prepare yourself, as much as you can, for what will come out of the detector," said Tim Hallman, Brookhaven group leader for the Solenoidal Tracker at RHIC, or STAR, one of two major experiments on the collider. Both Hallman and Kaplan were founding members of the STAR collaboration, which now includes 35 institutions and about 450 researchers worldwide.
STAR is a 1,200-ton, house-size instrument that will detect and record the heavy ion collisions. At the heart of the $60 million detector is the Time Projection Chamber, a fancy name for a three-dimensional digital camera. It will track the trajectory of each of the thousands of particles generated in a collision.
To do so, the camera includes about 140,000 wires; charged particles flying by a wire induce a current in the wire. By recording these "hits," the track of each particle can be reconstructed.
Computer simulations of gold-gold collisions performed at the Pittsburgh Supercomputing Center and elsewhere helped determine the best arrangement and combination of those wires, Kaplan said, to produce images containing the most information.
The Pittsburgh center, along with supercomputers in California and Japan, also generated simulations for a quarter million such events, which were used last year to test the RHIC facility's ability to handle the flood of data from STAR. Each event will produce 15 megabytes, or 15 million bytes, of information; if just one event is recorded each second, a year of operations will yield some 200 to 300 trillion bytes, Hallman said. The RHIC computing center will thus have to begin some analysis almost immediately.
"If you get behind," Kaplan added, "you will never catch up."
The tests, called Mock Data Challenge I and II, helped identify bottlenecks and led to software changes that sped up analysis and reduced the need for memory.
But even before computers start processing data from the events, decisions must be made about which events merit study. About 10,000 collisions might occur every second in the STAR detector and the computer system can only handle one a second, so Kaplan's group and their associates have designed a trigger system that will rapidly identify the most interesting events.
Unusual is the watchword, so Kaplan's trigger uses information that will be available almost instantaneously -- the total number of charged particles produced, the total energy produced -- to identify the extraordinary events.
In addition to STAR, there is one other major detector, called PHENIX, and two minor experiments, known as Brahms and Phobos. STAR is the most complete of the four, but all will be operational once the ion beams are turned on.
The RHIC accelerator ring occupies a tunnel originally built for Isabelle, an accelerator that the Department of Energy abandoned during its construction in the 1980s. The gold ion beams will be created and undergo their initial acceleration in several existing Brookhaven facilities, including the 40-year-old Alternating Gradient Synchrotron.
Inside the RHIC ring, more than 1,700 superconducting magnets, each nine meters in length, are strung together like beads on a string. Some of these magnets turn the beams, so they go round and round the ring building up speed, and others focus the beams. The beams actually consist of about 60 small bunches of ions and each must be as tightly packed as possible if collisions are to occur efficiently, Ludlam said. But the massive nuclei, all carrying a positive charge, repel each other and thus would blow apart if not for the magnets.
Those forces that blow up the beam actually decrease as the speed of the ions increases, Ludlam said. Once the beam enters the ring, operators have just a few minutes to accelerate it to relativistic speeds, or the bunches will start coming apart and reduce the beam's luminosity.
For several weeks now, the largest refrigeration system in the world has been working to cool these magnets to superconducting temperatures. That means several thousand tons of material must be chilled with liquid helium to a mere handful of degrees above absolute zero. One of the two guideways, the "blue" ring, has cooled and should be ready to begin accelerating ions early this week, Ludlam said. A leak in a valve room has placed the other, "yellow" ring behind schedule, but he predicted it too will be ready to accept beams later this week.
"It's coming down to the wire," Ludlam said, noting construction was to finish by June 30.
"We expect great things," Hallman said, predicting discoveries by RHIC could reshape thinking about the nature of matter and energy and the early universe. "It's a completely new environment. The very first data we see, even if it's not extraordinary in a fundamental sense, will be very interesting."
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