Beam Up, Running at CEBAF
First experiment begins at physics laboratory
After years of planning and construction, and months of rests and delays, the first actual experiment at the Continuous Electron Beam Accelerator Facility has finally begun.
The six-week project, which began this week, marks a milestone in the 20-year development of the $600 million Newport News physics lab, where researchers are studying the fundamental nature of matter - especially the subatomic particles at the core of atoms, the building blocks of all matter.
The Department of Energy has pinned much of its hopes for future research on Jefferson Lab, especially in the wake of Congress' decision in 1993 to scrap the far more ambitious Superconducting Super Collider project in Texas.
But the first experiment's start was delayed throughout the summer and fall while Jefferson Lab physicists struggled to produce an electron beam that was both accurate and continuous - a requirement to obtain usable data.
That hurdle, they say, has finally been passed.
"I think the people were under an enormous amount of pressure," said Roger Carlini, physicist and leader of Hall C, the underground cavity where the first experiment is taking place. I think people are in a decompressing mode now. We're enjoying the fruits of what we have done. The tension is down a bit."
Now, it's the users' turn to begin producing results.
In whistle-clean rooms under a huge, vegetation-covered dirt mound, casually dressed physicists - blue jeans and sweaters were the uniform of the day Friday - sidestepped what seems to be miles of white cables protruding from the backs of control panels and entered a room filled with computers and monitors. The screens displayed graphs, rows of numbers and live camera shots.
In one, red lights flashed inside the cavernous, equipment-laden underground target area. In another, a bright white spot showed where the high-powered hair-width beam was focused.
Scanning the screens were members of a group led by nuclear physicist Donald Geesaman of the Argonne National Laboratory near Chicago. The first order of business was calibrating the two detectors, known as spectrometers, that would record the results of subatomic collisions caused as the electron beam strikes various target materials.
By about Christmas, about 100 experimenters - most of them from university groups - will have had a hand in the collection of data from 7 million particle collisions.
It's twice the size of a normal experimental group. But many helped build the lab's components during the 10-year construction process and Geesaman said, bring a level of expertise about Jefferson Lab's components that his group doesn't possess.
Being first to use Jefferson Lab, an Energy Department manager joked last spring, is something "only an idiot wants." Geesaman acknowledged that most would prefer to let someone else go first" and make sure it works. But he said he was so anxious to get started that he was willing to put up with the potential problems.
"And for some of us," he added, "ironing out the kinks is part of the fun. It's creative."
Jefferson Lab experiments, like those at other national laboratories, are geared toward a far-reaching understanding of the subatomic world akin to the peek into the distant regions of the universe that the Hubble Space Telescope is giving astronomers.
Think of Jefferson Lab as "Hubble in reverse."
To take this journey into the infinitesimally tiny world of the atom, Jefferson Lab users have at their disposal a continuous stream of electrons that jets around an underground oval through supercooled cavities at close to the speed of light.
The beam, which generates 4-billion electron volts of energy during its 4.37 mile journey, then strikes its target. Geesaman's targets are ultra-thin sheets of carbon, iron or gold foil, chosen, he said, because they won't melt.
As electrons collide with the target's protons, highly sensitive spectrometers in one of Jefferson Lab's three halls - two are still being completed - record the speed, mass, energy and direction of the particle collisions.
From this data - which can be displayed as raw numbers, as a graph or as pictures of the results of individual collisions - physicists can tell if a particle is or has become electrically charged. Although they cannot see the actual collisions - they're too small, Geesaman said - physicists can deduce what has taken place.
It's somewhat, Carlini said, like being given a box of identical wristwatches and being told to figure out how they work. Instead of taking them apart, the watches must be thrown against a wall and shattered. By watching the direction, speed and eventual resting place of the various parts as they fly out, the inner workings of the watch can gradually be determined.
"We're measuring probabilities." Geesaman said.
Put another way, imagine hitting a cue ball into a group of billiard balls loosely packed into a triangle shape - the "rack" arrangement used to start a game of pool. Subatomic particles are loosely packed according to a theory known as quantum chromodynamics, which says that there's a space between each particle equal to half of the particle's diameter.
To an electron, that's a chasm. Fired at the nucleus of an atom, most fly straight through, untouched. But about one percent of the electrons strike a proton.
The physicists, however, can't see the cue ball strike the rack. "All we know is that the ball ended up in the pocket." Geesaman said.
Other variables further complicate the research. Sometimes, for instance, a proton can be knocked out of the nucleus without touching any of the adjacent particles. One of the aims of Geesaman's experiment is to find out why.
"This is not the kind of experiment where we're out to discover a new particle," Geesaman said. "What we're doing is really the why part of it. We're trying to answer why, when we knock a proton out of the nucleus, it interacts the way it does."
In the past, Carlini said, looking for ever-smaller particles - like the quarks that physicists believe are the base components of protons and neutrons - has increased the understanding of the nature of matter.
It has allowed scientists to devise more accurate models of the atom. But the process also opens new cans of worms.
"When you look smaller and smaller," Carlini said, "the simple models begin to break down. One of the basic problems in particle physics is, how far can it go?"
Physicists hope experiments like Geesaman's will allow them to determine whether quantum chromodynamics is the "final theory of atomic structure and behavior."
"We're not absolutely positive," Geesaman said. "So we need to test, to tug on it...to see if it breaks.
"If we've reached the end, we'll be ecstatic," Geesaman said. "But people have thought that they reached the end before, and they were wrong."
It may be months or years, physicists say, before conclusions can be drawn about the data being collected at Jefferson Lab. But while they note that it's premature to speculate on any practical applications for the research, they remain confident that some good will come of it - as, Carlini maintains, it almost always does.
"Ninety percent of the time, you get nothing," Carlini said. "But 10 percent of the time, you get how we made our modern world."