Collider Scientists Still Seek Elusive Zs
Scientists at the Stanford Linear Accelerator Center, more than a year behind schedule, are struggling to make one of the more sophisticated scientific instruments ever built work for more than a few minutes at a time.
Although the Stanford Linear Collider was supposed to have started producing an elusive subatomic particle called the “Z” in the spring of 1987, it has so far failed to capture even one.
The collider has pushed technology to new limits in an effort to break ground in high energy physics, and by working in brief spurts it has proved that the bold concept is on the right track, according to those who have built the $115-million machine.
Uncooperative Beast
But, said Nobel laureate Burton Richter, director of the center, “We can’t keep the miserable beast running long enough.”
Physicists at Stanford had hoped to win a high stakes race with European colleagues who are building a new facility to capture Zs at the center for high energy physics near Geneva, where the particle was discovered five years ago. It now appears less likely that Stanford will win that race, and the Europeans could be the first to collect enough Zs to unlock some of the innermost secrets of the atom, including the process that releases radioactivity.
The European collider is of a conventional design and is not expected to have many problems starting up next summer. But Stanford’s is a radically new concept that is unlike any collider ever built before.
It seems that all of the Stanford collider’s many parts have been made to work independently--and occasionally at the same time--but when they are asked to work together for an extended period, something invariably goes wrong.
The collider is so sensitive that scientists monitoring its performance even know when a truck rumbles down Interstate 280, which crosses over the 2-mile-long race track used to accelerate electrons and positrons to nearly the speed of light. Engineers who have worked day and night for the past couple of years to bring the pioneering machine into operation are constantly frustrated by minor glitches that degrade its performance.
“We’re getting close,” physicist Johnathan Dorfan said recently as he stood in the control room at the center in the hills above Stanford University south of San Francisco. “But we have to keep things together for longer than 10 minutes.”
To produce Z particles, the collider must run flawlessly for hours at a time. That singular goal has proved so elusive that earlier this month Richter took personal charge of the project and temporarily abandoned the quest for Zs. The current goal is simply to make the machine work.
“We had two goals, to prove the technology and get some physics done,” Richter said. “The first goal has been met,” by getting the beams to collide, even for short periods.
He noted that physicists in Japan and Europe are already planning the next generation of linear colliders because the work at Stanford has proved the technology is feasible. The concept is attractive because linear colliders can cause collisions at far greater energies at much less cost than conventional colliders.
“They no longer think we are a bunch of crazy, wild-eyed nuts,” he said.
But Richter admits he has been extremely disappointed that the facility has failed to produce any Zs, which are created when electrons collide with a positively charged sister particle, called a positron.
“This is giving us terrible heartburn at the moment,” Richter said. “There are 130 physicists sitting there waiting to take data.”
The Z particle is believed to be the bearer of the so-called “weak force,” the subatomic force that allows one element to decay into another, causing radioactivity. If enough Z particles can be captured, scientists should be able to determine their life span and how they fit into the complex subatomic structure.
Dreamers in the world of physics believe a fuller understanding might ultimately lead to ways of controlling radioactivity, but the primary goal of any collider is simply basic research.
Richter and others associated with the project insist the collider will eventually produce Z particles, but are not certain when. The delay is particularly agonizing for particle physicists like Dorfan who have been waiting for the machine to go into full operation so that they can do their science.
“There’s a need for people to see one Z,” Dorfan said. “It will make us all feel fantastic.”
Problem With Parts
The problems have grown partly out of the fact that the collider was built largely from materials cannibalized out of previous projects at the center. That helped hold the price down to about one-tenth the cost of the collider that is now under construction near Geneva. But some of that old equipment, Richter said, has not been up to the task.
In addition, the level of precision at which the collider must function has required near perfect performance throughout the system, and almost as soon as the facility is brought up to speed “things begin to degrade,” said Michael Riordan, a physicist who acts as spokesman for the center.
The collider fires two beams of particles--one of electrons and the other of positrons--down the 2-mile race track, which was built for another project 20 years ago. When they reach the end of the track, the two beams are channeled in opposite directions around a curved course. At the far end of that course, the beams are to collide. If the beams are small enough, and if they are packed with enough particles, collisions should occur and Z particles should be created.
But to work, the beams must be no wider than two microns (about one tenth the thickness of a human hair) and they must be focused so accurately that they will hit head on.
Source of Skepticism
In the beginning, there was considerable skepticism over whether the Stanford group could even get the beams to collide, because the degree of control is far beyond the level of precision required for any other type of particle accelerator.
“We have achieved that,” Riordan said. “People now believe in linear colliders.”
But the beams are still two to three times broader than they should be, and almost as soon as the system starts working properly, something goes wrong.
During a 48-hour trial in July, for example, the beams were colliding only 6% of the time, and only half that time was suitable for taking data.
“We’re not very proud of that,” Riordan said.
Unusually hot weather has contributed to the problems. The collider’s control system includes 30 microprocessors that are in areas where they cannot be cooled properly, including several along the two-mile race track. On one particularly hot day, all but one of them overheated and failed, Riordan said.
Some of those microprocessors will be moved to areas where they can be air conditioned, Riordan said, but some other problems remain very challenging.
A Matter of Control
If the beams are not under perfect control, for example, particles scrape the inside of the pipe through which they travel down the accelerator, creating debris that pushes the beam even farther off course.
Despite the problems, the federal Department of Energy, which funded the project, remains supportive, said David Nelson, a physicist and executive director of the federal agency’s office of energy research.
“We knew up front we were taking a risk,” Nelson said. “We think that risk was and is an extremely important risk to take.”
Nelson said the collider has not met all of its goals because it failed to produce Z particles last summer, but he insisted the federal government has already earned an acceptable return on its investment because the facility has proved the feasibility of the technology.
“We justified this (expenditure) as a research and development project, not a Z factory,” he said.
But even Nelson admits it would be nice to catch a few Zs.
Richter said he still hopes to have a few thousand Z particles in hand by the time the new European facility cranks up next summer, but he knows that will be difficult.
“We will make it work,” he said, “But the question is when.”
STANFORD’S COLLIDER: WILL IT WORK?
The Stanford Linear Collider differs radically from the accelerators that physicists have been using for decades.
1. Electrons are fired down the accelerator and into a target which produces positively charged electrons, called positrons.
2. The positrons are sent back to the “injector” end of the accelerator.
3. Beams of positrons and electrons are fired down the accelerator and into “damping rings”, which compress the size of each stream of particles.
4. The beams emerge from the damping rings and travel down the accelerator. The positrons and electrons are accelerated by an electric field, riding microweave pulses like surfers on the crest of a wave.
5. Electrons are sent down one arc and positrons are sent down another, placing them on a collision course.
6. The particle streams are further compressed to about one-tenth the thickness of a human hair, and the beams are focused for a head-on collision.
7. Traveling at nearly the speed of light, the two beams collide at tremendous energies. The debris from the at collision should tell scientists much about the force that allows particles to change from one kind to another: the so-called “weak force” of nuclear physics.
8. Most particles will not collide and must be captured because they will be slightly radioactive. They will be siphoned off to “particle dumps” buried in the hillsides along the arcs.
How a Conventional Collider, Like the One Under Construction in Europe, Works.
Particles are shot in opposite directions through a giant circular accelerator. The particles, launched from small booster accelerators, are accelerated by an electric field to nearly the speed of light. Giant magnets bend the particle streams around the circle. The particles collide near detectors, where debris from collisions could be captured and studied.
