Accelerator Planners Worry That SSC May Be A Hard Act To Follow

The next big project in high-energy particle physics is likely to use electrons and be cheaper to build Not an inch of tunnel for the 53-mile Superconducting Supercollider has been laid, but already the massive accelerator is casting a long shadow on the future of high-energy physics facilities. As Congress and the nation struggle with the estimated $7 billion cost of the huge machine, many scientists are coming to the conclusion that the SSC may signify the end of the line - a dinosaur as big and expensive as physics research machines will ever grow. But it's not clear what should come next, nor who should design and build it. That question is particularly relevant to the two laboratories most affected by the construction of the supercollider - the Stanford Linear Accelerator Center (SLAC) in Palo Alto, Calif., and Fermi National Laboratory (Fermilab) in Batavia, Ill. If the SSC represents a budgetary brontosaurus around which the rest of the community must tiptoe, then they and other U.S. labs may have to bid goodbye to increasing budgets. And if it represents a financial ceiling for physics projects, then the key to continued growth in the field will likely lie in a radical new approach to research. That next step must be a project that is both relatively cheap and scientifically compelling, researchers say. The answer may rest with using electrons instead of protons. But no one has yet come up with a machine that fills the bill. "At the moment there isn't anything on the horizon that can take us beyond the SSC except for a larger SSC," says University of Chicago physicist and 1988 Nobel laureate Leon Lederman. Scientists at a half-dozen labs across the country are working to change that situation. Probably the most intensive research effort is taking place at SLAC. Design teams planning for the next decade now constitute 10% of the center's 6,000 employees, and that percentage is expected to grow with every year. More than any other U.S. high-energy lab, SLAC believes that its existence depends on its ability to come up with a workable design for the next generation of particle accelerators. What might such an accelerator look like? The SSC uses protons, powerful magnets, and a racetrack-shaped ring to create collisions of unprecedented fury. SLAC, on the other hand, has historically finessed itself into the record books with humble but easy-to-manage electrons, and a simple three-mile tube known as a linear collider. But while many physicists believe that the SLAC approach represents the most likely direction for high-energy physics after the SSC, not all are convinced that SLAC will be there to see it happen. "I think it's clear that SLAC has a problem in coming up with something," says Lederman, the former director of Fermilab. "They can't do much to increase their energy, and their only claim to fame is that they use electrons." That difference is significant, believes Burton Richter, SLAC director and the leader in the quest to find the next step. "In the past," says Richter, "there has been considerable overlap in what you could do with the two different kinds of accelerators. But to go much beyond the [science] of the SSC it doesn't look like protons are even viable." Five years from now, scientists at the Stanford Linear Accelerator Center, in concert with the neighboring Lawrence Berkeley National Laboratory, hope to be finishing work on a facility that would let them search for the beauty, or bottom, quark. Nestled within the existing accelerator ring, it would be turned on just as the current Stanford Linear Collider (SLC), which is currently pumping out Z particles, would be reaching the end of its useful life. Under a plan that would have the SLC inject particles into the new accelerator, physicists believe that they can build such a b-factory for around $250 million. But 3,000 miles away, in Ithaca, N.Y., a team of physicists at Cornell University wants to build its own b-factory. Cornell has already demonstrated the most successful b-factory-type accelerator to date, known as the Cornell Electron Storage Ring. And its scientists have more experience in b-quark physics. They also say their facility would be cheaper to build. The machine not only would use an existing ring and injector, like at SLAC, but also would require so few modifications that the total cost would be under $100 million. The Sciulli panel, created by the Department of Energy to review plans for future accelerators, is expected to recommend the construction of only one b-factory. And science won't be the only criterion. The Cornell facility is supported by the National Science Foundation, where funding for high-energy physics has been notoriously tight in recent years. At the same time, SLAC gets its funding from DOE, which is understandably reluctant to adopt a new lab while it builds the SSC and tries to maintain support for a host of other labs. On balance, however, the DOE affiliation is seen as an advantage. "At this point being NSF-funded is not a great place to be. It's a strike against us," says Cornell physicist Robert Siemann, who is also a member of the HEPAP subpanel studying the issue. But Siemann believes the scientific strength of the Cornell proposal will influence the Sciulli panel to see things the university's way. (DOE has not decided whether Siemann himself - or panel member David Leith from SLAC - should be allowed to vote on the issue.) The proposed Cornell machine would have a high "luminosity," a measure of the number of collisions that will occur. Because b-quark physics requires sensitive measurements of subtle phenomenon, scientists must record vast numbers of collisions before their results are statistically significant. The edge, says Siemann, goes to the b-factory with the highest luminosity. Stanford, on the other hand, claims its own ace in the hole. The SLAC b-factory proposal calls for an asymmetrical collider, in which electrons would be accelerated one way around one ring, while positrons (positively charged electrons) would be spun the other way at an energy three times larger. The advantage, says project planner Andrew Hutton, is that the subsequent collision between the two particles is easier to read if they are moving with different momentum. "If two Fords hit each other head on, you get a pile of parts in the middle," explains Hutton. "But if a Ford hits a Fiat, the parts all come flying out one side, and you can see what happened," he says. What the Sciulli panel must decide is whether that feature compensates for SLAC's lower luminosity. A decision favoring one proposal will likely mean no b-factory at the losing lab, although collaborations are possible. Says SLAC director Burton Richter, "I've told the Cornell people that I think one [U.S.] b-factory is enough. I've invited them to join us, and I hope that if they win, we'll join them." --G.C.A. SLAC has its own problems, however. Linear accelerators are limited in the energy they can generate because they propel particles in one long spurt, from the beginning to the end of their straight beam tubes. Ring-shaped accelerators, on the other hand, send particles around the loop thousands of times each second, giving them a push with every turn. Although those energies are not boundless - the particles are "steered" around the accelerator's curves by a series of large and powerful magnets that are currently about as large as scientists know how to make - the rings can typically reach energies that are an order of magnitude higher than those in linear colliders. This month a unit of the Department of Energy's High Energy Physics Advisory Panel (HEPAP) will begin a survey aimed at setting priorities for the 1990s. The Energy Department has asked the independent subpanel to visit the top U.S. accelerator physics laboratories and rank their upgrade proposals based on an appraisal of their scientific validity and their value to the physics community. The report of the panel, chaired by Columbia University physicist Frank Sciulli, is due April 30. While the Sciulli panel will be reviewing many accelerator facilities, the SSC's shadow is expected to loom large in its thinking. As an electron accelerator, SLAC has no direct role in training scientists to work on the SSC. A negative or less than enthusiastic appraisal by the Sciulli panel, say many scientists, could seriously jeopardize the future of the Stanford lab. Fermilab, in contrast, has five-year-old Tevatron - a superconducting proton accelerator like the SSC - which will serve as a natural training ground for future SSC scientists. Fermilab is planning a series of scheduled upgrades that are expected to carry it safely through the next decade. But SLAC officials aren't sitting on their hands waiting for others to decide their fate. Facing what appears to be a do-or-die situation, they have prepared an ambitious plan to carry the lab into the next century. For the immediate future, SLAC plans to use polarized beams to continue its research into Z particles. The first months of its new linear collider last spring featured a spirited competition with the Large Electron Positron (LEP) collider at CERN to be the first to generate large quantities of rare Z particles. As the LEP's ability to generate large quantities of Zs enables it to move ahead of its U.S. counterpart, SLAC scientists have settled down to exploring certain properties of the particle in detail. But that work is expected to sustain the lab for only a few more years. Until recently, the next step was supposed to be a machine currently known only by the generic designation "Next Linear Accelerator" (NLC). But while many scientists believe that a NLC-like machine does indeed represent the future of high-energy physics after the SSC, most agree that current technology is not up to the task of designing and building one. Indeed, it may be another five years before they can know whether such a machine is even possible. But the mid-1990s, unfortunately, coincides with the peak period of SSC construction. Facing that kind of competition for funds, the NLC will probably have to wait until the SSC is finished, sometime around the turn of the century. This setback has left SLAC with a gaping hole in its program. The solution Richter and his staff have come up with is called a "beauty factory," or, more simply, a b-factory (see story on page 14). The machine takes its name from what are variously known as "beauty," or "bottom" quarks - the particles that scientists expect to produce in large quantities using such an accelerator. By examining thousands of b-quarks, researchers hope to find that slightly more than half tend to turn more one way than another. Such a finding would violate the laws of symmetry that until recently have been accepted as scripture. And that discovery, in turn, would help physicists untangle the complex interactions that govern the behavior of matter in the universe. Five years beyond the b-factory, SLAC planners hope to gain approval for their version of a next-generation linear collider. It would likely consist of two 3-mile-long beam lines laid head to head, "like two guys with rifles shooting at each other," says project planner Andrew Hutton. (The current linear accelerator features two groups of particles going down the same tunnel, branching off, and then coming together for a collision.) By accelerating electrons to two and a half times the 100 MeV of the beam now being generated in the linear accelerator, SLAC researchers hope to produce collisions strong enough to reveal for the first time the long-sought Higgs boson and the "top" quark. But some snags have already appeared in the Stanford plan. Japan, the Soviet Union, and Switzerland are all considering an NLC-like machine. With top-notch talent - and a potentially healthier funding climate - abroad, "it's not clear the NLC should even be built in the U.S.," says SLAC theoretical physicist Michael Peskin. Another complication is that the NLC, as currently envisioned by SLAC, would be several miles too long to fit on the land now occupied by the current linear collider. Although the university owns enough adjacent land to allow the new accelerator to be built at a diagonal to the current machine, little of the existing facility could be recycled. And that creates another problem for Richter. Without a physical foundation on which to base the NLC, there are no compelling reasons to build the new machine at SLAC. The arguments in favor of SLAC come down to such abstract qualities as institutional experience and an existing network of scientists and support personnel. But those reasons may not be enough to convince Congress, DOE, or the taxpayers. "If it can't fit here, then you talk about doing it somewhere " acknowledges Richter. Basing the NLC at another lab "is not my preferred solution, but it seems to me that the most important thing for the long term is that physics continue to advance," he says. The $1 billion cost of such a machine, says Richter, is secondary. "My pitch to the Sciulli panel is that we'll worry about [money] later on. Ten years from now, the [funding] situation may be very different." Whatever HEPAP concludes, the field of electron physics appears to have an edge over proton physics. If the SSC really represents the final stage of proton accelerators, scientists are expected to turn increasingly to electrons to explore atomic and subatomic features with greater detail. "What electrons offer is a clean initial state," explains Lederman. In contrast to the proton, which is made up of three relatively large quarks, the electron acts as if it has no spatial dimensions at all. The advantage is that electron collisions produce fairly simple tracks involving few by-products, compared to the virtual subatomic zoo that emerges from a proton collisions. "If an electron is a point," Lederman says, "then a proton is a garbage can." At Stanford, Richter and his colleagues are hoping to emphasize in their discussions with DOE that difference, as well as the chance with their approach to do science without the massive rings and multibillion-dollar budgets of proton accelerators. The future of SLAC may depend on their success. G. Christopher Anderson is a writer in the Washington, D.C., bureau of Nature.

Accelerator Planners Worry That SSC May Be A Hard Act To Follow

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