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06/28/94

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High-energy physics and the emperor's visit

STANFORD -- Lying just beneath the pomp and circumstance of the visit of the emperor and empress of Japan to the Stanford Linear Accelerator Center (SLAC) is high-stakes international science politics.

SLAC's director, Burton Richter, said he isn't absolutely certain what led the Japanese government to add SLAC to the royal couple's itinerary, but added: "We consider our collaboration with the Japanese very important. The fact that the Japanese asked to visit SLAC, and agreed with my suggestion that Hirotaka Sugawara [director of Japan's National Laboratory for High-Energy Physics, KEK] co-host the visit, may be a sign that the Japanese government feels the same way."

Richter used the occasion to publicly throw his support behind the proposition that one of the next two major high-energy physics accelerators, called generically the Next Linear Collider or NLC, be a Pacific Rim project.

With the cancellation of the controversial $11 billion Superconducting Super Collider (SSC) project in Texas earlier this year, the U.S. high-energy physics community has turned its attention from national to international projects as the most likely way to continue to make progress.

Closest to realization is a project called the Large Hadron Collider (LHC), which has strong support from a 19-member European consortium, the European Laboratory of Particle Physics. Last week a last-minute wrangle delayed approval for funds to begin construction on the $1.8 billion machine. Similar to the SSC, but only one-third as powerful, the LHC is designed to accelerate protons to high energies and crash them together with enough force to shatter them into even smaller particles called quarks. Quarks are thought to be one of two fundamental families of particles that serve as the basic building blocks of matter.

The NLC would be a complementary machine to the European LHC machine. It is designed to probe the structure of matter by colliding electrons and positrons, representatives of the second family of fundamental particles called leptons. While the LHC is designed to operate at slightly higher energies, the NLC should be capable of making more precise determinations of the properties of new particles. Detailed cost studies have not been made, but the NLC cost is estimated in the $2 billion to $4 billion range.

Joint project?

As Richter told the San Francisco Chronicle June 23, if the United States can't build the NLC by itself, "I think that we can put this together as a joint U.S.-Japanese project."

There are sound political as well as scientific reasons for taking this approach, Richter argued. Historically, the United States has given much more attention to Europe and Russia. With the end of the Cold War it is time to adjust that balance and increase contact with the Asian nations. Pure science is a perfect vehicle for this purpose "because it can lead to a closer relationship culturally and symbolically in a way that doesn't interfere with economic competition," Richter said.

Under the auspices of the U.S.-Japan agreement in high-energy physics that has been in effect since the 1980s, SLAC has forged strong ties with the Japanese high-energy research establishment. Scientists from Nagoya and Tohoku universities helped SLAC scientists build the 4,000-ton detector that is used in the Stanford Linear Collider.

In addition, KEK scientists have been collaborating with SLAC researchers on advanced designs for the NLC.

Essentially, the NLC would be a bigger version of the Stanford Linear Collider. It would be about 6 to 12 miles long, compared with the SLC's 2-mile length. SLAC's current collider requires about 50 megawatts of electrical generating capacity - about the same as the city of Palo Alto - while the NLC will take up to 200 megawatts. With all this power, the bigger machine would pump up to a trillion electron volts into the electrons and positrons that it accelerates up to nearly the speed of light before crashing them together, as opposed to SLC's 100 billion electron volts.

Smashing subatomic particles together in this fashion provides scientists with new information about the structure of matter and the interactions between its parts. In the 1960s and 1970s, for example, experiments of this type at SLAC provided the first experimental evidence that the protons and neutrons that make up the nucleus of the atom are, in turn, made up of even smaller particles called quarks. (This work led to a Nobel Prize for SLAC Professor Richard E. Taylor and his collaborators Jerome Friedman and Henry Kendall from the Massachusetts Institute of Technology.)

An important characteristic of the SLAC/KEK design is that it would pack its particle beam into a much smaller cross-section than the Stanford collider. The current beam is one-twentieth the diameter of a human hair. The NLC beam will fill an area only about a thousandth of that. The smaller the beam, the more densely the particles are packed together. The denser the beams, the more collisions are produced per second, making the machine more scientifically productive.

SLAC scientists, working with colleagues from KEK, already have demonstrated one of the techniques required to squeeze the beam down to this smaller size. In the final focus test beam project, to which the Japanese have contributed about $2 million worth of components, researchers have managed to compress the SLAC collider beam by a factor of 300 to 400, about what will be needed in the NLC.

Damping ring

Meanwhile in Japan, KEK scientists currently are constructing an "advanced damping ring." This is a special kind of storage ring that produces a starting beam that is substantially finer than those at SLAC. When combined with the focusing techniques described above, the damping ring should make it possible to produce the ultrafine beam called for in the NLC design.

Like SLAC, the NLC would use microwaves to accelerate particles. The particles "surf" on electromagnetic waves that propel them to higher and higher velocities. The SLAC collider uses microwaves that are about half the wavelength of those used in microwave ovens. The new machine would use microwaves that are one-quarter the length of those produced in the current collider. Because the shorter microwaves accelerate the particles more rapidly, they reduce the accelerator's length and thus reduce its cost. But these savings must be balanced against the fact that they demand greater precision in the collider's design. And that, in turn, means that the Japanese industry's capability for precision manufacturing would be highly beneficial.

Another area in which the Japanese are expected to make a major contribution is seismic isolation. The collider must be protected against seismic vibrations generated by passing trucks and other comparable environmental sound sources. The Japanese are currently world leaders in this technology.

Once the NLC is constructed, there would be a lot of interesting physics for it to explore, according to proponents.

  • Based on recent evidence for the mass of the top quark, the last of the quark family that remains to be discovered, the NLC should be able to create top quark/anti-top quark pairs. This will allow more precise measurements of the particle's properties.
  • The NLC will be involved in the search for a new class of supermassive particles, called Higgs particles, that are responsible for the phenomenon of mass. All that is currently known about Higgs particles is that, if they exist, they must lie at energies greater than 65 billion electron volts. If the smallest of these particles lies within the energy range of the next-generation machines, the European LHC is most likely to discover it. But the NLC would be able to provide more precise measurements.
  • Another area of investigation for the machine are so-called supersymmetry particles. Some theorists have predicted that each known particle has a corresponding supermassive particle - for every quark there is a squark and for every lepton a slepton - that creates a kind of mirror universe.

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