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New SLAC strategy on the B factory

Burton Richter, director of the Stanford Linear Accelerator Center, has proposed a way that the United States can build a revolutionary new particle collider without increasing the national budget.

Speaking Feb. 22 before a panel of scientists that will set the future priorities for high-energy physics, Richter announced that SLAC will request its budget for fiscal years 1994-98 be partly redirected to upgrade its electron-positron collider PEP to create billions of heavy subatomic particles known as B mesons.

The $150 million facility - dubbed PEP-II or the Asymmetric B Factory - would help keep SLAC at the forefront of high-energy physics research well into the next century.

Many physicists consider B mesons to hold an important key to why there is far more matter - the stuff we are made of - than antimatter in the universe.

The U.S. High Energy Physics Advisory Panel (HEPAP) strongly endorsed the physics goals of an asymmetric B factory last June, and urged the Department of Energy and the National Science Foundation to allocate funds to build one.

However, the federal deficit and a major national commitment to build the $8 billion Superconducting Supercollider in Texas makes funding a B factory difficult. Richter is proposing to fund the project by tightening belts at SLAC and building it within the existing budget.

"In these times of tight budgets, we have the responsibility to set our priorities and choose the directions that will yield the best possible scientific returns," Richter said. "SLAC has decided to proceed with the best, even if it means we have to sacrifice the good."

If approved by the Department of Energy, the plan would apply about a quarter of SLAC's annual budget of $140 million to upgrade the 12- year- old PEP collider.

The project, to be done jointly with Lawrence Berkeley Laboratory and Lawrence Livermore National Laboratory, would consist largely of installing a second storage ring inside the 2,200-meter PEP tunnel. Positrons circulating in this tube at an energy of 3.1 billion electron volts (3.1 GeV) would crash into 9 GeV electrons circulating the opposite direction in the existing ring. There also would be major improvements made to the systems that provide microwave power to the two rings and ensure a high level of vacuum inside them.

When complete, PEP-II would be the world's first electron- positron collider in which the two beams circulate at different energies. These energies are chosen to produce abundant pairs of B mesons; the fact that they are unequal means that the Bs are produced in motion rather than at rest.

First proposed by physicist Piermaria Oddone, currently the deputy director of Lawrence Berkeley Lab, this "asymmetric" feature has attracted great interest from physicists around the world. It permits experimenters to isolate individual B mesons and to obtain much more information about their behavior.

The stuff of galaxies

Physicists believe that at the birth of the universe, equal amounts of matter and antimatter began condensing out of a fireball of pure energy. However, at some point, there must have been a process or interaction that favored matter, resulting in a small excess of it. Virtually all the antimatter interacted with an equal amount of matter, annihilating both and leaving the excess matter to form galaxies, stars, planets and every other object that exists today.

A difference in the way matter and antimatter behave, which physicists call a "CP violation," was first observed in 1963 in the disintegrations of relatively light subatomic particles known as K mesons. This is the only proven instance of CP violation so far observed, despite many attempts by scientists to find other occurrences.

High-energy physicists widely believe that an asymmetric B factory offers the best opportunity to discover other examples of CP violation and to study this curious phenomenon in sufficient detail to provide a general explanation of its occurrence. Such an understanding would help explain how a difference between matter and antimatter could have developed in the early universe.

In order to build the B factory with existing budgets, Stanford's linear accelerator would be operated for only 6 months per year, beginning in the fall of 1993. This would mean cutting back on SLAC's current experimental program, including research on Z particles using the Stanford Linear Collider.

Scientific, engineering and technical staff would concentrate on the PEP-II upgrade during the remaining months. The money saved by this reduction in operations could be used to purchase new components needed for the B factory.

If Richter's plan is accepted, PEP-II would become the centerpiece of the SLAC program in 1998, drawing physicists from around the world.

At the Feb. 22 meeting, representatives of the Canadian and German physicists who have been working with SLAC, Lawrence Berkeley and Lawrence Livermore on the project stressed that their countries were prepared to make contributions worth millions of dollars toward a particle detector for PEP-II - and perhaps even to the collider itself. Once under way, Richter said, the project would be likely to attract many other contributions from other institutions and countries.

Richter, however, faces an uphill battle in convincing the Bush administration and the Congress to allow SLAC to proceed with the plan. He hopes to win a strong endorsement from the High-Energy Physics Advisory Panel, which has been asked to establish the overall priorities of the U.S. high-energy physics program for the 1990s.


Michael Riordan is assistant to the director of SLAC.


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