Raw energy: The latest on the high-energy particle accelerator

Physicists explain that, unlike the linear collider they seek, the LHC and Tevatron accelerate and collide protons and antiprotons. Because both particles are made of more-fundamental particles called quarks, each time a proton and an antiproton collide, the collision actually takes place between three quarks in the proton and three antiquarks in the antiproton. Thus, the collider's detectors record a confusing array of collision events that computers then must sift through to spot the one physicists seek.

"At the LHC, for every one interesting collision, you'll have 25 additional collisions in the same picture," Dr. Desch says.

Using electrons and positrons, which are thought to be fundamental particles, physicists at a linear collider can "tune" the collider to energies that generate the particles they want to study. Because the collisions are occurring with simpler particles, the results are cleaner and easier to analyze than those from debris-laden proton-antiproton collisions.

As an example, Desch cites the quest for a particle thought to impart mass to other particles. Called the Higgs boson, it is the last undetected particle in the grand edifice of physics known as the Standard Model, which describes the known fundamental particles and the forces that act on them. Depending on its detailed properties, the Higgs boson also is seen as a signpost pointing the way to a realm of physics beyond the Standard Model.

"Either the Tevatron or the LHC will discover the Higgs," Desch says. "But once the particle is discovered, you have to ask the questions: What is it? What are its properties? Precision information is the domain of electron-positron colliders" such as TESLA or the NLC.

This latest quest for a new linear collider is unique compared with past collider projects, according to Michael Witherell, director of the Fermi National Accelerator Laboratory in Batavia, Ill.

"For the first time, we're trying to get to a project in our field that is truly global - where different national groups all take ownership for this project," beginning with initial R&D, he says.

Here at DESY, collaborators have settled on an approach that uses superconducting components for TESLA. Superconductors are materials that, when chilled sufficiently, carry electrical currents with virtually no resistance.

The advantage, designers here say, is that key TESLA components can be smaller than those for a non-superconducting accelerator. And TESLA will be able to channel virtually all of its energy into accelerating beams of electrons and positrons instead of losing some to heating of the key accelerator's components.

In turn, this means that the particle beams can contain larger numbers of electrons and positrons, setting up conditions for larger numbers of collisions. The more collisions, the faster one can get results and the greater confidence in the statistical calculations physicists use to interpret the results.

The higher beam "luminosity" in TESLA's design has allowed the collaboration to include an instrument that designers say would broaden TESLA's appeal: a powerful free-electron laser. Molecular biologists, chemists, materials scientists, and geophysicists have expressed interest in using the laser, which would divert a portion of the electrons hurtling down the accelerator . The diverted electrons, which come in tightly spaced bunches, can be "tuned" to generate intensely focused, brilliant beams of radiation ranging from ultraviolet light to low-energy x-rays.

At SLAC, researchers are focusing on what Burke calls a "warm" machine, which would operate at normal temperatures. Then, by next June, an international panel is scheduled to present its technical evaluation of the various design approaches. Based on its evaluation, the international physics community is expected to make its choice. Then come the political challenges: selecting a site and convincing lawmakers to spend the money on the project.

"Politicians understand that the host country will pay most of the cost," Dr. Wetherill says. "We need to get at least one country - Germany, the US, or Japan - that is ready to adopt that host-country role and the additional financial burden it represents."

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