Raw energy: The latest on the high-energy particle accelerator
Teenagers crave the latest Sony Play Station. Adults pause at a Rolex watch display in the center of town. But Klaus Desch is holding out for raw energy. The physicist has his eyes set on building a new high-energy particle accelerator.
He's not alone. At a three-week meeting earlier this summer in Old Snowmass, Colo., particle physicists from around the world gathered to chart their field's future. What would they need, they asked, to help answer questions that lie on the frontier of physics and cosmology? The consensus: Physicists need a new linear collider, and they need to build it soon.
Such a machine, they say, would stretch for 20 miles underground - an arrow-straight "microscope" to probe the subatomic world. The device would send swarms of electrons and their antimatter counterparts, positrons, hurtling toward each other from opposite ends of the accelerator. The particles and antiparticles would meet in the middle, annihilating each other in a burst of energy.
From that energy, physicists say, new particles are expected to form - and quickly decay - that could open windows on the workings of the fledgling universe and perhaps explain such conundrums as the origins of particles' masses.
Results from such a machine also could point the way toward demonstrating that the four forces of nature physicists describe today are really low-energy manifestations of one force that existed when the universe burst into being in what has become known as the Big Bang.
Noting that a new machine would cost roughly $6 billion, "the world can afford only one linear collider," says Dr. Desch during an interview in his office at the German Electron-Synchrotron (DESY) accelerator lab here. "We should have it, we should have it fast, and we should choose the best technology to have it fast."
In the eyes of many here, that means adopting the design an international science and engineering team has been developing on the outskirts of this German port city. Dubbed TESLA, it is one of two major designs vying for selection as the next linear collider.
At first glance, building another high-price particle accelerator seems just as extravagant as buying a Rolex.
Europe's premier particle-physics lab, CERN, is building a multi-billion-dollar ring-shaped collider that, when finished, will be the most powerful in the world. Dubbed the Large Hadron Collider (LHC), the machine is scheduled to begin operation in 2006 near Geneva.
Meanwhile, in the United States, the Fermi National Accelerator Lab has substantially overhauled its "Tevatron" collider, in Batavia, Ill., which now is hot on the trail of new particles. On New York's Long Island, the Brookhaven National Laboratory's Relativistic Heavy Ion Collider is busy smacking the nuclei of gold atoms into each other in an attempt to recreate and study conditions thought to have existed in the early universe. And results from Stanford University's linear accelerator, forerunner of the new designs under consideration, are highlighting shortfalls in currently accepted theories about fundamental particles and the forces that act on them.
Yet the need for a new, more powerful linear collider stems from the science machines like the ones in Illinois and Switzerland are expected to yield, physicists say.
"The urgency is to have a new linear collider in operation concurrent with the LHC" and with similar energy levels, says David Burke, assistant director of the Stanford Linear Accelerator's (SLAC) technical division in Palo Alto, Calif.
SLAC is the focal point for designing and developing what it calls the Next Linear Collider. Along with a similar design being evaluated at Japan's high-energy physics lab, KEK, in Tskuba City, the NLC appears to be TESLA's main design rival.
To get "the full story" behind discoveries at the LHC in Geneva, Dr. Burke says, "we will need to have both machines running at the same time."
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."