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Physicists in the United States have been understandably timid about asking for a major new accelerator. The debacle of the Superconducting Super Collider (SSC), which Congress canceled in 1993 when it was already under construction, is still fresh in their minds. And it took years of negotiation to arrange a consolation prize: U.S. participation in what will be the highest energy collider ever built, the Large Hadron Collider (LHC) at CERN in Geneva, Switzerland. But the message was clear in two U.S. reports on the state of particle physics released last week: Another collider will be needed if physicists are to assemble a complete picture of the particles and forces that constitute the world.
Plans for some kind of successor or companion to the LHC--which hurls protons against protons--have been in the works since the mid-1980s. But in a once-a-decade review of the field prepared by the National Research Council (NRC) and a draft report to the Department of Energy's (DOE's) influential High Energy Physics Advisory Panel (HEPAP), physicists have made their most public plea yet for a new machine. "The LHC is set, so the stage is now open for the next [machine]," says Columbia University physicist William Willis, a member of the HEPAP panel.
On the wish list are three very different devices: a scaled-up LHC, called a Very Large Hadron Collider; a 30- to 50-kilometer-long Next Linear Collider (NLC) that would smash electrons together; or an even more fanciful device that would collide muons--the electron's short-lived, heavy brothers (Science, 9 January, p. 169). Physicists have been exploring all three possibilities, and some portions of the NLC have even been bench-tested. Any of the three options will cost over a billion dollars and take a decade or more to plan and build.
But such a behemoth will be essential for moving beyond the existing picture of the subatomic world, says University of Chicago physicist Bruce Winstein, who chaired the NRC committee. The missing piece of the decades-old theory called the Standard Model is an account of how particles get mass. Most physicists think a hypothetical particle called the Higgs boson works behind the scenes to confer mass. Although physicists expect the Higgs to tumble out of the collisions in the LHC, sightings are expected to be rare at the energies the accelerator can achieve. And occasional glimpses won't be enough, because the Higgs is expected to lead the way beyond the Standard Model to an even more fundamental theory of particles and forces.
Finding out which, if any, of several candidate theories is right will take a more detailed investigation of the Higgs--and any other particles that turn up--than will be possible at the LHC, the reports say. The electron and muon colliders would give physicists a cleaner environment for studying the new particles, as these collisions produce less debris than do the proton collisions of the LHC. And a supersized LHC would generate higher energy collisions, allowing physicists to search for still more particles.
But the specter of the failed SSC clearly darkens the pages of both reports. The SSC cost rose billions of dollars over initial estimates, to nearly $12 billion just before its demise, and the project came up short in attracting funding from other countries. The SSC "cannot happen again," says Cornell physicist Persis Drell, who helped draft the NRC report. "If nothing else, that is branded on our foreheads."
The key to success, physicists hope, will be an affordable price tag and a global effort. Building even the NLC (the furthest along of the three options) will be "damn difficult and very, very expensive," says Donald Shapero, director of the NRC board on physics and astronomy. "This is going to be an international game from now on. No one country is going to be able to contemplate doing it alone."
Yet the NLC camp is already divided, members of the NRC committee note. While physicists at the Stanford Linear Accelerator Center (SLAC) and Japan's KEK laboratory have been working in close concert on a design that uses conventional radio-frequency cavities to accelerate the electrons, Germany's DESY lab is hard at work on a plan that uses superconducting technology. That's a bad omen for future collaboration, some say. "Each lab gets committed to its own technology," explains Michael Riordan, assistant to the director at SLAC. "You have this kind of technological inertia."
Overcoming that inertia, and bringing all the labs together, may take the creation of a "world HEPAP," says Peter Rosen, DOE's associate director for high-energy and nuclear physics. It may have to come soon. Already, DESY director Bjorn Wiik has made the rounds at towns near Hamburg to build support for an underground tunnel that could house the NLC. Wiik says he is simply preparing the ground for DESY's proposal. But many say that raising the issue of the NLC's site at this early stage is risky, because each lab is likely to want the new accelerator in its own backyard. "I have to say, quite frankly, it's not good for international collaboration," says Rosen.
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