The architecture of the Inner Detector is determined by three factors, says Gilchriese: "First, the angular space you want to cover—the LHC's physics require detectors to cover a large angular region. Then the number of particles, much higher in the LHC than ever before—so as not to confuse particle tracks you have to have finer granularity and finer resolution, many more elements. And finally, radiation levels—they will be much higher in the LHC, especially near the interaction region." The Inner Detector employs three different tracking technologies. Farthest from the beam line is the Transition Radiation Tracker, 400,000 gas-filled plastic "straws," each four millimeters in diameter with a gold-plated wire running down its axis.

Closer in is the Semiconductor Tracker, strips of silicon each 12 centimeters long, mounted on overlapping lattices—more than five million strips bundled with their electronics into 4,100 modules. Innermost is the pixel detector, consisting of tiny rectangles of silicon only 50 microns wide—about the width of a human hair—by 300 microns long. All these barely visible pixels must be connected by means of tiny "bump" contacts to electronics mounted directly on top of them—140 million sensing elements totaling two square meters of integrated circuits—more than ten billion transistors bathed in the most intense radiation ever produced by an accelerator.

"The cost of the pixels and their electronics is driven by area, not by the number of elements," Gilchriese says, "so we are making as many as we can, as small as we can." One reason for having tens of millions of micro-elements near the interaction region is that each will be hit by energetic particles less often. While the straw tracker, the silicon strips, and most of the pixel detector should last ten years, the innermost pixel layer will probably have to be replaced annually. The Inner Detector will be shipped to Geneva in pieces; 60 institutions all over the world are involved in what Gilchriese calls "a United Nations approach to detector design and construction." But some of the most advanced elements, the pixels and silicon strips and their electronics, are being assembled and tested at Berkeley Lab. "We are working with the companies that will manufacture the units to push the state of the art."

Gilchriese's ATLAS team of physicists, engineers, and technical personnel includes several with major responsibilities in the overall U.S. ATLAS collaboration, among them theorist Ian Hinchliffe, U.S. ATLAS physics coordinator; Michael Barnett, U.S. ATLAS education coordinator; and Stewart Loken, a coordinator of the U.S. ATLAS computing team.

When the LHC sees first light and the ATLAS detector begins to collect data in 2005, George Trilling and Gil Gilchriese will have reached the end of a long quest. The grail they seek, alongside thousands of other researchers from around the world, is the last undiscovered particle predicted by the standard model, the Higgs boson. Beyond lies unexplored country.