is not likely to provide a complete understanding of electroweak symmetry breaking or to completely unravel the origins of mass.

Because facilities that must operate at the highest energies may take two decades to plan, design, and construct, it is essential to try to anticipate now what will be learned from the LHC and other colliders by the end of the first decade of the next century and to understand what questions post-LHC facilities may have to explore. In discussing the technologies being developed now in order to address physics issues anticipated in 2010, it is important to consider both nearterm technology (for a collider that might be built in the first decade of the next century) and long-term technology (for colliders that might not operate until 2020 or later). The LHC can be built today because the planning phase of the project, as well as much of the research and development work to solve technical problems and reduce costs, started more than a decade ago in the early 1980s. This chapter considers questions that will have to be answered even after the LHC program is mature. The accelerator technologies and possible collider facilities needed to address these questions are considered here. As described in Chapter 6, much of the necessary research and development is already in progress to develop the technologies that will define the colliders and the physics program of the future.

THE LANDSCAPE IN 2010

This section describes three scenarios for the physics that may have been discovered at the LHC at the end of its first 5 years of operation. By this time, experiments currently running or being built at LEP, the Tevatron, the Stanford Linear Accelerator Center (SLAC), and the Cornell Electron Storage Ring (CESR) (as described in the previous chapter) should be finished or near completion. Although one cannot predict the future with certainty, making projections is essential in planning for long-term evolution of the field. The scenarios chosen, at least from today's perspective, effectively bracket the possibilities of the state of our understanding in 2010.

The primary missions of the LHC and of the Toroidal LHC Apparatus (ATLAS) and Compact Muon Solenoid (CMS) detectors are to find evidence for the mechanism of electroweak symmetry breaking and to uncover and explore the origins of particle masses. The simplest model of how gauge bosons, leptons, and quarks acquire mass, and how electroweak symmetry is broken, includes a single neutral boson, the Higgs particle. This mechanism makes sense only if the mass of the Higgs particle is less than about 1,000 GeV (1 GeV = 109 electron volts). However, with this model the mystery remains as to why the mass of the Higgs boson should be so small compared with the grand unification scale discussed in Chapter 3. A possible alternative model is supersymmetry, which in its simplest version replaces a single Higgs boson with five bosons—three neutral and two charged. For supersymmetry to be the source of electroweak



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