Towards Very High Energy Accelerators

The development of a new generation of colliders beyond the LHC is central to the future of high-energy physics. As was clearly pointed out by Lykken [1], the prospects for high-energy physics may be limited by our ability to build them. This paper describes possibilities for these colliders and some of the challenges we face in building them. None of the fundamental ideas that underly the advanced accelerator work reported herein are new; they have been suggested, and studied in various levels of detail, for the last two decades. What is new, and encouraging, are the technological advances, the level of understanding and detailed modeling, and the initial experimentation. These changes are affecting a wide variety of concepts. For example, W-band structures are being tested at SLAC, a large collaboration, numberingmore than 100 individuals, is working on a neutrino factory andmuon collider, and laser-driven plasma accelerators have matured to the point where numerous groups worldwide can accelerate nC of charge to MeV energies. Such progress may lead one to wonder why none of these options offers an unimpeded path to a high-energy collider. A simplified answer is that the desired luminosity (even extrapolating our ability to produce, accelerate, and focus high quality beams) demands high average power in the colliding beams. Overall system efficiency is thus vital. Preserving the beam quality needed for focusing to very small spot sizes requires that great care be paid to critical engineering issues such as the fabrication of structures, focusing fields, and jitter. Accommodating these numerous and severe constraints requires a mature technology. Many of the beam interactions with plasma are complicated and relevant experimental studies are in an early stage. The expression for the luminosity illustrates the previous comments, and also makes clear the different approaches that the e+e− and μ+μ− colliders take. We can write the luminosity