R&D toward neutrino factories and muon colliders

R&D aimed at the production, acceleration, and storage of intense muon beams is under way in the U.S., in Europe, and in Japan. Considerable progress has been trade in the past few years toward the design of a "Neutrino Factory" in which a beam of 20-50 GeV μ^- or μ⁺ is stored. Decay neutrinos from the beam illuminate a detector located roughly 3000 km from the ring. Here, we briefly describe the ingredients of a Neutrino Factory and then discuss the current R&D program and its results. A key concept in the design is "ionization cooling," a process whereby the muon emittance is reduced by repeated interactions with an absorber material followed by reacceleration with high-gradient rf cavities. Plans to test this concept in the Muon Ionization Cooling Experiment (MICE) are well along and are described briefly.