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Report from The Snowmass 2001 Working Group M1: Muon Based Accelerators

Recent results from the SNO collaboration [1] coupled with data from the SuperK collaboration [2] have provided convincing evidence that neutrinos oscillate and that they very likely do so among the three known neutrino species. Experiments currently under way or planned in the near future will shed further light on the nature of neutrino mixing and the magnitudes of the mass differences between them. Neutrino oscillations and the implied non-zero masses represent the first experimental evidence of effects beyond the Standard Model, and as such are worthy of our utmost attention. This working group reviewed the international effort on establishing an ongoing program of research in accelerator and experimental physics that can be implemented in an incremental fashion. At each step, one opens up new physics vistas, leading eventually to a Neutrino Factory and a Muon Collider. One of the first steps toward a Neutrino Factory is a proton driver that can be used to provide intense beams of conventional neutrinos in addition to providing the intense source of low energy muons from pion decay that must be cooled to be accelerated and stored. While the proton driver is being constructed, one could simultaneously engage in R&D on collecting and cooling muons. A source of intense cold muons can be immediately used to do physics on such items as measuring the electric and magnetic dipole moments of the muon to higher precision, muoniumantimuonium oscillations, rare muon decays and so on. Once the capability of cooling and accelerating muons is developed, the storage ring for such muons will be the first Neutrino Factory. Its precise energy and its distance from the long-baseline experiment will be chosen using the knowledge of neutrino oscillation parameters gleaned from the present generation of solar and accelerator experiments (Homestake, Kamiokande, SuperKamiokande, SAGE, GALLEX, K2K, SNO), the next generation experiments (MiniBooNE, MINOS, CNGS, KamLAND, Borexino), and the high-intensity conventional beam experiments that would already have taken place. A Neutrino Factory provides intense beams of both νμ and νe from stored μ − beams, and their charge conjugate beams for stored μ beams. In addition, it provides beams having smaller divergence than conventional neutrino beams of comparable energy. These properties permit the study of non-oscillation physics at near detectors and the measurement of structure functions and associated parameters in non-oscillation physics to unprecedented accuracy. They also permit long-baseline experiments that can determine oscillation parameters. Depending on the value of the parameter sin 2θ13 in the three-neutrino oscillation formalism, one can expect to measure the oscillation νe → νμ. By comparing the rates for this channel with its charge-conjugate channel νe → νμ, one can determine the sign of the leading mass difference in neutrinos, ∆m 2 32, by making use of their passage through matter in a long-baseline experiment. Such experiments can also shed light on the CP violating phase, δ, in the lepton mixing matrix and enable us to study CP violation in the lepton sector. It is known that CP violation in the quark sector is insufficient to explain the baryon asymmetry of the Universe. Perhaps the lepton sector CP violation played a crucial role in creating this asymmetry during the initial phases of the Big Bang. While the Neutrino Factory is being constructed, R&D can be performed to make the Muon Collider a reality. This would require orders of magnitude more cooling than the Neutrino Factory. Muon Colliders, if realized, provide a tool to explore Higgs-like objects by direct s-channel fusion, much as LEP explored the Z . They also provide a means to reach higher energies (3–4 TeV in the center of mass) using compact collider rings. These concepts and ideas have aroused significant interest throughout the world scientific community. In the U.S., a formal collaboration of some 140 scientists, the Neutrino Factory and Muon Collider Collaboration (MC) [3], has undertaken the study of designing a Neutrino Factory, along with R&D activities in support of a Muon Collider design.