Neutrino Factory and Muon Collider Fellow, Final Technical Report for DOE Award DE-FG02-03ER41267

Abstract

By providing an intense, well controlled, well characterized, narrow beam of muon neutrinos (νμ’s) and electron antineutrinos (–νe’s) from the decay of muons (μ⁻’s) in a storage ring, a neutrino factory can advance neutrino physics beyond the current round of approved and proposed experiments using conventional neutrino beams produced from a beam of decaying pions and kaons [1, 2]. There is no other comparable single clean source of electron neutrinos (from the decay of μ+’s) or antineutrinos. A muon storage ring producing 1019 to 1021 muon decays per year should be feasible. These intense neutrino beams can be used to study neutrino oscillations and possible CP violation. An entry-level muon storage ring that could provide 1019 decays per year would allow a determination of the sign of Δm²31and a first measurement of sin22θ13 for favorable values of this parameter. An improved muon storage ring system that could provide 1020 muon decays per year would allow measurement of sin22θ13 to ~10⁻4. A high performance muon storage ring capable of providing more than 1020 muon decays per year would allow the exciting possibility of a measurement of CP violation in the leptonic sector. An intense cold muon beam at the front end of a neutrino factory could enable a rich variety of precision muon physics, such as a more precise measurement of the muon anomalous magnetic moment (g – 2) and searches for μ -> e γ and μ⁻N -> e⁻ N conversion [3]. In addition, colliding beams of μ⁺ and μ⁻ in a muon collider can provide an effective “Higgs factory” or multi-TeV center-of-mass energy collisions [4]. A muon collider will be the best way to study the Higgs bosons associated with supersymmetric theories and may be necessary to discover them. Two neutrino factory feasibility studies have been carried out in the U.S. [5, 6]. International design efforts are now under way. The International Neutrino Factory and Superbeam Scoping Study (ISS) [7] began at the NuFact05 Workshop in June 2005 with the goals of elaborating the physics case, defining the baseline options for such a facility and its neutrino detectors, and identifying the required R&D program to lay the foundations for a complete design study proposal, and an International Design Study of the Neutrino Factory is beginning. These studies entail iterative cost and technical difficulty evaluations, thereby providing guidelines for the advancing R&D program. One of the central subsystems of a neutrino factory or muon collider is the muon cooling system. The muon beam is cooled to increase the phase space density and allow the muons to pass through smaller apertures, thus reducing the cost of the following accelerator systems. This cooling is accomplished through ionization cooling, in which the beam is passed through liquid hydrogen absorbers and then accelerated in RF cavities to restore the longitudinal momentum. Ionization cooling was proposed more than twenty years ago [8] but has not yet been demonstrated in practice. The International Muon Ionization Cooling Experiment (MICE) [9, 10] seeks to build and operate a muon-cooling device of a design proposed in Feasibility Study-II [6]. In addition to cooling the muons, MICE includes apparatus to measure the performance of the device. The experiment will be carried out by a collaboration of physicists from the U.S., Europe, and Japan at the Rutherford Appleton Laboratory in the U.K. MICE will begin operation in late 2007. Successful performance of the MICE experiment will provide the understanding needed to design a complete neutrino factory, in which the muons are cooled, accelerated, circulated in a storage ring, and decay to produce the neutrino beam. The first neutrino factory might be built in the U.S., Europe, or Japan. A Muon Collider Task Force (MCTF) has recently been organized at Fermilab.