Muon Cooling, Muon Colliders, and the MICE Experiment

Muon colliders and neutrino factories are attractive options for future facilities aimed at achieving the highest lepton-antilepton collision energies and precision measurements of parameters of the Higgs boson and the neutrino mixing matrix. The performance and cost of these depend on how well a beam of muons can be cooled. Recent progress in muon cooling design studies and prototype tests nourishes the hope that such facilities can be built during the coming decade. The status of the key technologies and their various demonstration experiments is summarized. MUON COLLIDERS AND NEUTRINO FACTORIES Discussed since the 1960s [1, 2], muon colliders (Fig. 1) are now reaching the threshold at which their construction can be realistically contemplated. Their interest stems from the important advantages over electrons that muons confer for high-energy lepton colliders: suppression of radiative processes by the 200-times greater mass of the muon, enabling the use of storage rings and recirculating accelerators, and of “beamstrahlung” interactions, which limit e+e−-collider luminosity as energy increases [3]. The smaller size of a muon collider (Fig. 2) eases the siting issues and suggests that the cost will be less as well. Furthermore, the muon/electron cross-section ratio for s-channel annihilation to Higgs bosons, (mμ/me) = 4.3 × 10, gives the muon collider unique access to precision Higgs measurements [4, 5, 6, 7]. For example, at the≈ 126 GeV/c mass measured by ATLAS and CMS [8], only a muon collider can directly the observe the (4 MeV) width and lineshape of a Standard Model Higgs boson [4] (see Fig. 3). Furthermore, should the Higgs have closely spaced supersymmetric partner states at higher mass, only a muon collider has the mass resolution required to distinguish them. (The same argument applies as well to closely spaced scalar states in any other new-physics scenario.) The neutrino factory (Fig. 1) is a newer idea [9]. A muon storage ring is an ideal source for long-baseline neutrino-oscillation experiments: via μ− → eνμνe and μ → eνμνe, it can provide collimated, high-energy neutrino beams with well-understood composition and properties. The clean identification of final-state muons in far detectors enables low-background appearance measurements using νe and νe beams. Distinguishing oscillated from nonoscillated events requires a magnetized detector: if μ− are stored in the ring, the oscillated events contain μ, and ∗Work supported by the U.S. DOE and NSF. † [email protected] Figure 1: (top) Muon collider and (bottom) neutrino factory schematic diagrams.