Muon cooling: a key R&D towards Neutrino Factories and Muon Colliders

Muon beams of high brilliance are basilar for future facilities such as a Neutrino Factory, an Higgs-factory or a multi-TeV Muon Collider. Along this R&D path, cooling of the incoming muon beams is essential: cooling factors up to ∼ 10 6 are needed in the Muon Collider case. Due to the need for fast cooling (as the muon lifetime is ∼ 2.2μs) the only practical solution is ``ionization cooling'', realized via energy loss in a suitable absorber and re-acceleration of muons by r.f. cavities in the longitudinal direction. Ionization cooling is effective only in the transverse directions. For a 6-D cooling, where also a longitudinal cooling is needed, the only viable solution is by ``emittance exchange''. The fundamental R&D on cooling is the subject of this review.