Muon ionization cooling experiment: results and prospects

A high-energy muon collider could be the most powerful and cost-effective collider approach in the multi-TeV regime, and a neutrino source based on decay of an intense muon beam would be ideal for measurement of neutrino oscillation parameters. Muon beams may be created through the decay of pions produced in the interaction of a proton beam with a target. The muons are subsequently accelerated and injected into a storage ring where they decay producing a beam of neutrinos, or collide with counter-rotating antimuons. Cooling of the muon beam would enable more muons to be accelerated resulting in a more intense neutrino source and higher collider luminosity. Ionization cooling is the novel technique by which it is proposed to cool the beam. The Muon Ionization Cooling Experiment collaboration has constructed a section of an ionization cooling cell and used it to provide the first demonstration of ionization cooling. Here the observation of ionization cooling is described. The cooling performance is studied for a variety of beam and magnetic field configurations. The outlook for an experiment to measure muon ionization cooling in all six phase-space dimensions as part of the demonstrator facility being considered by the interna-tional Muon Collider collaboration will also be discussed. INTRODUCTION Muons are considered excellent beam particles for a collider applications due to their unique properties. Entire muon energy, being fundamental particle, is available for production of secondary particles. The design of such Muon collider is strongly influenced by radiative effects. The large muon mass also offers an increased coupling to Higgs boson compared to electron. Muon colliders can thus employ rings of small circumference for acceleration and collisions, reducing facility footprints and construction and operating costs. Production of high quality muon beams is challenging. Muon beam production starts by sending the high power proton beam to the target, where pions are produced subsequently decaying into muons. This way muons emerge as a tertiary beam with a very large initial emittance and energy spread, which requires a significant beam cooling in order to be able to achieve a sufficient luminosity in the collider applications. Due to the shortness of the muon lifetime the only cooling technique fast enough to be applicable to the muon beams is ionization cooling [1–3]. In muon ionization cooling, muons are passed through energy-absorbing material where the transverse and longitudinal momentum is reduced, reducing the normalised beam emittance and cooling the beam. Multiple Coulomb scattering from atomic nuclei induces an increase in transverse momentum and heats the beam. By focussing the beam tightly onto the absorber and using materials having low atomic number the heating effect may be suppressed, resulting in overall cooling. The ionization cooling of muons has been demonstrated for the first time experimentally by Muon Ionization Cooling Experiment (MICE) at RAL [4].