An important activity toward advancing accelerator capability is research and development for a high luminosity muon collider. Muons have important properties compared to electrons and protons when considering a possible next step for a high energy physics facility. Since radiation by muons is much suppressed compared to electrons, it is feasible to consider a relatively compact circular collider in the TeV energy range for muons. However the muon, like the electron, is a point particle and therefore can explore the same physics regime as protons with approximately ten times higher energy. Accordingly the muon collider has attracted increasing interest over the past couple of years. Since the muon is an unstable particle there are many new facets of accelerator physics and technology that need to be examined before one can say with confidence that such a device will work.
The International Muon Ionization Cooling Experiment (or MICE) is a high energy physics experiment dedicated to observing ionization cooling of muons for the first time. Such cooling is a necessary step toward a muon collider. Cooling is a process whereby the emittance of a beam is reduced in order to reduce the beam size, so that more muons can be accelerated in smaller aperture accelerators and with fewer focussing magnets. This might enable the construction of high intensity muon accelerators, for example for a Neutrino Factory or Muon Collider, which potentially can attain higher energies than electron-positron colliders.
Researchers looking for a method of cooling muons designed the MICE experiment to test the effectiveness of a technique called ionization cooling. Ionization cooling takes place when muons are sent through an absorber in which they lose momentum via ionization energy loss. They are then reaccelerated in a linear accelerator where their energy is restored only in the forward direction. MICE will use a single particle beam, with not more than one muon passing through the detector about every 10 nanoseconds. The experiment needs to balance the cooling from energy loss with the heating from multiple scattering of the muons.
The Muon Ionization Cooling Experiment will make detailed measurements of muon ionization cooling using a newly constructed low-energy muon beam at the Rutherford Appleton Laboratory (RAL). The experiment is a single particle experiment and utilizes many detector techniques from high energy physics experiments. To characterize and monitor the muon beamline, newly developed scinitillating fiber profile monitors and scintillator paddle rate monitors are employed. In order to monitor the purity of the beam and tag the arrival time of individual muons, a dual aerogel Cherenkov system is used, and a plastic scintillator time-of-flight system will be used.
Particle accelerators have been key drivers for a broad spectrum of fundamental discoveries and transformational scientific advances since the early 20th century. Since their inception, particle accelerators have been core components of technological innovation and economic competitiveness. Each generation of particle accelerators builds on the previous one, raising the potential for discovery and pushing the level of technology ever higher. Up to now, accelerators have used protons (and its anti-particles) and electrons (and its anti-particles) as beam particles. There has been a novel idea of using muons (massive cousins of electrons) as beam particles. Because muons are massive and elementary particles, there are many advantages if we make them work. Unfortunately, due to its short lifetime (2.2 microseconds), it has not been possible to make muon accelerators. What is required to make muon accelerators? Firstly we need to create lots of muons using a proton accelerator. A proton accelerator steers protons into a target. The collisions create short-lived particles called pions. Within 50 meters or so, the pions decay into muons and neutrinos. At this point, muons are like a hot gas. To accelerate those muons, we need them organized in a cold beam (called "muon cooling"), in another word, those muons have to stay narrow (to fit through the beam pipe) and to be accelerated. All of these steps have to be done before the muons decay. The biggest scientific and technical challenge for muon accelerators is cooling muons. Ionization cooling is a new technique that can accomplish such cooling. The beam of muons passes through material in which it loses energy by ionizing nearby atoms. The energy loss reduces the muon velocity in all three directions (horizontal, vertical, and longitudinal). The beam is then accelerated by radio-frequency (RF) cavities, replacing only the longitudinal component of velocity. The horizontal and vertical components are increasingly reduced while the longitudinal component is maintained. The muon beam becomes increasing collimated, while focusing magnets reduce the beam size, thus achieving the needed intensities. The Muon Ionization Cooling Experiment (MICE) is a project to demonstrate the ionization cooling from the first principle. The MICE experiment is under construction by an international effort at the Rutherford Appleton Laboratory (RAL) in Oxfordshire, England. The apparatus that forms the muon beam to be studied in MICE has been installed and is working well. The development of muon ionization cooling involves advancements in the theory, simulation, and design of cooling configurations. In addition, suitable spectrometers positioned upstream and downstream of the cooling channel are needed to measure the amount of cooling achieved by a specific channel configuration. Given the modest amount of cooling expected in MICE and the need to extrapolate the MICE observations to a roughly twenty-fold longer cooling lattice, the quality of the data from the two spectrometers is paramount. We have participated in the software development of spectrometers needed to measure the muon beams, cool them, and measure them again to determine the effects of the cooling, in aligning spectrometers using cosmic ray data, in the development of the RF cavities that accelerate the muon beams, and in studying beam properties and determining the process to improve the purity of the muon beam. Both undergraduate and graduate students have been involved in the project. Their involvement ranged from theory, simulation, software programing, hardware development, data taking and data analysis. One of the significant aspects of this project is the possibility for students to become involved in forefront accelerator physics research at universities, thus to educate and train the next generation of accelerator scientists, who will lead innovations in the field and will form the foundation of the nation's highly trained accelerator workforce. Their experience in the design, simulation, construction, and/or testing of accelerator components, and in data analysis and interpretation gives them the ability to work in many areas of science and technology.
