The Muon Ionisation Cooling Experiment (MICE) project, an accelerator research experiment for a major component of a future neutrino factory, has achieved an important milestone with the successful transport of muons along the MICE muon beamline. The international team can now turn its attention to tuning the beam and working towards the demonstration of ionization cooling.
Neutrinos, though challenging to detect because they are only weakly interacting, have already proved to harbour indications of physics beyond the Standard Model. Observations of atmospheric and solar neutrinos have shown that they oscillate between three forms – electron, tau and muon. This can only occur if they have mass, although in the Standard Model they have no mass. To learn more about these mysterious particles requires a new way to generate high-intensity, high-energy beams of neutrinos of known characteristics, such as composition and energy.
Image credit: STFC.
A neutrino factory would store muons inside a decay ring with long straight sections pointing to large detectors hundreds, or thousands, of kilometres away. Neutrinos produced in the decay of the muons within these straight sections would travel through the Earth to the distant detectors. Studies have shown that such a facility can be built, but a number of challenges must be solved before a technical design can be completed. One major challenge arises because the muons, produced in the decays of pions, will need “cooling” to form bunches of particles with similar momentum and direction if they are to be accelerated and stored. The problem is that muons decay in about 2 μs.
Ionization cooling is the only technique that can cool the muons fast enough. In this process, passage through matter (liquid hydrogen) reduces the momentum of the muon, and one component of the momentum is then restored by acceleration with RF electric fields. Understanding the efficiency of this cooling technique requires a detailed knowledge of the behaviour of muons in many materials, for example in the windows of the vessel containing the liquid hydrogen.
The MICE project aims to demonstrate the technologies required for ionization cooling and prove that muons can be assembled into cold bunches small enough to allow the muon beam to be accelerated and stored. The MICE collaboration is designing, building and testing a section of a realistic cooling channel on a beamline on the ISIS facility at the Science and Technology Facilities Council’s Rutherford Appleton Laboratory (RAL). Achieving this will give confidence that a full ionization-cooling channel, consisting of many cooling sections, can be designed and built economically.
The successful transport of the first muons along the new beamline is the latest of several significant steps the MICE team has taken since the formation of the collaboration in 2001, and more recently in commissioning the beamline. They have completed the installation and testing of the pion-production target in the ISIS proton synchrotron, built the pion decay line and installed beam counters and other equipment in the experimental hall. Over the coming months, the MICE spectrometer system will be installed and the experiments will finally begin. The cooling channel will be built over the next two or three years, culminating in the demonstration of ionization cooling in 2010.
• The MICE project is a major collaboration involving 150 scientists and engineers from across the world, with collaborators in Bulgaria, China, Italy, Japan, the Netherlands, Switzerland, the UK and the US.
