Proposal for continuation of UK participation in the International Muon Ionization Cooling Experiment

Neutrinos are three different but related particles; their ability to turn into each other has given physicists their first glimpse of the physics which they know must lay beyond the Standard Model. Investigation of the physics which underlies their properties will: deepen our understanding of how the Universe developed after the Big Bang; how the current asymmetry between matter and anti-matter developed from a situation where they were created in equal amounts in the Big Bang; and help us to understand what happens when a supernova explodes showering the cosmos with the heavy elements necessary for planets and life itself to form.

In order to understand their properties, we must build an accelerator capable of creating neutrinos in immense numbers. They must have energy between well-defined limits and the mixture of different types must be very precisely known. Such a facility, known as the Neutrino Factory, would be revolutionary and to build one is a challenging project, both from the point of view of the particle detectors which must be built, and the engineering problems which must be overcome. This programme needs a world-wide collaboration, but it is one in which physicists and engineers from the UK are playing a leading role.

Neutrinos are created from a beam of muons and the muons themselves are produced from the decay of pions produced by the collision of protons with a metal target. A machine to make an intense beam of neutrinos needs to take the beam of muons, which is large and diverges rapidly, and reduce its size and divergence. The resulting beam can be accelerated, stored and when it decays produces an intense beam of neutrinos. The muons only live for 2.2 microseconds when at rest, and even when they are accelerated and their lifetime is extended by the effect of relativity, there is little time to manipulate the muons so that they are in a state to be accelerated.

MICE is an international collaboration based at the Rutherford Appleton Laboratory in Oxfordshire, which uses a beam of muons created by the ISIS accelerator and aims to show that it is feasible to create such an intense beam. It will do this by creating a beam of muons of much lower intensity and tracking each one individually through one part of the system which has been designed to perform this beam compression at the Neutrino Factory. This process where the random sideways motions of the muons are reduced and we are left with the longitudinal motion is referred to as cooling the beam; the system which performs the cooling is known as the cooling channel.

The first stage was to build a system capable of producing a muon beam whose size and divergence could be adjusted before it enters the cooling channel. This was completed last year and measurements have been made to show that the beam has the flexibility and intensity for MICE to perform the required measurements.

The second stage is to finish construction of the cooling channel itself and to provide a system to measure very accurately the position and momentum of each muon before and after it has passed through the cooling channel. By looking at many muons produced in many different conditions, it will be possible to determine how much cooling has been produced by the channel. In the channel itself the muons will be slowed by passing through a suitable material, such as liquid hydrogen, liquid helium or lithium hydride. As they slow they lose momentum both longitudinally and transversely to the beam axis. Then they are accelerated with high field radio frequency cavities, replacing only the longitudinal momentum.

This experiment which is pushing the boundaries of what is possible with materials, magnets and cooling technologies, represents a collaboration between particle physicists, and accelerator physicists and will demonstrate the UK's ability to host an experiment at the forefront of science and engineering.

MICE is a large, capital construction project. A significant part of the investment in the project has been used to source products and materials in British industry. For example, of the £4.50M non-staff spend in Phase I of the project, approximately £4.09M was used to source materials and equipment in the UK.

The UK is responsible for the procurement of the focus-coil modules that focus the muon beam at the centre of the liquid-hydrogen absorbers. Each focus-coil module contains two coils capable of producing a field of 5 T on axis. The coils will be kept cold using closed-cycle refrigerators (cryocoolers), a novel technique requiring considerable development. TESLA Engineering based in Surrey won the contract to provide the focus coils. Engineers and physicists from Technology Department at RAL and the University of Oxford are working with TESLA to ensure that the design and its implementation will yield a magnet fit for purpose in MICE. The expertise gained by TESLA will be applicable in the construction of magnets for other applications. In addition, the UK is responsible for the provision of liquid hydrogen to each of the three absorber-modules. The hydrogen delivery system uses state-of-the-art hydride-bed technology and liquefaction is performed with cryocoolers. The development of the systems for MICE will be carried out in collaboration with industry to the benefit of the hydrogen economy.

The MICE programme is varied, encompassing the development of numerical methods and simulation techniques, the development of accelerator hardware (from conventional transfer lines to the development of novel superconducting magnets), the construction of a novel, pion-production target, the development of hydrogen-handling systems, the provision of RF power, and the construction of state of the art diagnostics. Further, the implementation of the large and complicated project has allowed members of the collaboration to develop project management and integration engineering skills.

MICE is an integral part of the Proton Accelerators for Science and Innovation initiative, putting the UK firmly at the heart of the community that seeks to develop the technology required to produce high intensity muon beams suitable for use in the Neutrino Factory and Muon Collider. MICE has created a UK community capable of delivering large and complex projects in an international environment.

Substantial contributions to the experiment in equipment, personnel, and intellectual input are being made by the international collaboration. To date, the international collaboration has provided the muon decay solenoid, beam-line instrumentation (scintillators and scintillating-fibre based beam-position monitors), three time-of-flight hodoscopes, two Cherenkov detectors, a lead-scintillator pre-shower detector, a prototype of the Electron Muon Ranger, the tracker readout and cryogenic systems, three high-power RF amplifiers, and the data-acquisition system. Over the period of the award, the international collaboration will complete the time-of-flight and calorimeter systems and provide the spectrometer solenoids, the liquid-hydrogen absorbers, and the RF-cavity/coupling-coil modules. The total value of these contributions, through hard to estimate, is in excess of £25M.

By making a success of the MICE project, the UK has gained substantial influence in the international Neutrino Factory community. The leverage opportunity for the future will be to forge appropriate partnerships with those laboratories or collaborations wishing to develop the Neutrino Factory and Muon Collider as options for the field. This is being taken forward through the EC FP7 Preparatory Phase Project TIARA in which funds have been secured to turn the infrastructure that supports the MICE experiment into the Ionization Cooling Test Facility (the ICTF).