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.