Proposal for UK participation in Phase II of the International Muon Ionisation Cooling Experiment

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One of the most challenging questions in understanding the origin of the Universe is why it is almost entirely made of matter. We believe the Big Bang which created it made equal amounts of matter and antimatter. It is possible that the answer to this puzzle lies in the properties of particles known as neutrinos. We know very little about neutrinos. They are fundamental particles; they weigh almost nothing and they hardly interact with ordinary matter. It is possible that neutrinos and their 'antiparticles' (antineutrinos) behave differently -- a phenomenon known to physicists as 'CP violation'. Because they interact are so weakly, intense beams of neutrinos are needed to study their properties. Machines called 'Neutrino Factories' promise intense neutrino beams The neutrinos would come from the decay of parent particles called muons. These would be stored in a special ring -- the 'decay ring' -- and allowed to decay naturally to neutrinos. Beams of neutrinos would then travel several thousands of kilometres to large detectors where their interactions would be studied. Muons beams can be produced quite easily, but they are large and have a wide spread in angle. A fundamental challenge in the design of a neutrino factory is to reduce the size of the muon beam so that it can fit into the decay ring. The process of reducing the size and spread of a beam of particles is known as 'cooling'. There are a number of commonly used ways of cooling beams of particles but these take a long time and cannot be used for muons because they decay to neutrinos in a few microseconds. The development of a new technique called 'ionisation cooling' is essential. If a beam of muons is passed through some material the muons will slow down. If they are then all reaccelerated in the same direction, the size and spread of the beam will be reduced. If this process is repeated many times, the beam can be made narrow and parallel enough to fit into a storage ring. The Muon Ionisation Cooling Experiment (MICE) at the Rutherford Appleton Laboratory in the UK aims to demonstrate the ionisation cooling of muons for the first time. It consists of a short 'cooling section' which contains 'absorbers' where the muons lose energy and 'cavities' where the muons are accelerated by high electric fields. The muons will be guided by strong magnetic fields from superconducting magnets. The size the muon beam will be measured before and after it has passed through the cooling section. Because the cooling section is short the cooling effect will be quite small and the before- and-after measurements must be made with a precision of one part in a thousand. The high electric and magnetic fields, and the possibility that liquid hydrogen will be used as an absorber material, mean that the engineering of MICE is extremely challenging.