Muon Ionization Cooling Experiment

Our understanding of the nature of matter, the forces that bind it and the history of the Universe itself have all been shaped by experiments at particle accelerators. Most experiments employ beams of stable particles; their collisions, with other particle beams or with ordinary matter, create secondary particles. Measuring the properties of these secondaries and how they decay gives us information about the fundamental structure of matter and the forces by which particles interact.

Precise acceleration and control of particle beams requires sophisticated magnet and microwave systems. The delivery of measurements at the cutting edge of particle physics pushes accelerator technology beyond the state of the art. Present-day accelerators employ stable particles; protons, anti-protons, electrons and positrons; while incremental development of these techniques will continue to deliver first-rate science, a revolution in sensitivity and precision could be delivered if it were possible to deliver ice-cold beams of “muons”.

The muon is a fundamental particle identical an electron, except that it is 200 times heavier and it is unstable. For the particle physicist, the advantage of beams composed of muons is that the large mass makes it straightforward to accelerate the beams to extremely high energy. What's more, the theoretical description of the muon's decay is so exquisitely precise that the beams of neutrinos created when muons decay can be exploited to investigate why all the anti-matter created in the Big Bang has been removed from the Universe we inhabit.

A beam of primary particles, for instance protons, can be created all heading in the same direction rather like a military marching band. Muons, by contrast, are created in processes that yield a “cloud” of particles. Capturing the muon cloud to produce a beam results in something which is may be likened to a mixed group of pedestrians crossing a bridge; they are all crossing the bridge, but they are moving from one side of the bridge to the other as they go, to see the sights. If present-day focussing techniques were to be used, a beam that is tens of centimetres in diameter would be produced. An accelerator that could accept such a wide beam would need to have extremely large magnets, which would be very expensive to build and to run. Furthermore, the resulting particle intensity would be insufficient to make many of the most exciting measurements in which we are interested.

The problem then is to take a beam with a large diameter and squeeze it to produce a much narrower beam that can be fed into an accelerator.

In the early 1980's an analogous problem was solved for anti-protons (a stable particle produced in collisions of protons with a solid target). The technique, referred to as “stochastic” cooling, earned its inventor, Simon Van Der Meer, a Nobel Prize. Applying this technique to muons would not yield acceptable results; the reduction in beam size produced by stochastic cooling takes seconds to build up. In this time, all the muons, which live for only a few millionths of a second, would decay.

The object of the MICE experiment is to take a beam of muons created by protons from the ISIS accelerator hitting a titanium target and to show that it is possible to create a narrow intense beam.

A beam of muons can be characterised by its velocity along the direction in which the beam is travelling and its velocity at right angles to the direction of travel. The width of the beam depends on the ratio of the velocity at right angles to the beam to that along the beam. A particle that passes through a material loses energy and slows down, but the ratio of the velocities remains constant and the width of the beam is unchanged. Such a beam can be accelerated by an electric field and with the direction of the field parallel to the beam direction only the velocity along the beam direction is increased. This reduces the ratio of velocities and the width of the beam falls.

Why is a narrow beam ‘cool’? This is really a picture more than a rigorous description. A physicist will think of temperature as the random motion of particles in all directions, where each particle has energy somewhere between zero and some maximum. A crowd of people in a park moving around might strike a physicist as having a temperature and they might comment that a group of children rushing around would have a higher temperature than a group of older people. For those same groups crossing a bridge, each person will have some velocity taking him or her across the bridge; this component of motion will be in the same direction for everyone. But people may meander from one side of the bridge to the other in a random way. The second group, the one that contains children, may have a higher average “meandering velocity” from one side of the bridge to the other. To physicists talking among themselves, the second group has a higher temperature conveying the idea of a random motion of varying size distributed among the members of the group. For a particle beam a small side-to-side motion means we can create a narrow, intense beam. The objective of MICE is to demonstrate a method to reduce the side-to-side motion of a muon beam and hence reduce its size. For someone without a physics training it is probably easier to describe this reduction directly; to a physicist “cooling the beam” is a convenient shorthand to describe the size-reduction process in a single phrase.

August 25, 2015