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

理解宇宙起源最具挑战性的问题之一是为什么它几乎完全由物质组成。我们相信创造它的大爆炸产生了等量的物质和反物质。这个谜题的答案可能就在于中微子的性质。我们对中微子知之甚少。它们是基本粒子;它们几乎没有重量，也几乎不与普通物质相互作用。中微子和它们的“反粒子”（反中微子）的行为可能不同-物理学家称之为“CP破坏”的现象。由于它们之间的相互作用非常微弱，因此需要强中微子束来研究它们的性质。被称为“中微子工厂”的机器承诺强烈的中微子束中微子将来自于被称为μ子的母粒子的衰变。这些中微子将被储存在一个特殊的环--“衰变环”中，并被允许自然衰变为中微子。然后，中微子束将行进数千公里到达大型探测器，在那里研究它们的相互作用。介子束可以很容易地产生，但它们很大，并且有很宽的角度分布。中微子工厂设计中的一个基本挑战是减小μ子束的尺寸，使其能够适应衰变环。减小粒子束的尺寸和扩散的过程被称为“冷却”。有许多常用的冷却粒子束的方法，但这些方法需要很长时间，并且不能用于μ子，因为它们在几微秒内衰变为中微子。开发一种称为“电离冷却”的新技术至关重要。如果一束μ子穿过某些物质，μ子就会减速。如果它们都在同一个方向上被重新加速，那么光束的大小和分布就会减小。如果这个过程重复多次，光束可以变得足够窄和平行，以适合储存环。英国卢瑟福阿普尔顿实验室的μ子电离冷却实验（MICE）旨在首次展示μ子的电离冷却。它由一个短的“冷却部分”组成，其中包含“吸收器”，在那里μ子失去能量，以及“空腔”，在那里μ子被高电场加速。μ子将由超导磁体产生的强磁场引导。μ子束的大小将在其通过冷却段之前和之后进行测量。由于冷却段很短，冷却效果将非常小，因此必须以千分之一的精度进行前后测量。高电场和磁场，以及液态氢将被用作吸收材料的可能性，意味着MICE的工程设计极具挑战性。

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.