MICE-UK & UKNF Programmes

物理学目前已知的粒子——中微子有三种类型：电子、μ子和τ子——可能是最有趣但最不为人所知的粒子之一。中微子的质量很小，与物质的相互作用非常弱。迄今为止，所有粒子物理实验的结果都可以很容易地用20世纪60年代发展起来的粒子物理“标准模型”来解释：也就是说，除了少数几个研究中微子行为的实验中的一小部分结果。中微子在恒星和地球大气层中自然地不断产生。探测这些中微子的实验总是发现的中微子比产生的要少。最近，人造中微子的实验也证明了类似的中微子振荡行为。对于自然界中的每一种粒子类型，都有相应的反粒子类型——反粒子加在一起被称为反物质。特别令人感兴趣的是，反中微子的振荡是否与中微子的振荡相同。这个问题被认为与这样一个事实有关：今天的宇宙几乎完全由物质组成，而大爆炸应该创造出等量的物质和反物质。反物质的去向之谜可以用中微子和反中微子振荡行为的不同来解释。这个问题只能通过使用高强度人造中微子束对中微子振荡进行超灵敏测量来回答。这项拟议研究的最终目标是在“中微子工厂”中产生有史以来人类创造的最强烈的中微子束。这些中微子束将以不同的角度穿过地球。这将使我们能够比以往更精确地确定中微子的性质，并使我们能够回答中微子和反中微子的振荡是否相同的基本问题。这应该能解开大爆炸时产生的反物质去了哪里的谜团，从而有助于解释我们今天所知道的宇宙的存在。中微子工厂将从称为μ子的母粒子的衰变中产生强烈的中微子。在中微子工厂能够实现之前，有许多重要的问题必须解决。首先，必须解决产生足够质子束的问题。其次，有必要开发新的技术来快速加速μ子，因为μ子的衰变寿命（静止）为2.2微秒。最重要的问题之一是在介子束进入介子加速系统之前压缩（冷却）它所占据的空间体积。通常用于冷却粒子束的技术不能用于μ子，因为它们衰变成中微子的速度太快了。开发一种名为“电离冷却”的新技术至关重要。如果一束μ子穿过物质，μ子的速度就会减慢。如果它们在同一方向上再次加速，那么光束的大小和传播将会减小。如果这个过程重复很多次，光束就可以变得狭窄和平行，足以容纳一个存储环。英国卢瑟福阿普尔顿实验室的μ子电离冷却实验（MICE）旨在首次演示μ子的电离冷却。它包括一个短的“冷却部分”，其中包含μ子损失能量的“吸收器”和μ子被高电场加速的“空腔”。介子将被超导磁体产生的强磁场所引导。在通过冷却段之前和之后，将测量μ子束的大小。由于冷却段较短，冷却效果很小，前后的测量精度必须达到千分之一。

The species of particles currently known to physics, the neutrinos, which come in three types: electron, muon, and tau, are probably among the most intriguing but least understood. The neutrinos have tiny masses and interact with matter only very weakly. The results of all particle physics experiments to date have been readily explained by the 'Standard Model' of particle physics, developed in the 1960s: that is, except, for a small set of results from a select few experiments investigating the behaviour of neutrinos. Neutrinos are naturally produced continuously in stars and the earth's atmosphere. Experiments to detect these neutrinos have consistently found fewer neutrinos than are produced. More recently, experiments involving man-made neutrinos have demonstrated similar neutrino oscillation behaviour. For each particle type in nature, there is a corresponding anti-particle type - the anti-particles together are known as anti-matter. Of particular interest is the question of whether the oscillations of anti-neutrinos are the same as those of neutrinos. This question is believed to be related to the fact that the universe today is composed almost entirely of matter, whereas the Big Bang should have created equal amounts of matter and anti-matter. The mystery of where the anti-matter has gone could be explained by a difference in the oscillation behaviour of neutrinos and anti-neutrinos. This question can only be answered by making ultra-sensitive measurements of neutrino oscillations using high-intensity man-made beams of neutrinos. The ultimate goal of the proposed research is to produce the most intense beams of neutrinos ever created by man in a 'Neutrino Factory'. These beams of neutrinos will be directed at various angles through the Earth. This will allow the properties of the neutrinos to be determined far more precisely than ever before, and will allow us to answer the fundamental question of whether the oscillations of neutrinos and anti-neutrinos are the same or not. This should solve the puzzle of where the anti-matter created at the Big Bang has gone, and therefore help to explain the existence of the universe as we know it today. The Neutrino Factories will produce intense neutrino from the decay of parent particles called muons. There are a number of important issues that must be addressed before the Neutrino Factory can be realized. First, the issue of producing sufficient proton-beam must be dealt with. Second it is necessary to develop novel techniques to accelerate the muons very rapidly since muons decay with a lifetime (at rest) of 2.2 microseconds. One of the most important problems is that of squeezing down (cooling) the volume of space occupied by the muon beam before it enters the muon-acceleration system. The commonly used techniques for cooling beams of particles cannot be used for muons because they decay to neutrinos so quickly. The development of a new technique called 'ionisation cooling' is essential. If a beam of muons is passed through material the muons will slow down. If they are then 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.