Mu2e : A proposal to extend the sensitivity to charged lepton flavour violation by 4 orders of magnitude.

电子是最轻、最稳定的带电粒子，它的性质被很好地测量，通过它在化学反应中的作用支撑着生命。1937年，在宇宙射线中发现了一种类似但更重的带电粒子--μ介子。在过去的80年里，人们一直在研究μ介子&它的行为似乎像一个更重的电子，其性质只会因为它重了大约220倍而有所改变。它看起来像电子一样，没有结构，不是电子的激发态，而是一种独特的基本粒子。这种区别体现在一种叫做轻子味的性质中：电子和μ子（和τ）都是带电轻子，我们说电子是带电子味的带电轻子，而μ子是带μ子味的带电轻子。中性轻子也是类似的情况：中微子。它们似乎也有三种不同的味道：电子，μ子和τ子。2015年的诺贝尔奖是因为观察到中微子在空间中移动时会改变味道：一个电子味的中微子（所谓的电子中微子）会变成一个μ子味的中微子（μ子中微子）。这说明轻子味的量不是神圣不可侵犯的，也就是说，它并不总是守恒的：人们可以从1个单位的电子味开始，以零结束，而是1个单位的μ子味。与电子相比，μ介子的质量更大，这意味着它是不稳定的，衰变的寿命为2 x 1/百万分之一秒。到目前为止，我们只看到μ子以一种方式衰变：电子（和正电子）、中微子和反中微子（偶尔会有一个额外的光子）。在这些衰变中，当我们考虑衰变粒子的组合轻子味时，它总是一个单位的μ子味，就像最初的衰变μ子一样。中微子、电子和正电子的轻子味总是相互抵消。鉴于中微子的情况并非如此，我们预计带电轻子的情况也不会总是如此&我们预计一些μ子的衰变方式不会保持轻子的味道。我们从1947年就开始寻找这种衰变了！对于粒子物理学标准模型中的已知粒子，这一过程有可能发生，但每10^50（1有50个0）个μ子只有一次：把这一点放在上下文中，10^50大约是我们整个星球上的原子数量，所以观察到这样一个不太可能发生的事情是不可能的。相反，我们试图以每10^17 μ ons一次异常衰变的速率观察这种衰变，这要容易得多：这只相当于观察地球上所有沙漠和海滩中的一粒沙子的行为奇怪！然而，由于技术的进步，我们现在可以产生大量的μ子：大约每秒10亿个。如果观测到μ介子的这些异常衰变之一，它将表明存在额外的新粒子或相互作用，而这些粒子或相互作用在我们目前的理论中没有体现出来。遗憾的是，我们目前的理论是不够的：它不能描述原子尺度上的引力，不能解释暗物质的存在，也不能解释为什么我们的宇宙是由物质主导的，而且几乎没有反物质。要解释这一点，需要有新的粒子或相互作用，对μ介子反常衰变的观测将证明这种新的粒子和相互作用确实存在。三个英国研究所（利物浦，曼彻斯特和伦敦大学学院）将为寻找这种异常μ子衰变做出关键贡献。我们将在费米实验室为Mu 2 e实验建造一个探测器，它将测量实验中产生的μ子数量，这样如果观察到异常衰变，我们就可以确定它的速率。没有这个检测器，我们就没有测量的标准化。我们将在未来3年内建造探测器，并在2020年开始研究μ子衰变，希望能够回答是否确实存在新的、以前从未见过的粒子。

The electron is the lightest, stable charged particle & its properties are extremely well measured & underpin life through its role in chemical reactions. In 1937 a similar but heavier charged particle, the muon, was discovered in cosmic rays. The muon has been studied for the past 80 years & it seems to behave like a heavier version of the electron with its properties only modified by virtue of it being approximately 220 times heavier. It appears, like the electron, to have no structure & is not an excited state of the electron but a distinct fundamental particle. This distinction is embodied in a property called lepton-flavour: both the electron & muon (& tau) are charged leptons & we say that the electron is a charged lepton with electron-flavour & the muon, a charged lepton with muon-flavour. It is a similar case for the neutral leptons: the neutrinos. They also appear to come in three distinct flavours: electron, muon & tau. The 2015 Nobel Prize was awarded for the observation that neutrinos change flavour as they move through space: a neutrino of electron flavour (so called electron-neutrino) changes into one with muon-flavour (a muon-neutrino). This illustrates that the quantity of lepton flavour is not sacrosanct i.e. that it's not always conserved: one can start with 1 unit of electron-flavour & finish with zero but instead 1 unit of muon-flavour. The larger mass of the muon compared to the electron means it is unstable & decays with a lifetime of 2 x 1/millionth of a sec. To date we have only seen the muon decay in one way: to electrons (& positrons) & neutrinos & anti-neutrinos (with occasionally an additional photon). In each of these decays when one considers the combined lepton flavour of the decay particles it is always one unit of muon-flavour just like the initial decaying muon. The lepton-flavours of the neutrinos, electrons & positrons always cancel out. Given that this isn't the case for the neutrinos, we expect it will not always be the case for charged leptons & we expect some muons to decay in a way that does not conserve lepton-flavour. We have been searching for such decays since 1947 ! With the known particles in the Standard Model of particle physics it is possible for the process to occur, but only once for every 10^50 (1 with 50 zeros) muons: to put this into context 10^50 is approximately the number of atoms on our entire planet & so observing such an unlikely occurrence is impossible. We are instead trying to observe this decay at rate of one anomalous decay per 10^17 muons which is much easier: it is only the equivalent of observing one of the earth's grains of sand across all its deserts & beaches behave strangely! However this is now possible with muons thanks to technological advances that allow us to produce muons in huge quantities: approximately a billion every second. If one of these anomalous decays of the muon is observed it would signal that there are additional new particles or interactions that are not embodied in our current theory. Our current theory is sadly inadequate: it fails to describe gravity on the atomic scale, cannot explain the existence of dark-matter nor why our universe is dominated by matter & has very little anti-matter. To explain this requires there to be new particles or interactions & the observation of the anomalous decay of the muon would prove that such new particles & interactions do exist. Three UK institutes (Liverpool, Manchester & UCL) will be making a key contribution to the search for this anomalous muon decay. We will be building a detector for the Mu2e experiment at Fermilab that will measure the number of muons being produced in the experiment such that if an anomalous decay is observed we can determine its rate. Without this detector we have no normalization for the measurement. We will build the detector in the next 3 years & start examining the muon decays in 2020 & hope then to answer whether there are indeed new, previously unseen particles.