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，它的性质可以很好地测量，并通过它在化学反应中的作用来支撑生命。1937年，在宇宙射线中发现了一种类似但更重的带电粒子——介子。在过去的80年里，人们一直在研究μ子，它的行为似乎就像一个更重的电子，其性质只是因为它的质量大约是电子的220倍而有所改变。像电子一样，它似乎没有结构，也不是电子的激发态，而是一种独特的基本粒子。这种区别体现在一种被称为轻子味的特性中：电子和μ子（和tau）都是带电的轻子，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（中微子）也是类似的情况。它们似乎也有三种不同的味道：电子、介子和tau。2015年诺贝尔奖授予了中微子在太空中运动时改变味道的观察：电子味的中微子（所谓的电子中微子）变成了介子味的中微子（介子中微子）。这说明了轻子味的数量并不是神圣不可侵犯的，也就是说，它并不总是守恒的：一个人可以从一个单位的电子味开始，以零结束，而不是一个单位的介子味。与电子相比，μ子的质量更大意味着它不稳定，衰变寿命为2 × 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（偶尔还有一个额外的光子）。在每一次衰变中，当人们考虑到衰变粒子的轻子味道时，它总是一个单位的μ子味道，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。鉴于中微子不是这种情况，我们预计带电的轻子也不会总是如此，我们预计一些μ子会以一种不保留轻子味道的方式衰变。自1947年以来，我们一直在寻找这样的衰变！对于粒子物理标准模型中已知的粒子，这一过程是可能发生的，但每10^50（1加50个零）μ子才会发生一次：在这个背景下，10^50大约是我们整个星球上原子的数量，所以观察到这种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子一次异常衰变的速率来观察这种衰变，这要容易得多：它只相当于观察地球上所有沙漠和海滩上的一粒沙子的行为奇怪！然而，由于技术的进步，我们现在可以大量生产μ子：每秒大约10亿个μ子。如果观察到其中一种介子的异常衰变，这将表明存在我们目前理论中没有体现的额外的新粒子或相互作用。可悲的是，我们目前的理论是不充分的：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，而且反物质很少。要解释这一点，就需要有新的粒子或相互作用&对μ子异常衰变的观察将证明这样的新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）将为寻找这种异常的介子衰变做出关键贡献。我们将在费米实验室为Mu2e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，我们就无法对测量进行归一化。我们将在未来3年内建造探测器，并在2020年开始检查μ子衰变，希望届时能回答是否确实存在以前看不见的新粒子。

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.