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

Mu2e：一项将带电轻子风味违规的灵敏度扩大 4 个数量级的提案。

• 批准号: ST/P002854/1
• 负责人: Mark Lancaster
• 金额: $ 8.32万
• 依托单位: University College London
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2017
• 资助国家: 英国
• 起止时间: 2017 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=ST%2FP002854%2F1
• 关键词: Mu2e; proposal; extend; sensitivity; charged

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.

电子是最轻、最稳定的带电粒子，其特性得到了很好的测量，并通过其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 子。过去 80 年来人们一直在研究 μ 子，它的行为似乎就像电子的较重版本，其性质只是由于其重约 220 倍而发生了变化。它看起来像电子一样没有结构，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称为轻子味的属性中：电子和μ子（以及τ）都是带电轻子，我们说电子是具有电子味的带电轻子，而μ子是具有μ子味的带电轻子。中性轻子（中微子）的情况类似。它们似乎也具有三种不同的特征：电子、μ子和τ子。 2015 年诺贝尔奖获奖者是因为观察到中微子在太空中移动时味道会发生变化：电子味中微子（所谓的电子中微子）变成了μ子味中微子（μ介子中微子）。这说明轻子味的数量并不是神圣不可侵犯的，即它并不总是守恒的：可以从 1 个单位的电子味开始，以 0 单位结束，但改为 1 个单位的 μ 味子味。与电子相比，μ 子的质量较大，这意味着它不稳定且会以 2 x 1/百万分之一秒的寿命衰变。迄今为止，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（偶尔还有额外的光子）。在每一种衰变中，当人们考虑衰变粒子的组合轻子味时，它总是一个μ子味单位，就像最初衰变的μ子一样。中微子、电子和正电子的轻子味道总是相互抵消。鉴于中微子并非如此，我们预计带电轻子也不会总是如此，并且我们预计一些μ子会以不守恒轻子风味的方式衰变。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的已知粒子，该过程有可能发生，但每 10^50（1 个有 50 个零）μ 子只能发生一次：将其置于上下文中，10^50 大约是原子数在我们整个星球上，因此观察到这种不可能发生的情况是不可能的。相反，我们试图以每 10^17 μ 子一次异常衰变的速率观察这种衰变，这要容易得多：这相当于观察地球上所有沙漠和海滩上的一粒沙粒表现得很奇怪！然而，由于技术进步使我们能够大量生产 μ 子（每秒大约 10 亿个），现在 μ 子已经成为可能。如果观察到μ子的这些异常衰变之一，则表明存在我们当前理论中未体现的其他新粒子或相互作用。令人遗憾的是，我们目前的理论是不够的：它无法描述原子尺度上的引力，无法解释暗物质的存在，也无法解释为什么我们的宇宙由物质主导而反物质很少。要解释这一点，需要存在新的粒子或相互作用，并且对 μ 子的异常衰变的观察将证明这种新粒子和相互作用确实存在。三个英国研究机构（利物浦、曼彻斯特和伦敦大学学院）将为寻找这种异常μ子衰变做出重要贡献。我们将为费米实验室的 Mu2e 实验建造一个探测器，该探测器将测量实验中产生的 μ 子数量，这样如果观察到异常衰变，我们就可以确定其速率。如果没有这个检测器，我们就无法对测量进行标准化。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，并希望能够回答是否确实存在新的、以前未见过的粒子。

项目成果

Delivering the world's most intense muon beam

提供世界上最强的μ子束

• DOI: http://dx.10.1103/physrevaccelbeams.20.030101
• 发表时间: 2017
• 期刊: Physical Review Accelerators and Beams
• 影响因子: 1.7
• 作者: Cook S