A measurement of the anomalous magnetic moment of the muon to 0.14 ppm using the FNAL g-2 experiment.

使用 FNAL g-2 实验测量 0.14 ppm 的 μ 子反常磁矩。

• 批准号: ST/L001888/1
• 负责人: Mark Lancaster
• 金额: $ 31.4万
• 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: 2014
• 资助国家: 英国
• 起止时间: 2014 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=ST%2FL001888%2F1
• 关键词: measurement; anomalous; magnetic; moment; muon

The electron is the lightest, stable charged particle and its properties are extremely well measured and 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 and it seems to behave like a heavier version of the electron with its properties only modified by virtue of it beingapproximately 220 times the mass of the electron. It appears, like the electron, to have no structure and is not an excited state of the electron but a distinct fundamental particle.Its larger mass means it is unstable and decays with a lifetime of 2 x 1/millionth of a second. Like the electron, the muon is charged and has the quantum mechanical property of spin. This in turn means that the muon acts like a subatomic magnet and has a property called a magnetic moment. This microscopic magnetic moment in the case of an electron ultimately determines the macroscopic magnetic properties of a material. The size of this magnetic moment determines the size of the torque that an external magnetic field will exert on the muon. This torque causes the direction of the muon's spin to precess around the direction of the magnetic field with a certain frequency. This frequency is determined by the muon's magnetic moment and it this frequency and hence magnetic moment that we will measure in this project.We are seeking to measure the magnetic moment of the muon to a precision of 0.14 parts per million which will be over a factor of 4 better than the previous measurement. The reason for making such a precise measurement is that the value of the muon's magnetic moment is very precisely predicted in quantum mechanics and so we can use the measurement to test the predictions of quantum mechanics to a very high level of precision.We presently know there are 4 types of force or interaction: the strong nuclear force, the electromagnetic force, the weak nuclear force and the gravitational force. The muon is subject to all these forces (interactions) and these in turn affect its magnetic moment. The gravitational contribution is too tiny to be measured but the others are not. Since we know the properties of these forces very well then using quantum theory, we can then predict the magnetic moment of the muon and compare it to experiment. Should the prediction and the measurement differ significantly then that would be evidence that there are new types of interaction or that the muon is not a fundamental particle after all and has some sort of structure. The previous measurement of the muon's magnetic moment from data taken in 2001 was at odds with the prediction such that the probability of them being consistent was only 0.05%. However in science the benchmark for inconsistency is that the chance of them being consistent has to be extremely small (0.0001%). By making a more precise measurement we can better examine this consistency of the measurement and the prediction and determine whether there is indeed evidence of new physics or not.We will make this measurement by injecting a beam of muons into a circular storage ring (of 7m radius) which is subject to a 1.45 T magnetic field. By examining the direction of the electrons from the muon decay as a function of time (and measuring very precisely i.e. to better than 0.1 parts per million) the magnetic field we can measure the magnetic moment. This will be done in 2016 at Fermilab in the USA.The UK institutes (Liverpool, UCL, Oxford, QMUL, RAL) will be making key contributions to this measurement. We will build the detectors that measure the muon beam's trajectory, the device to measure the magnetic field and the magnet that injects the beam into the circular storage ring. We hope by 2018 to have completed the measurement and so know whether there is new physics beyond the four known interactions or not.

电子是最轻、最稳定的带电粒子，其性质得到了极好的测量，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。在过去的80年里，人们一直在研究介子，它的行为似乎像是一个更重的电子版本，它的性质只是因为它的质量大约是电子的220倍而被改变。和电子一样，它看起来没有结构，不是电子的激发态，而是一种独特的基本粒子。它的较大质量意味着它是不稳定的，衰变寿命为百万分之一秒的2x1。像电子一样，Muon带电，并具有自旋的量子力学性质。这反过来意味着，Muon的行为就像一个亚原子磁铁，具有一种称为磁矩的性质。在电子的情况下，这种微观磁矩最终决定了材料的宏观磁性。这个磁矩的大小决定了外部磁场将施加在介子上的扭矩的大小。这个力矩会导致介子的自旋方向以一定的频率绕着磁场的方向前进。这个频率是由Muon的磁矩决定的，因此我们将在这个项目中测量这个频率和磁矩。我们正在寻求测量Muon的磁矩，精度为百万分之0.14，这将比以前的测量好4倍以上。之所以进行如此精确的测量，是因为在量子力学中，介子的磁矩的值被非常精确地预测，所以我们可以用这种测量来检验量子力学的预测，达到非常高的精度。我们目前知道有四种力或相互作用：强核力、电磁力、弱核力和引力。介子受到所有这些力(相互作用)的影响，而这些力又反过来影响它的磁矩。引力的贡献太小，无法测量，但其他的都不能测量。由于我们非常了解这些力的性质，然后使用量子理论，我们就可以预测Muon的磁矩，并将其与实验进行比较。如果预测和测量结果有很大不同，那么这将是存在新类型相互作用的证据，或者是Muon毕竟不是基本粒子，具有某种结构的证据。之前根据2001年的数据测量的介子磁矩与预测不符，因此它们保持一致的可能性只有0.05%。然而，在科学中，不一致的基准是它们保持一致的机会必须非常小(0.0001%)。通过进行更精确的测量，我们可以更好地检验测量和预测的一致性，并确定是否确实存在新物理的证据。我们将通过将一束µ子注入半径为7米的圆形存储环中来进行测量，该存储环受到1.45T的磁场的影响。通过检查来自Muon衰变的电子的方向作为时间的函数(并且非常精确地测量磁场，即好于百万分之零点一)，我们可以测量磁矩。这项研究将于2016年在美国费米实验室完成。英国的研究机构(利物浦、伦敦大学学院、牛津大学、昆士兰大学、拉尔大学)将为这一测量做出重要贡献。我们将建造测量介子束轨迹的探测器，测量磁场的装置，以及将束流注入圆形存储环的磁铁。我们希望在2018年前完成测量，从而知道在这四个已知相互作用之外是否存在新的物理现象。

项目成果

Measurement of the Positive Muon Anomalous Magnetic Moment to 0.20 ppm

• DOI: 10.1103/physrevlett.131.161802
• 发表时间: 2023-10-20
• 期刊: PHYSICAL REVIEW LETTERS
• 影响因子: 8.6
• 作者: Aguillard, D. P.;Albahri, T.;Zhang, C.
• 通讯作者: Zhang, C.

The New FNAL Muon g-2 Experiment

新的 FNAL Muon g-2 实验

• 发表时间: 2014
• 期刊: Proceedings of Science
• 作者: Lancaster, M

The muon EDM in the g-2 experiment at Fermilab

• DOI: 10.1051/epjconf/201611801005
• 发表时间: 2016-04
• 作者: R. Chislett

The Measurement of the Anomalous Magnetic Moment of the Muon at Fermilab

费米实验室μ子反常磁矩的测量

• DOI: 10.1063/1.4917553
• 发表时间: 2015
• 期刊: Journal of Physical and Chemical Reference Data
• 影响因子: 4.3
• 作者: Logashenko I