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

电子是最轻、最稳定的带电粒子，它的性质被很好地测量出来，并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——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，它的性质只是因为它的质量大约是电子的220倍而有所改变。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。它较大的质量意味着它不稳定，衰变寿命为2 × 1/百万分之一秒。像电子一样，介子是带电的，具有自旋的量子力学性质。这反过来意味着介子的行为就像亚原子磁铁，并具有称为磁矩的特性。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对μ子施加的扭矩的大小。这个扭矩使μ子的自旋方向以一定的频率围绕磁场的方向进动。这个频率是由μ子的磁矩决定的，这个频率和磁矩，我们将在这个项目中测量。我们正在设法测量μ子的磁矩，精度达到百万分之0.14，这将比以前的测量好4倍以上。进行如此精确的测量的原因是，在量子力学中，μ子的磁矩的值是非常精确的，所以我们可以用这个测量来测试量子力学的预测，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，而这些力反过来又影响它的磁矩。引力的作用太小，无法测量，但其他因素则不然。由于我们很好地了解了这些力的性质，然后使用量子理论，我们就可以预测μ子的磁矩，并将其与实验进行比较。如果预测和测量结果有显著差异，那么这将是存在新型相互作用的证据，或者证明介子毕竟不是基本粒子，而是具有某种结构。先前根据2001年采集的数据对μ子磁矩的测量结果与预测结果不一致，结果一致的概率只有0.05%。然而，在科学中，不一致的基准是它们一致的机会必须非常小（0.0001%）。通过进行更精确的测量，我们可以更好地检查测量和预测的一致性，并确定是否确实存在新物理学的证据。我们将通过将一束μ子注入一个圆形存储环（半径为7米）来进行测量，该存储环受到1.45 T磁场的影响。通过检查电子从介子衰变的方向作为时间的函数（并非常精确地测量磁场，即优于百万分之0.1），我们可以测量磁矩。这项工作将于2016年在美国的费米实验室完成。英国的研究机构（利物浦、伦敦大学学院、牛津大学、QMUL、RAL）将对这一测量做出关键贡献。我们将建造测量介子束轨迹的探测器，测量磁场的装置和将介子束注入圆形存储环的磁铁。我们希望在2018年之前完成测量，从而知道在四种已知相互作用之外是否存在新的物理。

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.