Precision Measurements in Intermediate Energy Physics

µ子非常像我们熟悉的电子，有许多相同的特征(例如本征自旋和电荷)，但质量大约是前者的200倍。这些相似之处和不同之处使得介子成为探索新物理的敏感工具，并将在该奖项支持的两个实验中得到利用：保罗谢勒研究所(瑞士维利根)的MuSun实验和费米国家加速器实验室(FNAL，芝加哥附近)的Muon g-2实验。质子-质子聚变是一系列反应中的初始核反应，这些反应是太阳产生的能量的来源。这种聚变反应的速度既不能在实验室里测量，也不能从第一性原理计算出来。在MuSun实验中，介子将被用于反向研究质子-质子聚变之类的东西。当一个缪子被一个由一个中子和一个质子组成的重核俘获时，重离子分解成一对中子和一个介子中微子。测量重氢俘获的介子将提供必要的信息，使第一性原理计算成为可能。这一计算也将为有关中微子特性的结果提供更坚实的理论基础。法国国家航空航天局新的介子g-2实验旨在以前所未有的精度测量介子的反常磁矩。由于介子具有固有的角动量(自旋)和电荷，它还具有磁矩，即它的行为就像一块微小的磁铁。角动量和磁矩之间的关系用旋磁比或g因子来描述。对于电子和介子，g略大于2，这与2之差就是所谓的反常。无论是电子还是µ子的反常现象都可以非常精确地测量和计算。两者之间的任何重大分歧都是新物理的迹象。测量捕获率的技术很简单。负缪子被阻止在一个充满超纯氢气的探测器中。该实验将测量介子的消失率，类似于任何放射性衰变实验。由于俘获过程，消失率将略大于自由Muon衰变为电子加中微子(电子和Muon两种类型)的消失率--不同之处是捕获率。关于捕获率的1.5%的灵敏度目标将需要收集大约200亿个Muon消失事件。为了测量这种反常现象，他们将µ子注入一个存储环中，这是一个圆周约44米的甜甜圈形状的装置，引导它们沿大致圆形的轨道运行。当介子绕着存储环旋转时，它们的自旋矢量就像陀螺仪一样，比它们的动量矢量转动得更快。从介子衰变电子的时间分布中提取的进动速率与反常成正比。实验的目标是，对反常现象的微小误差大约为千万分之一，这应该会为可能的新物理提供严格的测试。

Muons are very much like the familiar electron, with many of the same characteristics (e.g. intrinsic spin and electric charge) but approximately 200 times more mass. Those similarities and that difference make muons a sensitive tool for exploring new physics and will be exploited in two experiments supported by this award: the MuSun experiment at the Paul Scherrer Institut (Villigen, Switzerland) and on the muon g-2 experiment at the Fermi National Accelerator Laboratory (FNAL,, near Chicago). Proton-proton fusion is the initial nuclear reaction in a chain of reactions which are the source of the energy produced by our sun. The rate of this fusion reaction cannot be measured in the laboratory nor has it been calculated from first principles. In the MuSun experiment, muons will be used to study something like proton-proton fusion in reverse. When a muon is captured by a deuterium nucleus, consisting of a neutron and a proton, the deuterium breaks apart into a pair of neutrons and a muon neutrino. The measurement of muon capture by deuterium will provide the necessary information to make possible a first principles calculation. That calculation will also provide a more solid theoretical foundation for the results concerning neutrino characteristics. The new muon g-2 experiment at FNAL aims to measure the anomalous magnetic moment of the muon with unprecedented precision. Because the muon has intrinsic angular momentum (spin) and an electric charge, it also possesses a magnetic moment, that is, it behaves like a tiny magnet. The relationship between the angular momentum and magnetic moment is described by the gyromagnetic ratio or g factor. For the electron and the muon, g is very slightly greater than 2. This difference from 2 is the so-called anomaly. The anomaly, for both electrons and muons, can be measured and calculated with great precision. Any significant disagreement between the two is a hint of new physics.The technique for measuring the capture rate is simple. Negative muons are stopped in a detector filled with ultra-pure deuterium gas. The experiment will measure the disappearance rate of the muons, similar to any radioactive decay experiment. Because of the capture process, the disappearance rate will be slightly larger than that of free muon decay to electrons plus neutrinos (of both electron and muon types) - the difference is the capture rate. The sensitivity goal of 1.5 percent on the capture rate will require the collection of approximately 20 billion muon disappearance events. To measure the anomaly, muons are injected into a storage ring, a doughnut shaped device, roughly 44 m in circumference, which guides them in roughly circular orbits. As the muons circle the storage ring, their spin vectors, which act like gyroscopes, turn faster than their momentum vectors. The rate of the precession, which is extracted from the time distribution of muon decay electrons, is directly proportional to the anomaly. The experimental goal, a fractional error on the anomaly of approximately one part in ten million, should provide a stringent test of possible new physics.