Precision Measurements in Intermediate Energy Physics

中间能量物理的精密测量

• 批准号: 1506374
• 负责人: Robert Carey
• 金额: $ 21.91万
• 依托单位: Trustees of Boston University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2015
• 资助国家: 美国
• 起止时间: 2015-08-15 至 2018-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1506374&HistoricalAwards=false
• 关键词: Precision; Measurements; Intermediate; Energy; Physics

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.

μ子非常像我们熟悉的电子，具有许多相同的特性（例如固有的自旋和电荷），但质量大约是电子的200倍。这些相似性和差异使μ子成为探索新物理学的敏感工具，并将在该奖项支持的两个实验中使用：Paul Scherrer Institut（Villigen，瑞士）的MuSun实验和费米国家加速器实验室（FNAL，芝加哥附近）的μ子g-2实验。 质子-质子融合是一系列反应中的初始核反应，这些反应是太阳产生能量的来源。这种聚变反应的速率无法在实验室中测量，也无法从第一原理计算出来。在MuSun实验中，μ子将被用来研究类似质子-质子聚变的反向过程。 当一个μ子被一个氘核（由一个中子和一个质子组成）俘获时，氘分裂成一对中子和一个μ子中微子。氘对μ介子俘获的测量将提供必要的信息，使第一原理计算成为可能。这一计算也将为中微子特性的研究提供更坚实的理论基础。 FNAL的新μ子g-2实验旨在以前所未有的精度测量μ子的异常磁矩。由于μ介子具有内禀角动量（自旋）和电荷，它也具有磁矩，也就是说，它的行为像一个微小的磁铁。角动量和磁矩之间的关系由旋磁比或g因子描述。对于电子和μ介子，g略大于2。这个与2的差就是所谓的异常。电子和μ子的异常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学的暗示。测量捕获率的技术很简单。负μ介子被阻止在充满超纯氘气的探测器中。该实验将测量μ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μ子衰变为电子加中微子（电子和μ子类型）的消失率-差异是俘获率。 捕获率的1.5%的灵敏度目标将需要收集大约200亿μ子消失事件。为了测量异常，μ子被注入一个储存环，一个甜甜圈形状的装置，周长约44米，引导它们在大致圆形的轨道上运行。当μ子绕着储存环旋转时，它们的自旋矢量就像陀螺仪一样，比它们的动量矢量转得更快。 从μ子衰变电子的时间分布中提取的岁差率与异常成正比。实验的目标是使反常现象的误差达到大约千万分之一，这应该是对可能的新物理学的严格检验。