Fundamental Physics with Cold Polarized Neutrons

冷极化中子的基础物理

• 批准号: 0555432
• 负责人: Timothy Chupp
• 金额: $ 46.5万
• 依托单位: Regents of the University of Michigan - Ann Arbor
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2006
• 资助国家: 美国
• 起止时间: 2006-05-01 至 2012-04-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0555432&HistoricalAwards=false
• 关键词: Fundamental; Physics; Cold; Polarized; Neutrons

The neutron is a subatomic particle that comprises more than half of the matter in the world. Within the nuclei of most atoms, the neutron remains stable, but when freed from the nucleus, it is unstable. Free neutrons are an important tool for study of subatomic physics, because the decay and interactions of free neutrons reveal the interactions of its constituents and decay products. Neutrons also have spin, a quantum mechanical property of fundamental particles. Spin is responsible for the nuclear magnetism exploited, for example, in NMR and MRI. The spin states also affect the decay and interactions of neutrons, and so the control of neutron spin becomes useful for more detailed study of subatomic interactions. In the proposed research, free neutrons produced at Los Alamos National Laboratory, at the new Spallation Neutron Source at Oak Ridge National Laboratory and at the nuclear reactor at NIST will be used in experiments that control the neutron spin. The results will reveal weak interactions and possible new physics beyond the Standard Model that summarizes known elementary particle interactions. The neutron spins in a beam of neutrons will be manipulated by selecting predominantly one spin state with a spin filter based on laser polarized 3He. The neutron spin can be reversed using magnetic fields that couple to the nuclear magnetic moment. Very precise measurement of the neutron polarization (i.e. the excess fraction of the selected spin state) and precise spin reversal are required to mitigate false effects that may arise in these experiments. Several of the techniques used in these experiments will be greatly improved over those of previous experiments.Three major experiments will be undertaken. The objective of the first is measurement of the weak interaction effects on the absorption of neutrons by hydrogen. The weak interaction is characterized by absorption and emission of particles that have a handedness that correlates the spin orientation with the direction of motion. This handedness allows observation of the weak interaction amidst the much stronger interactions of neutrons and protons. Definitive characterization of the weak interaction between the neutron and proton is crucial to fully understanding the forces that bind the nucleus. This experiment will take data at Los Alamos before moving to Oak Ridge in 2007. An experiment at NIST aims to report the discovery of a rare mode of neutron decay, radiative decay, in which a photon is produced along with the neutrino, electron, and proton emitted in most neutron decays. The third experiment will be designed and apparatus built to detect the dependence of the direction of proton emission with respect to the decaying neutron spin. This proton direction dependence depends on the relationship of the multiple quantum paths from the initial neutron to the final decay products. It is affected by the intrinsic interactions among the fundamental particles that make up the neutron and proton and the emitted electron and neutrino. Advanced techniques of spin state selection, precision polarimetry, and spin flipping will be applied with the objective of precisely measuring the handedness of neutron decays. When combined with other neutron decay measurements these measurements will also probe physics beyond the Standard Model.This work probes deep intellectual questions about fundamental issues in science. The aim is to collect data that will aid in understanding elementary particles and their interactions, but the techniques have much broader impact. Laser polarized 3He and 129Xe experiments can be applied to biomedical research, materials science, and quantum information research. This project is an unusual training ground for undergraduate and graduate students. The technical challenges combined with the deep intellectual issues provide motivation and develop technical skills. Undergraduates will gain research experience working along with graduate and post doctoral fellows. Graduate students emerge broadly capable and move on to prepare for faculty or national lab positions. This work has also led to development of new courses for nonphysics majors, to a set of public lectures on Nuclear Magnets and Neutrinos and to communicating science to the interested general population. In the context of probing fundamental problems of physics, exciting in its own right, the hardest problems produce the most innovative solutions with spin-offs unimaginable at the outset. Atomic clocks, enhanced MRI, and probing the origin of matter all follow from the control of nuclear magnetism and neutron spins.

中子是一种亚原子粒子，占世界物质的一半以上。在大多数原子的原子核中，中子保持稳定，但当它从原子核中释放出来时，它就不稳定了。自由中子是研究亚原子物理的重要工具，因为自由中子的衰变和相互作用揭示了其成分和衰变产物的相互作用。中子也有自旋，这是基本粒子的量子力学特性。自旋是核磁被利用的原因，例如，在核磁共振和核磁共振中。自旋态也影响中子的衰变和相互作用，因此对中子自旋的控制对亚原子相互作用的更详细的研究是有用的。在拟议的研究中，在洛斯阿拉莫斯国家实验室、橡树岭国家实验室的新散裂中子源和NIST的核反应堆中产生的自由中子将用于控制中子自旋的实验。结果将揭示弱相互作用和可能超越标准模型的新物理，标准模型总结了已知的基本粒子相互作用。中子束中的中子自旋将通过激光极化3He自旋滤波器选择一个主要自旋态来控制。中子自旋可以用与核磁矩耦合的磁场来逆转。需要非常精确地测量中子极化（即所选自旋态的多余部分）和精确的自旋反转，以减轻这些实验中可能出现的错误效应。在这些实验中使用的一些技术将比以前的实验有很大的改进。将进行三项主要实验。第一个目标是测量弱相互作用对氢吸收中子的影响。弱相互作用的特点是吸收和发射具有旋向的粒子，旋向与运动方向相关。这种旋向性允许在中子和质子之间强得多的相互作用中观察弱相互作用。明确描述中子和质子之间的弱相互作用对于充分理解束缚原子核的力是至关重要的。这项实验将在洛斯阿拉莫斯收集数据，然后于2007年转移到橡树岭。NIST的一项实验旨在报告发现一种罕见的中子衰变模式，即辐射衰变，在大多数中子衰变中，光子与中微子、电子和质子一起产生。第三个实验将设计并建造仪器来检测质子发射方向与衰变中子自旋的依赖关系。这种质子方向依赖性取决于从初始中子到最终衰变产物的多重量子路径的关系。它受到构成中子和质子以及发射的电子和中微子的基本粒子之间的内在相互作用的影响。先进的自旋态选择、精密偏振法和自旋翻转技术将被应用于精确测量中子衰变的手性。当与其他中子衰变测量相结合时，这些测量也将探测超出标准模型的物理。这部作品探讨了关于科学基本问题的深刻的智力问题。其目的是收集有助于理解基本粒子及其相互作用的数据，但这些技术的影响要广泛得多。激光偏振3He和129Xe实验可应用于生物医学研究、材料科学和量子信息研究。这个项目对本科生和研究生来说是一个不同寻常的训练基地。技术挑战与深刻的智力问题相结合，提供了动力和发展技术技能。本科生将获得与研究生和博士后一起工作的研究经验。研究生表现出广泛的能力，并继续为教师或国家实验室职位做准备。这项工作还促成了为非物理专业学生开设新课程、一系列关于核磁体和中微子的公开讲座，以及向感兴趣的普通大众传播科学。在探索物理学基本问题的背景下，最难的问题本身就令人兴奋，产生了最具创新性的解决方案，并带来了最初难以想象的副产品。原子钟、增强的核磁共振成像（MRI）和探测物质的起源，都源于对核磁和中子自旋的控制。