Fundamental Physics with Cold Polarized Neutrons

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内，中子保持稳定，但当脱离原子核时，它是不稳定的。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，这是基本粒子的一种量子力学性质。例如，在核磁共振和核磁共振中，自旋是利用核磁的原因。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于更详细地研究亚原子相互作用是有用的。在这项拟议的研究中，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核反应堆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，并可能在总结已知基本粒子相互作用的标准模型之外发现新的物理。中子束中的中子自旋将通过使用基于激光极化的3He的自旋过滤器来选择主要的一个自旋态来操纵。利用与核磁矩相耦合的磁场，可以逆转中子的自旋。需要非常精确地测量中子极化(即所选自旋态的过剩分数)和精确的自旋反转，以减轻在这些实验中可能出现的错误效应。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。将进行三个主要的实验。第一个目标是测量弱相互作用对氢吸收中子的影响。弱相互作用的特征是吸收和发射具有将自旋方向与运动方向相关联的左旋手的粒子。这种利手效应使我们能够在中子和质子的强得多的相互作用中观察到弱相互作用。确定中子和质子之间弱相互作用的特征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2007年转移到橡树岭之前，在洛斯阿拉莫斯获取数据。NIST的一项实验旨在报告发现一种罕见的中子衰变模式--辐射衰变，在这种模式中，光子与在大多数中子衰变中发射的中微子、电子和质子一起产生。第三个实验将被设计和建造用于检测质子发射方向与衰变的中子自旋的依赖关系的装置。这种质子方向的依赖关系取决于从初始中子到最终衰变产物的多个量子路径的关系。它受到组成中子和质子的基本粒子以及发射的电子和中微子之间的内在相互作用的影响。将应用先进的自旋态选择、精密偏振测量和自旋翻转技术，以精确测量中子衰变的利手性。当与其他中子衰变测量相结合时，这些测量也将探索超越标准模型的物理。这项工作探索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，但这些技术具有更广泛的影响。激光偏振3He和129Xe实验可应用于生物医学、材料科学和量子信息研究。对于本科生和研究生来说，这个项目是一个不同寻常的训练场。技术挑战与深层次的智力问题结合在一起，提供了动力并培养了技术技能。本科生将获得与研究生和博士后研究员一起工作的研究经验。研究生具备广泛的能力，并继续为教师或国家实验室的职位做准备。这项工作还为非物理专业的学生开发了新课程，举办了一系列关于核磁体和中微子的公开讲座，并向感兴趣的普通民众传播科学。在探索物理学基本问题的背景下，最难的问题产生了最具创新性的解决方案，其副产品在一开始是不可想象的，这本身就令人兴奋。原子钟，增强的核磁共振成像，以及探索物质的起源，都来自于对核磁和中子自旋的控制。

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.