Precision Measurements with Polarized Cold Neutron Beams and 3He

中子是一种亚原子粒子，构成了世界上一半以上的物质。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组分和衰变产物之间的相互作用。中子也有自旋，这是最基本的物质的量子力学性质，它区分了两种状态，即上自旋和下自旋。自旋是核磁共振和核磁共振等核磁共振技术的基础。自旋态也影响中子的衰变和相互作用，因此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。在拟议的研究中，由洛斯阿拉莫斯国家实验室的粒子加速器和NIST的核反应堆产生的自由中子将被用于控制中子自旋的实验，以揭示弱相互作用和超越标准模型的可能的新物理，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基于激光偏振的aspin过滤器选择一个主要的自旋状态来操纵。3美元。中子的自旋可以用与核磁矩耦合的磁场来逆转。非常精确的测量中子极化（{\it i.e.}所选自旋状态的过量部分）和精确的自旋反转来减轻可能出现的假效应。He的极化、自旋过滤和自旋翻转等技术将随着这些实验的进行而得到很大的改进。洛斯阿拉莫斯实验的目的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旋向的粒子，这种旋向将自旋取向与运动方向联系起来。这种旋向允许在中子和质子的强得多的相互作用中观察弱相互作用。中子和质子之间弱相互作用的连续表征对于充分理解束缚原子核的力是至关重要的。NIST的实验测量了中子衰变对自旋态的依赖性。在一个专用的探测器中，通过同时探测质子和电子来观察中子衰变。第三种粒子是中微子，它太难以捉摸了，无法高效探测。电子和质子的相对运动对中子自旋态的依赖性将揭示在时间反转下不对称的相互作用，这是一个特殊的信号，可以揭示标准模型之外的新物理学，可以解释大爆炸中物质的起源。先进的自旋态选择、精密偏振测量和自旋翻转技术将被应用于一个计划中的实验，其目标是精确测量中子衰变的旋向性。当与其他中子衰变测量相结合时，这些测量也将探索标准模型之外的物理学。这项工作探索了关于物质最基本部分的深层智力问题。其目的是收集数据，这将有助于完成基本粒子及其相互作用的图片。这些技术有着更广泛的影响。激光极化氦和氦在相关实验中的应用现在被用于生物医学研究、材料科学，并可能应用于量子计算的进展。本研究为本科生和研究生提供了一个可行的训练基地。技术挑战与深层的智力问题相结合，提供了动力并发展了技术技能。 一些本科生将沿着研究生和资深研究人员一起工作，获得研究经验。研究生们表现出广泛的能力，并继续为教师职位和跨学科研究做准备。这项工作还导致了非物理专业新课程的开发和一系列关于核磁体的公开讲座。面向公众的主题是，在探索物理学基本问题的背景下--这些问题本身就令人兴奋，最困难的问题产生了最具创新性的解决方案，并在一开始就产生了不可估量的副产品。原子钟、增强的核磁共振成像和探索物质起源都是由核磁控制而来的。

The neutron is a sub--atomic particle thatcomprises more than half of the matter in theworld. Within the nuclei of most atoms, theneutron remains stable, but when freed from thenucleus, it is unstable. Free neutrons are thusan important tool for study ofsub--atomic physics, because the decay andinteractions of free neutrons reveal theinteractions of its constituents and decayproducts. Neutrons also have spin, the quantummechanical property of the most fundamentalpieces of matter that distinguishes two states calledspin--up and spin--down. Spin is responsiblefor the nuclear magnetism exploited, for example,in NMR and MRI. The spin states also affect thedecay and interactions of neutrons, and so thecontrol of neutron spin becomes useful for moredetailed study of sub--atomic interactions. Inthe proposed research, free neutrons produced bya particle accelerator at Los Alamos NationalLaboratory and at the nuclear reactor at NISTwill be used in experiments that control theneutron spin to reveal weak interactions andpossible new physics beyond the Standard Model that summarizes known elementaryparticle interactions.The neutron spins in a beam of neutron will be manipulated byselecting predominantly one spin state with aspin filter based on laser polarized $^3$He. Theneutron spin can be reversed usingmagnetic fields that couple to the nuclearmagnetic moment. Very precise measurement of theneutron polarization ({\it i.e.} the excessfraction of the selected spin state) and precisespin reversal are required to mitigate falseeffects that may arise. The techniques of$^3$He polarization, the spin filter, and spinflipping will be greatly improved as we undertakethese experiments.The objective of the Los Alamos experiment ismeasurement of the weak interaction effects onthe absorption of neutrons by hydrogen. The weakinteractionis characterized by absorption and emission ofparticles that have a handedness that correlatesthe spin orientation with the direction of motion.This handedness allows observation of the weakinteraction amidst the much stronger interactionsof neutrons and protons. Definitive characterization of the weakinteraction between the neutron and proton iscrucial to fully understanding the forces thatbind the nucleus. The experiment at NIST measuresthe dependence of neutron decays on the spinstate. In a dedicated detector, neutron decaysare observed by simultaneously detecting the proton andelectron. Thethird particle emitted, the neutrino, is too elusive todetect with high efficiency. A dependence ofthe electron's and proton's relative motion on the neutron spinstate would reveal interactions that are notsymmetric under reversal of time, a specialsignature that could reveal new physics beyondthe Standard Model, physics that mayexplain the origin of matter in the Big Bang. The advancedtechniques of spin state selection, precisionpolarimetry, and spin flipping will be applied toa planned experiment with the objective of preciselymeasuring the handedness of neutron decays. Whencombined with other neutron decay measurementsthese measurements will also probe physics beyond theStandard Model.This work probes deep intellectual questionsabout the most fundamentalpieces of matter. The aim is to collect datathat will help complete the picture of elementaryparticles and their interactions. The techniqueshave much broader impact. Laser polarized$^3$He and $^{129}$Xe used in related experimentsare now used in biomedical research, materialsscience, and may be applied in advances ofquantum computation. This research is a remarkabletraining ground for undergraduate and graduatestudents. The technical challenges combined withthe deep intellectual issues provide motivationand develop technical skills. Several undergraduates willgain research experience working along with graduate students andsenior researchers. Graduate students emergebroadly capable and move on to prepare forfaculty positions as well as interdisciplinaryresearch. This work has also led to developmentof new courses for non--physics majors and to aset of public lectures on Nuclear Magnets.The theme for the public isthat, in the context of probing fundamentalproblems of physics -- exciting in their ownright, the hardest problems produce the mostinnovative solutions with spin--offs unimaginableat the outset. Atomic clocks, enhanced MRI, andprobing the origin of matter all follow from thecontrol of nuclear magnetism.