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它的成分和衰变产物的相互作用。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，称为自旋向上和自旋向下。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的粒子加速器和美国国家科学研究院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可能的新物理，而不是总结已知的元素粒子相互作用的标准模型。中子束中的中子自旋将通过选择主要一个自旋状态来控制，该过滤器基于激光偏振氦。利用与核磁矩相耦合的磁场，可以使中子自旋反转。为了减少可能出现的假效应，需要非常精确地测量中子极化(即所选自旋态的过多部分)和精确的反转。随着我们进行这些实验，He极化、自旋滤光器和自旋滑移的技术将得到极大的改进。洛斯阿拉莫斯实验的目的是测量氢吸收中子的弱相互作用效应。弱相互作用的特征是吸收和发射具有与自旋方向和运动方向相关的惯用手的粒子。这种惯用手允许在中子和质子之间更强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。NIST的实验测量了中子衰变与初生态的关系。在专用探测器中，通过同时探测质子和电子来观察中子衰变。发射出的第三个粒子，即中微子，太难以捉摸了，无法高效率地探测到。电子和质子的相对运动对中子源的依赖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，可能揭示标准模型以外的新物理，即可能解释大爆炸中物质起源的物理。先进的自旋态选择、精密偏振和自旋翻转技术将被应用于一个计划中的实验，目的是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出标准模型的物理。这项工作探索关于最基本物质的深层智力问题。其目的是收集数据，帮助完成元素粒子及其相互作用的图景。这些技术的影响要广泛得多。在相关实验中使用的激光偏振He和Xe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。这项研究为本科生和研究生提供了一个很好的培训场地。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。一些本科生将获得与研究生和高级研究人员一起工作的研究经验。研究生表现出广泛的能力，并继续为教师职位和跨学科研究做准备。这项工作还导致了非物理专业的新课程的开发和一系列关于核磁的公开讲座。面向公众的主题是，在探索物理学基本问题的背景下--本身就令人兴奋，最难的问题产生最具创新性的解决方案，其副产品从一开始就是不可想象的。原子钟、增强的核磁共振成像和探索物质的起源都源于核磁的控制。

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.