Beta Decay Measurements with Cold Neutron Beams and Ultracold Neutrons

该奖项将支持中子β衰变的三个重要实验，中子β衰变是由于弱核力（或弱相互作用）而衰变为质子，电子和反中微子的过程。 弱核力很重要，因为它为我们的太阳提供动力，并帮助确定宇宙中元素的丰度。 中子衰变特别重要，因为它在早期宇宙中氦的形成中起着关键作用。 此外，中子衰变只涉及单个核子，这使得从基本粒子和相互作用的标准模型计算非常精确的预测成为可能，而不会受到其他核子的影响。 通过非常精确的中子衰变测量，我们可以进行一种粒子物理取证，寻找标准模型中的差异，这些差异可以表明新力量的存在，但不使用高能粒子束。 这项研究是物理学“精确前沿”的一部分，定义了我们对弱核力和可能的新相互作用的知识的前沿。这项研究的一个重要方面是培养学生和博士后。 这有助于传播技术方法和专门知识。 这种培训来自项目的具体性质，使用核物理分析和技术，以及学生在我们的国家实验室和核设施进行研究的许多机会。所使用的一些技术在能量生成和医学成像中具有现实应用。该奖项支持的实验之一Nab将在橡树岭国家实验室的Spectron中子源上使用非偏振冷中子束来测量衰变电子和中微子之间的角相关性，目标是将这种相关性的精度提高一个数量级以上。 另外两个实验，UCNA和UCNTau，位于洛斯阿拉莫斯国家实验室（LANL）的超冷中子（UCN）源。UCNA实验测量中子自旋和衰变电子动量之间的角相关性（称为β不对称性），并旨在将轴向形状因子的全球数据集的精度提高3倍。UCNTau旨在测量中子的寿命，目标是在提案期间实现1秒的不确定性。 这些方案将影响：（1）在与超允许衰变相同的水平上从中子建立CKM矩阵元Vud所需的水平上的轴向形状因子的知识，（2）在一定水平上的中子寿命的知识，使得它在一段时间内不会限制宇宙学4He丰度模型的测试，以及（3）在CERN预期的水平上或以上的张量耦合的限制。 对于这些实验中的每一个，该小组通过开发实验所需的仪器和分析，将其在β衰变测量方面的经验用于系统误差的关键来源。

This award will support three important experiments in neutron beta decay, a process in which a neutron decays because of the weak nuclear force (or weak interaction) into a proton, an electron and an anti-neutrino. The weak nuclear force is important because it powers our sun and helps determine the abundance of elements in the universe. Neutron decay in particular is important because it plays a key role in the formation of helium in the early universe. Additionally, neutron decay involves only a single nucleon and that makes it possible to calculate very precise predictions from the Standard Model of fundamental particles and interactions without complications from other nucleons. With very accurate measurements of neutron decay, we can perform a kind of particle physics forensics, looking for discrepancies in the Standard Model that can indicate the presence of new forces but without using high energy particle beams. This research is a part of the "precision frontier" for physics, and defines the cutting edge of our knowledge of the weak nuclear force and possible new interactions.An important aspect of this research is the training of students and post-docs. This contributes to the spreading of technical methods and expertise. This training comes from the specific nature of the projects, which use nuclear physics analysis and technology, and from the many opportunities students have to perform research at our national laboratories and nuclear facilities. Some of the techniques used have real-world applications in energy generation and medical imaging. One of the experiments supported by this award, Nab, will use an unpolarized beam of cold neutrons at the Spallation Neutron Source from the Oak Ridge National Laboratory to measure the angular correlation between the decay electron and the neutrino, with the goal of improving the precision of this correlation by over an order of magnitude improvement. The other two experiments, UCNA and UCNtau, are situated at the ultracold neutron (UCN) source at Los Alamos National Laboratory (LANL). The UCNA experiment measures the angular correlation between the neutron spin and the decay electron momentum (called the beta-asymmetry), and aims for a factor of 3 improvement over the precision of the global data set for the axial form factor. UCNtau is designed to measure the lifetime of the neutron, with the goal of a 1 second uncertainty during the proposal period. These programs will impact: (1) knowledge of the axial form factor at the level required for a the establishment of the CKM matrix element Vud from the neutron at the same level as the super-allowed decays, (2) knowledge of the neutron lifetime at a level such that it will not limit tests of cosmological 4He abundance models for some time, and (3) limits on tensor couplings at or above the level expected from CERN. For each of these experiments, the group brings its experience in beta decay measurements to bear on key sources of systematic error by developing instrumentation and analysis required by the experiments.