High Precision Measurements of Beta Decay Using Stored Ultracold Neutrons and Cold Neutron Beams

该奖项支持对中子衰变的高精度测量。中子衰变是中子由于弱核力（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，因为它为太阳提供能量，并帮助确定宇宙中元素的丰度。中子衰变也很重要，因为它只涉及一个核子，使得从粒子物理学的“标准模型”进行非常精确的预测成为可能，而不会因为其他核子的存在而产生复杂性。通过对中子衰变的非常精确的测量，我们可以进行一种粒子物理“取证”，在我们的标准模型中寻找可以表明新力量存在的差异，但不使用高能粒子束，比如欧洲核子研究中心的大型强子对撞机。这项研究是物理学“精确前沿”的一部分，并定义了我们对弱核力和可能的新相互作用的知识的前沿。PI和他的团队为中子寿命的新测量（UCNtau）和角相关测量（Nab）做出了贡献。寿命和角相关测量探测弱相互作用的不同方面，如果成功，将增强我们对弱相互作用关键组成部分的认识，并有助于对标准模型的扩展做出强有力的限制，达到或超过欧洲核子研究中心正在建立的那些。该项目的一个重要方面是培养学生和博士后。这种培训部分是因为研究项目的特殊性，这些项目利用核物理分析和技术（例如，在能源产生和医学成像方面也有实际应用），部分是因为在这个项目中工作的学生有很多机会在国家实验室和核设施进行研究。该奖项支持的实验之一Nab将使用橡树岭国家实验室散裂中子源的非极化冷中子束来测量衰变电子和反中微子之间的角相关性（称为电子-反中微子相关性），其目标是将该参数的精度提高一个数量级以上。另一个是UCNtau，位于洛斯阿拉莫斯国家实验室（LANL）的超冷中子（UCN）源，目标是在当前资助期内精确水平低于0.3秒。这些计划成功的影响是：(1)在确定CKM矩阵元素Vud所需的水平上了解轴向形状因子，其水平与当前电弱辐射修正的理论不确定性相同；(2)在一定程度上了解寿命，从而在一段时间内限制我们的宇宙氦-4丰度模型的测试；(3)张量耦合的限制达到或高于CERN预期的水平。在过去的一年里，电弱辐射修正和轴向耦合常数的晶格计算的理论进展，扩大了这些测量的动机，两者都为新的物理学开辟了新的、更高精度的探针。对于每一项实验，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应用于系统误差的关键来源。以Nab为例，该小组正在分析不完全收集衰变电子能量和校准程序的影响，这是该实验系统误差预算的关键组成部分。UCNtau的设计是将ucn存储在磁引力阱中，ucn不与阱壁接触。陷阱的顶部是开放的，允许探测器进入陷阱来测量UCN的数量和确定寿命。该小组将为此目的继续开发UCN探测器，并将额外的精力集中在理解由于具有足够高的能量的UCN缓慢逃离陷阱并改变寿命测量值而引起的系统误差上。这些任务是所计划的实验的基本和正在进行的部分，在提议期间已完成的测量和分析已纳入出版物。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

This award supports high precision measurements of 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, in that it powers our sun and helps determine the abundance of elements in the universe. Neutron decay is also important because it involves only a single nucleon, making possible very precise predictions from the "standard model" of particle physics without the complications due to the presence of other nucleons. With very accurate measurements of neutron decay, we can perform a kind of particle physics "forensics", looking for discrepancies in our standard model which can indicate the presence of new forces, but without using high energy particle beams such as those at the Large Hadron Collider at CERN. 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. The PI and his group contribute to a new measurement of the neutron's lifetime (UCNtau) and an angular correlation measurement (Nab). The lifetime and angular correlations measurements probe different aspects of the weak interaction, and if successful, will sharpen our knowledge of critical components of the weak interaction and contribute to strong limits on extensions the standard model, at or above those which are being established at CERN. An important aspect of the project is the training of students and post-docs. This training comes in part because of the specific nature of the research projects, which utilize nuclear physics analysis and technology (which also has real-world applications in energy generation and medical imaging, for example) and in part from the many opportunities students working in the program have to perform research at national laboratories and nuclear facilities. One of the experiments supported by the 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 anti-neutrino (called the electron-anti-neutrino correlation), with the goal of over an order of magnitude improvement in the precision of this parameter. The other, UCNtau, is situated at the ultracold neutron (UCN) source at Los Alamos National Laboratory (LANL), and targets precision levels below 0.3 seconds during the current funding period. The impact of these programs being successful are: (1) knowledge of the axial form factor at the level required to determine the CKM matrix element Vud at the same level as the current theoretical uncertainty for the electro-weak radiative corrections, (2) knowledge of the lifetime at a level such that it will limit tests of our models for the cosmological He-4 abundance for some time, and (3) limits on tensor couplings at or above the level expected from CERN. Motivation for these measurements has been amplified over the last year by theoretical progress on the electroweak radiative corrections and lattice calculations of the axial coupling constant, with both opening new, higher precision probes for new physics. For each of these experiments, this 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. For Nab, for example, the group is analyzing the effects of incomplete collection of the energy of decay electrons and the calibration procedures, a critical component of the systematic error budget for this experiment. UCNtau is designed to store UCNs in a magneto-gravitational trap, where the UCNs do not contact with the trap walls. The top of the trap is open, permitting detectors to be lowered into the trap to measure UCN populations and determine the lifetime. The group will continue to develop UCN detectors for this purpose and focus additional effort on understanding systematic errors due to UCNs with high enough energies to slowly escape the trap and shift the measured value of the lifetime. These tasks are essential and ongoing parts of the planned experiments, with completed measurements and analysis incorporated into publications during the proposal period.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.