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

该奖项支持对中子贝塔衰变进行高精度测量的研究，这是一种由弱核力(或弱相互作用)引起的过程。虽然许多原子核都经历了β衰变，但中子衰变是重要的，因为它只涉及一个核子，这简化了计算，并使粒子物理“标准模型”的非常精确的预测成为可能，而不会因为其他核子的存在而复杂化。有了非常精确的中子衰变测量，我们可以进行一种粒子物理学的“取证”，在我们的标准模型中寻找可以表明新力存在的差异，但不需要使用像欧洲核子研究中心大型强子对撞机上的那些高能粒子束。这项研究是物理学“精确前沿”的一部分，并定义了我们关于弱核力和可能的新相互作用的知识的前沿。当中子贝塔衰变发生时，衰变过程导致质子、电子和反中微子的发射。该计划包括测量中子β衰变发生后发射粒子的方向分布(NAB)和测量中子的平均寿命(UCNτ)。综合起来看，这两个测量结果完全定义了β衰变过程。在获奖期间，这两个实验将提供迄今为止最精确的弱相互作用特征，可能会增加对标准模型不正确的重要确认。该计划的一个重要方面是对学生和博士后的培训，在这方面，该小组有助于传播一般核科学，特别是中子科学的技术和专业知识。这种培训在一定程度上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。这在一定程度上也是因为学生参与了国家实验室和核设施的研究。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上的一束未极化的冷中子束来测量衰变电子和发射的反中微子之间的角关联(β-中微子关联，a_βν)，目标是将该参数的精度提高近一个数量级。另一个是Ucnτ，它位于洛斯阿拉莫斯国家科学中心的超冷中子源，目标是在当前供资期间将寿命的精确度提高两倍。这些精度水平允许对根据超允许衰变确定的CKM矩阵元素V_UD的值进行有意义的交叉检查，确认标准模型的统一性测试中的差异(称为“Cabibbo异常”)。在新的粒子物理数据和对介子衰变的理论分析的改进的推动下，这种异常的重要性在过去几年里稳步增加。2022年的分析表明，在大于3σ的水平上，需要对上夸克进行新的物理耦合。这项裁决的结果可以证实这一结果，并最终消除作为不确定源的核结构。对于这些实验中的每一个，该小组都通过开发实验所需的仪器和分析，将其在贝塔衰变测量方面的经验应用于系统误差的关键来源。例如，对于NAB，该小组正在分析不完全收集衰变电子能量的影响，这是该实验系统误差预算的关键组成部分。这项工作一直在进行，为当前的能量校准和解释来自硅探测器的信号提供了基础。在今后一段时期内，该小组将在NAB实验运行期间应用在当前时期开发的分析工具和校准基础设施。UCNτ被设计成将超冷中子UCN存储在磁引力陷阱中，在那里UCN不与材料陷阱壁接触，从而极大地减少了系统的不确定性。2021年，UCNτ对中子寿命进行了迄今为止最精确的测量，测量不确定度为0.36秒。现在的焦点是名为Ucnτ+的升级，它将把不确定因素减少两倍，这是对高精度检查卡比博异常的关键贡献。国家空间研究中心小组目前正在领导一项绝热加载程序的模拟工作，该程序对ucnτ+的成功至关重要。这项工作将与他们正在进行的表征系统不确定性来源的工作相结合，例如UCN频谱的变化和探测器响应的细节。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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 research on high precision measurements of neutron beta decay, a process caused by the weak nuclear force (or weak interaction). Although many nuclei undergo beta decay, neutron decay is important because it involves only a single nucleon, which simplifies the calculations and makes very precise predictions from the "standard model" of particle physics possible 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 that 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. When neutron beta decay occurs, the decay process results in the emission of a proton, an electron and an antineutrino. This program involves a measurement of the distribution of directions for the emitted particles after neutron beta decay occurs (Nab) and a measurement of the average lifetime of the neutron (UCNτ). When taken together, these two measurements completely define the beta decay process. During the award period, these two experiments are set to provide the most precise characterization of the weak interaction to date, potentially adding important confirmation that the standard model is not correct. An important aspect of the program is the training of students and post-docs, where the group contributes to a dissemination of the techniques and expertise of nuclear science in general, and neutron science in particular. This training comes in part because of the specific nature of the research projects, which utilize nuclear physics analysis and technology. It also comes in part through the students’ involvement in research at national laboratories and nuclear facilities. 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 emitted anti-neutrino (the beta-neutrino correlation, a_βν), with the goal of almost an order of magnitude improvement in the precision of this parameter. The other, UCNτ, is situated at the ultracold neutron (UCN) source at Los Alamos National Science Center (LANSCE), and targets a factor of two improvement in the precision of the lifetime during the current funding period. These precision levels enable a meaningful cross-check of the value of CKM matrix element V_ud determined from super-allowed decays, confirming discrepancies in the unitarity test of the standard model (referred to as the “Cabibbo Anomaly”). The significance of this anomaly has steadily increased over the past few years, driven by new particle physics data and refinement of theoretical analysis of kaon decays. Analysis in 2022 suggests that new physics couplings to up quarks are required at greater than the 3σ level. The results of this award can confirm this result and finally eliminate nuclear structure as source of uncertainty. 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. For Nab, the group is analyzing, for example, the effects of incomplete collection of the energy of decay electrons, a critical component of the systematic error budget for this experiment. This work has been on-going, with contributions from efforts providing the basis for the current energy calibration and interpretation of the signals from the Si detectors. In the coming period the group will apply analysis tools and calibration infrastructure developed during the current period to during operation of the Nab experiment. UCNτ is designed to store ultracold neutrons, UCN, in a magneto-gravitational trap, where UCN do not make contact with material trap walls, dramatically reducing systematic uncertainties. In 2021, UCNτ produced the most precise measurement of the neutron lifetime to date, with an uncertainty of 0.36 seconds. The focus now is an upgrade called UCNτ+, which will reduce uncertainties by another factor of two, a critical contribution to a high precision check of the Cabibbo Anomaly. The NCSU group is currently leading a simulation effort for the performance of an adiabatic loading procedure which is essential for the success of UCNτ+. This work will be integrated with their ongoing effort to characterize sources of systematic uncertainty such as variations in the UCN spectrum and details of detector response.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.