RUI: Precision Measurements of Free Neutron Decay Observables

这一奖项将支持PI和与他一起工作的本科生了解中子衰变的方式，以及自由中子在衰变之前存在多长时间。没有束缚在原子核中的中子将在大约880秒内衰变成质子、电子和反中微子。以越来越精确的方式研究中子的这一特性，可以更深入地了解宇宙建立在其上的框架。一项名为橡子核的实验在美国国家标准与技术中心中子研究中心收集了数据，以比以往任何时候都更精确地测量电子和反中微子发射方向之间的相关性。另一项名为UCNtau的实验正在使用磁铁和重力来捕获非常低能量的中子，以测量中子的寿命不到一秒。捕获极低能中子并在一段时间后计算未衰变的幸存者的实验往往会给出与涉及更高能量的中子束和探测衰变过程中产生的质子的实验显著不同的结果。找到这种差异的原因是非常有兴趣的，因为它有可能揭示中子衰变的一个新特征。这一奖项将允许首席调查员(PI)和德堡大学的本科生研究人员为这两个实验做出重要贡献。橡子实验一直是PI研究计划的旗舰项目，在许多方面，该计划一直是他的学生为研究和教学做准备的训练场。参与这项研究的学生获得了广泛的硬件和软件技能，为他们在STEM领域的职业生涯或攻读STEM领域的研究生学位做好准备。更精确地测量电子-反中微子关联的动机包括改进的电弱标准模型的模型独立测试，以及测量不依赖于中子偏振的弱轴向矢量与矢量耦合常数的比率。此外，这一测量加上更精确的中子寿命，将允许仅根据自由中子衰变确定VUD--Cabbibo-Kobayashi-Maskawa(CKM)矩阵的一个元素，因此不会因核结构相关的修正而变得复杂。由于CKM矩阵被要求是么正的，这将提供对标准模型的测试。更好地了解中子寿命也可以对大爆炸核合成进行严格的测试。橡子实验依靠同时探测电子和质子来推断反中微子的相对发射方向。PI开发了一个质子跟踪模拟，该模拟确定了橡子质子聚焦系统对第一个数据集的效率，这对于确定由于质子探测不完善而产生的系统误差的大小是重要的。PI将把这一分析应用于第二个数据集，并全面分析NG-C数据。橡子质子模拟中使用的技术将被用于设计UCNtau实验中的质子探测器系统，以便通过观察质子来确定中子寿命，这更接近中子束寿命实验的实际测量结果。这样的测量可能有助于理解这些实验之间的差异。

This award will support the PI and undergraduate students working with him to understand the ways in which neutrons decay and how long a free neutron will exist before decaying. Neutrons which are not bound in an atomic nucleus will decay in about 880 seconds into a proton, electron and an antineutrino. Studying characteristics of this property of neutrons with increasing precision allows a deeper knowledge about the framework upon which the cosmos was built. One experiment, named aCORN, has collected data at the National Institute of Standards and Technology Center for Neutron Research to measure the correlation between the emission directions of the electron and antineutrino more precisely than ever before. Another experiment, UCNtau, is using magnets and gravity to trap very low energy neutrons to measure the neutron's lifetime to less than a second. Experiments trapping very low energy neutrons and subsequently counting the undecayed survivors after a period of time are tending to give results significantly different than experiments which involve a higher energy beam of neutrons and the detection of protons produced in the decay. Finding the cause of this discrepancy is of great interest as it has the potential of uncovering a new feature of neutron decay. This award will allow the Principle Investigator (PI) and DePauw University Undergraduate Researchers to make important contributions to these two experiments. The aCORN experiment has been the flagship project of the PI's research program, which in many ways has acted as a training ground for his students to prepare them for research and teaching. The students who participate in this research gain a broad range of hardware and software skills that prepare them for careers in STEM fields or to pursue graduate degrees also in STEM areas.The motivations for a more precise measurement of the electron-antineutrino correlation include an improved model independent test of the Electroweak Standard Model and a measurement of the ratio of weak axial vector to vector coupling constants that does not depend on neutron polarimetry. In addition, this measurement, along with a more precise neutron lifetime, would allow a determination of Vud - an element of the Cabbibo-Kobayashi-Maskawa (CKM) matrix - solely from free neutron decay and thus not complicated by nuclear structure dependent corrections. As the CKM matrix is required to be unitary, this would provide a test of the Standard Model. Knowing the neutron lifetime better also allows for a stringent test of Big Bang Nucleosynthesis. The aCORN experiment depends on the simultaneous detection of electrons and protons to infer the relative emission direction of the antineutrino. The PI has developed a proton tracking simulation that determines the efficiency of the aCORN proton focusing system for the first data set which is important in establishing the size of the systematic error that would result from imperfect proton detection. The PI will apply this analysis to the second data set and fully analyze the NG-C data. The techniques used in the aCORN proton simulation will be employed to design a proton detector system in the UCNtau experiment to allow determination of the neutron lifetime by observation of the protons which is closer to what the neutron beam lifetime experiments actually measure. Such a measurement could be instrumental in understanding the discrepancy between these experiments.