The Nab Experiment: An ultraprecise measurement of the electron-antineutrino angular correlation coefficient in neutron beta decay

NaB合作旨在对冷中子β衰变中的电子-中微子角关联参数“a”和Fierz干涉项“B”进行世界上最精确的测量。该实验采用了一种新型的4 π场膨胀谱仪，位于美国橡树岭国家实验室的高能中子源（SNS）的基本中子物理束线（FNPB）上。Nab合作最近通过了几个重要的里程碑，在FNPB上交付和安装了4米长的超导光谱仪磁体，并交付了几个高度分段的Si探测器。 该合作预计将于2019年春季/夏季在SNS进行全面实验，物理数据将在未来2年内开始。中子β衰变中角相关的测量提供了最灵敏的手段来评估轴向与矢量耦合常数的比率（λ（L）= gA/gV）。在弱相互作用理论的许多应用中，特别是在天体物理学中，“L”的精确值是非常重要的，例如星星的中微子产生与L2密切相关。参数“a”提供了一个独立的“L”的测量方法，与使用相关参数“A”的测量方法相比，具有相当的灵敏度和根本不同的系统学，目前使用相关参数“A”提供了“L”的最佳精度。Nab实验的主要组成部分包括超导光谱仪，一套高度分段的硅（Si）探测器和冷中子束监测器。光谱仪的功能是通过飞行时间（TOF）测量来测量质子动量的大小。至关重要的是，质子硅探测器在安装到Nab光谱仪之前必须使用已知的质子源进行充分表征。为此，马尼托巴大学开发了一个测试设施，以确定30千电子伏质子探测器的特性。 过去三年的大部分工作都集中在准备好这个设施上，其中包括使用大面积荧光屏探测器来确定适当的光束转向设置，以成像光束，以及建造大面积微通道板（MCP）探测器及其相关电子器件，作为硅探测器的参考探测器。 该设施将于2018-19年冬季/春季开始表征硅探测器。 自2016年以来，一个博士学位。学生和三名本科生在建造这一设施时接受了探测器制造/测试方面世界级培训。 鉴于该项目在未来三年的范围扩大，我们预计将为一名博士和一名硕士提供额外的培训。这是一个很好的例子，因为它是一个学生，预计比往年增加旅行预算，以协助调试，物理数据采集和Nab实验的分析。因此，我们预计未来三年的预算要求是14万美元，14.1万美元，12.1万美元 **。

The Nab collaboration aims to make the world's most precise measurement of the electron-neutrino angular correlation parameter "a" and the Fierz interference term "b" in cold neutron beta decay. The experiment employs a novel 4 Pi field expansion spectrometer and is located at the Fundamental Neutron Physics Beam line (FNPB) of the Spallation Neutron Source (SNS) at Oak Ridge National Lab in the USA. The Nab collaboration has recently passed several significant milestones with the delivery and installation of the 4 m long superconducting spectrometer magnet on the FNPB and the delivery of several highly segmented Si detectors. The collaboration is anticipating commissioning of the full experiment at the SNS in the spring/summer of 2019, with physics data to begin shortly after for the next 2 years.******The measurement of angular correlations in neutron beta-decay provides the most sensitive means to evaluate the ratio of axial to vector coupling constants (Lambda (L) = gA/gV). The precise value of "L" is very important in many applications of the theory of weak interactions, especially in astrophysics, e.g. a star's neutrino production is closely related to L2. Parameter "a'' offers an independent measure of "L'' with comparable sensitivity and radically different systematics than measurements using the correlation parameter “A”, which currently provides the best precision on “L”.******The main components of the Nab experiment include a superconducting spectrometer, a set of highly segmented silicon (Si) detectors, and cold neutron beam monitors. The function of the spectrometer is to measure the magnitude of the proton momentum via time of flight (TOF) measurements. It is crucial that the proton silicon detector(s) be fully characterized with a known source of protons before installation into the Nab spectrometer. To that end, a test facility has been developed at the University of Manitoba to characterize the detectors for 30 keV protons. Much of the work over the past three years has focused on getting this facility ready, which includes determining the appropriate beam steering settings using an large area phosphor screen detector to image the beam as well as the building of a large area micro-channel plate (MCP) detector and its associated electronics to act as a reference detector for the silicon detector. The facility will begin characterizing the silicon detector(s) in the winter/spring of 2018-19. Since 2016, one PhD. student and three undergraduate students have received world class training in detector manufacturing/testing while working to build this facility. Given the expanded scope of the project in the coming three years, we anticipate providing additional training to one PhD and one MSc. student and anticipate an increase over previous years travel budget to assist in the commissioning, physics data taking, and analysis of the Nab experiment. Thus our projected budget request is $140k, $141k, $121k over the next three years.**