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

NAB的合作旨在对冷中子β衰变中的电子-中微子角关联参数“a”和菲尔兹干涉项“b”进行世界上最精确的测量。该实验使用了一台新型的4pI场扩展能谱仪，位于美国橡树岭国家实验室散裂中子源的基础中子物理束线(FNPB)。NAB的合作最近跨越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硅探测器。这项合作预计将于2019年春夏在SNS投入使用，物理数据将在接下来的两年里很快开始。*中子β衰变中角关联的测量提供了评估轴向与矢量耦合常数之比的最灵敏手段(波长(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，例如恒星的中微子的产生与L2密切相关，“L”的精确值是非常重要的。参数“a‘”提供了对“L”的独立测量，具有与使用关联参数“A”的测量结果相当的灵敏度和完全不同的系统学，而关联参数“A”目前在“L”上提供了最好的精度。*NAB实验的主要部件包括超导光谱仪、一套高分段硅(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(S)在安装到NAB光谱仪之前，必须充分利用已知的质子源进行表征。为此，在马尼托巴大学开发了一个测试设备，以表征30keV质子的探测器。过去三年的大部分工作都集中在准备这一设施上，其中包括使用大面积荧光屏探测器对光束成像来确定适当的波控设置，以及建立大面积微通道板(MCP)探测器及其相关电子设备作为硅探测器的参考探测器。该设施将于2018-19年冬/春开始对硅探测器(S)进行表征。自2016年以来，一名博士。学生和三名本科生在建造该设施的同时接受了世界级的探测器制造/测试培训。鉴于未来三年该项目的范围扩大，我们预计将为一名博士和一名硕士提供额外的培训。学生并预计将比前几年增加旅行预算，以协助NAB实验的调试、物理数据获取和分析。因此，我们预计未来三年的预算需求分别为14万美元、14.1万美元和1.21亿美元。

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.**