Precision Neutron Decay Measurements

中子是普通物质的关键组成部分。地球质量的一半以上来自中子，但当脱离原子的限制时，中子是不稳定的，并衰变为其他基本粒子：质子，电子和反中微子（一种轻质中性粒子），平均寿命约为15分钟。中子衰变是一个有用的实验室，可以用来研究弱核力的细节，弱核力是自然界四种基本力之一。该奖项将支持使用自由中子束通过专用探测器的实验，这些探测器将精确测量中子衰变寿命和发射粒子之间的角度。这些结果测试并导致弱相互作用理论的改进，并更好地理解太阳，恒星，大爆炸和重要核反应的物理学。这项工作在粒子和核物理学的“精确前沿”，补充了“高能前沿”的研究，例如在欧洲的大型强子对撞机。该计划包括培养中子科学的一般方法和理论的本科生和研究生的绝佳机会，适用于物理，化学，材料科学和生物学研究的不同领域，在世界各地的主要中子源。自由中子的β衰变是原型半轻子弱相互作用和最简单的核β衰变。核结构没有复杂性，衰变能量与核子质量相比很小，因此反冲阶弱形状因子进入0.1%以下的水平。因此，中子衰变是一个有吸引力的系统，用于精确的低能弱相互作用测量。中子寿命确定了大爆炸后不久核子“冻结”的时间尺度和温度，从而确定了原始核合成时代的中子与质子比以及氦的丰度，并间接限制了轻中微子的有效数量。电子-反中微子相关系数a给出了λ，轴向矢量（GA）和矢量（GV）弱核子耦合的比率。有了中子寿命，它可以用来确定Vud，Cabbibo-Kobayashi-Maskawa矩阵的第一个元素，并限制标准模型之外的新物理，如弱标量和张量力。最近的中子寿命实验存在一个严重的问题：使用传统的束方法和使用较新的超冷中子储存方法获得的结果不一致约1%（4个标准偏差）。这项研究计划的一个重要目标是解决这一令人不安的差异。该计划包括两个主要项目：1）完成目前在NIST中子研究中心运行的BL 2中子寿命实验; 2）开发和运行新的BL 3下一代束流中子寿命实验，该实验将调查和测试系统效应，以帮助解决中子寿命差异，并将束流方法的精度提高到0.04%以下该奖项反映了NSF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。

The neutron is a key building block of ordinary matter. More than half of the Earth's mass is from neutrons, but when freed from the confines of an atom, the neutron is unstable and decays into other elementary particles: a proton, an electron, and an antineutrino (a lightweight neutral particle), with an average lifetime of about 15 minutes. Neutron decay is a useful laboratory for studying details of the force responsible - the weak nuclear force - one of the four fundamental forces of nature. This award will support experiments using beams of free neutrons passing through specialized detectors that will precisely measure the neutron decay lifetime and the angles between the emitted particles. These results test and lead to refinements of the weak interaction theory and a better understanding of the physics of the sun, stars, the Big Bang, and important nuclear reactions. This work, at the "precision frontier" of particle and nuclear physics, complements research at the "high energy frontier", for example at the Large Hadron Collider in Europe. This program includes excellent opportunities to train undergraduate and graduate students in the general methods and theory of neutron science that are applicable to diverse fields in physics, chemistry, materials science, and biology research at major neutron sources around the world.The beta decay of the free neutron is the prototype semileptonic weak interaction and simplest nuclear beta decay. There are no complications from nuclear structure, and the decay energy is small compared to the nucleon mass, so recoil-order weak form factors enter below the 0.1% level. Therefore, neutron decay is an attractive system for precise low energy weak interaction measurements. The neutron lifetime establishes the time scale and temperature of nucleon "freeze out" shortly after the Big Bang, which sets the neutron to proton ratio during the era of primordial nucleosynthesis and thence the helium abundance, and indirectly constrains the effective number of light neutrinos. The electron-antineutrino correlation a-coefficient gives lambda, the ratio of axial vector (GA) and vector (GV) weak nucleon couplings. With the neutron lifetime it can be used to determine Vud, the first element of the Cabbibo-Kobayashi-Maskawa matrix, and constrain new physics beyond the Standard Model such as weak scalar and tensor forces. There is a serious problem with recent neutron lifetime experiments: results obtained using the traditional beam method, and those using the newer ultracold neutron storage method, disagree by about 1% (4 standard deviations). An important goal of this research program is to resolve this troubling discrepancy. The program consists of two main projects: 1) completing the BL2 neutron lifetime experiment now running at the NIST Center for Neutron Research; 2) development and operation of the new BL3 next-generation beam neutron lifetime experiment that will investigate and test systematic effects that can help resolve the neutron lifetime discrepancy and improve the precision of the beam method to less than 0.04%.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.