Precision Neutron Decay Measurements

中子是普通物质的关键组成部分。地球一半以上的质量来自中子，但当中子从原子的束缚中释放出来时，它就不稳定了，会衰变成其他基本粒子：质子、电子和反中微子（一种轻的中性粒子），平均寿命约为15分钟。中子衰变是研究弱核力的有用实验室，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门的探测器进行实验，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。这些结果验证并导致了弱相互作用理论的改进，以及对太阳、恒星、大爆炸和重要核反应的物理学的更好理解。这项工作，在粒子和核物理的“精确前沿”，补充了在“高能前沿”的研究，例如在欧洲的大型强子对撞机。该项目为本科生和研究生提供了在中子科学的一般方法和理论方面的培训机会，这些方法和理论适用于全球主要中子源的物理、化学、材料科学和生物学等不同领域的研究。自由中子的衰变是原型半光子弱相互作用和最简单的核衰变。由于没有核结构的复杂性，并且衰变能量相对于核子质量较小，因此反冲阶弱形式因子进入0.1%以下的水平。因此，中子衰变是一种具有吸引力的低能弱相互作用精确测量系统。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由此产生的氦丰度，并间接限制了轻中微子的有效数量。电子-反中微子相关系数给出了轴向矢量（GA）和矢量（GV）弱核子耦合的比值lambda。中子寿命可以用来确定Vud， Cabbibo-Kobayashi-Maskawa矩阵的第一个元素，并约束标准模型之外的新物理，如弱标量力和张量力。最近的中子寿命实验存在一个严重的问题：使用传统的束流方法和使用较新的超冷中子储存方法得到的结果相差约1%（4个标准差）。这个研究项目的一个重要目标就是解决这个令人不安的矛盾。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BL2中子寿命实验；2)开发和运行新的BL3下一代束流中子寿命实验，研究和测试有助于解决中子寿命差异的系统效应，并将束流方法的精度提高到0.04%以下。该奖项反映了美国国家科学基金会的法定使命，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.