Precision Neutron Decay Measurements

中子是普通物质的关键组成部分，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（自然界四种基本力之一）细节的有用实验室。在这个项目中，研究小组将把自由中子束通过专门的探测器，精确测量中子衰变寿命和发射粒子之间的角度。从这些结果可以检验和完善弱核力理论，更好地理解太阳、恒星、大爆炸和其他重要核反应的物理性质。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。本项目是培养本科生和研究生在中子科学的一般方法和理论，适用于不同领域的物理，化学，材料科学和生物研究在世界各地的主要中子源的绝佳机会。自由中子的衰变是半轻子弱相互作用的原型，是最简单的核衰变。不受核结构的影响，衰变能量相对于原子核质量较小；因此，后坐力顺序弱形状因子进入低于0.1%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。然而，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获得的结果与使用较新的超冷中子储存方法获得的结果相差约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矛盾。该计划包括三个项目：1)完成目前在NIST中子研究中心运行的BL2中子寿命实验；2)设计和建造新的BL3束中子寿命实验，研究和测试有助于解决中子寿命差异的系统效应，并将束法的精度提高到0.04%以下；3)分析和发表最近完成的aCORN实验，该实验测量电子-反中微子相关a系数，并进行改进，以便在未来运行时将不确定性降低到1%。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, 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 ‘weak’ nuclear force - one of the four fundamental forces of nature. In this project the group will pass beams of free neutrons through specialized detectors that precisely measure the neutron decay lifetime and the angles between the emitted particles. From these results it is possible to test and refine the theory of the weak nuclear force and better understand the physics of the sun, stars, the Big Bang, and other 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 is an excellent opportunity 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 semi-leptonic weak interaction and the 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. However, 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 discrepancy. The program consists of three projects: 1) completing the BL2 neutron lifetime experiment now running at the NIST Center for Neutron Research; 2) design and construction of the new BL3 beam neutron lifetime experiment that will investigate and test systematic effects that could help resolve the neutron lifetime discrepancy and improve the precision of the beam method to less than 0.04%; and 3) analysis and publication of the recently completed run of the aCORN experiment, which measures the electron-antineutrino correlation a-coefficient, and develop improvements to reduce the uncertainty to 1% in a future run.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.