Precision Neutron Decay Measurements

中子是普通物质的关键组成部分，但当脱离原子的限制时，中子是不稳定的，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。中子衰变是一个有用的实验室，可以用来研究负责的力的细节--弱核力--自然的四种基本力量之一。该奖项将支持一项实验，在该实验中，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。根据这些结果，科学家们可以测试和完善弱核力理论，并更好地理解太阳、恒星、大爆炸和重要核反应的物理学。这项处于粒子和核物理“精确前沿”的工作是对“高能前沿”研究的补充，例如在欧洲的大型强子对撞机上。该项目也是培训本科生和研究生学习适用于世界主要中子源物理、化学、材料科学和生物研究各个领域的中子科学一般方法和理论的绝佳机会。自由中子的贝塔衰变是半轻子弱相互作用的原型，也是最简单的核贝塔衰变。由于原子核结构不存在复杂情况，且衰变能与核子质量相比较小，因此反冲阶弱形式因子进入0.1%以下。因此，中子衰变是一种很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，这设定了原始核合成时代的中子与质子的比率，从而确定了氦的丰度，并间接限制了轻中微子的有效数量。电子-反中微子关联“a”给出了λ，即轴向矢量(GA)和矢量(GV)弱核子耦合的比率。根据中子寿命，它可以用来确定Cabbibo-Kobayashi-Maskawa矩阵的第一个元素VUD，并约束标准模型以外的新物理，如弱标量力和张量力。目前关于中子寿命和波长的实验情况并不令人满意，以前的测量结果之间的一致性相对较差。拟议的计划包括三个项目：1)继续在NIST中子研究中心进行橡子实验，以实现测量不确定度达到0.5%(提高10倍)的最终目标；2)分阶段改进使用束流方法测量中子寿命；以及3)开发3He气体闪烁绝对中子通量计数器，以支持中子寿命计划。

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 force responsible - the weak nuclear force - one of the four fundamental forces of nature. This award will support an experiment in which beams of free neutrons pass through specialized detectors that precisely measure the neutron decay lifetime and the angles between the emitted particles. From these results the scientists can test and refine the theory of the weak nuclear force and better understand 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 project is also 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 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 hence the helium abundance, and indirectly constrains the effective number of light neutrinos. The electron-antineutrino correlation "a" 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. The present experimental situations with both the neutron lifetime and lambda are unsatisfactory, there is relatively poor agreement among previous measurements. The proposed program consists of three projects: 1) A continuation of the aCORN experiment at the NIST Center for Neutron Research toward an ultimate goal of measuring "a" to 0.5% uncertainty (a factor of 10 improvement); 2) a phased program to improve the neutron lifetime measurement using the beam method; and 3) development of a 3He gas scintillation absolute neutron flux counter to support the neutron lifetime program.