Precision Experiments with Cold and Ultracold Neutrons

基础核物理和粒子物理的研究分为三个领域：在“能量前沿”，物理学家希望通过碰撞具有越来越高能量的粒子来发现新的物理，例如新的对称性或粒子。在“宇宙前沿”，物理学家将宇宙作为一个实验室，其中包含有关新物理学的可检测信息。最后，在“精确前沿”，物理学家试图测量被认为是非常精确的已知量，并将已知物理预测的偏差解释为新物理学的信号。该奖项支持第三种类型的测量，即对自由中子β衰变中的几个可观测量进行精确测量，以更好地理解基本粒子之间的基本相互作用。β衰变过程已被纳入基本粒子物理学的标准模型中，它为我们成功地描述了基本粒子过程。新物理学的发现将对核物理和高能物理以及宇宙学产生重大影响。 该项目处于科学研究的前沿，处于技术可行性的边缘，将成为未来几代科学家的培训基地。大部分研究将由本科生和研究生完成。这项拨款支持设计，建造，调试和操作Nab光谱仪的努力。该谱仪将在位于田纳西州橡树岭的新的Sputterus中子源（SNS）中使用最先进的中子束线（FNPB），以确定中子β衰变中衰变电子和质子的二维能量分布。中子β衰变过程的标准模型描述中固有的冗余允许对模型的有效性和限制进行独特敏感的检查，这在天体物理学中具有强烈的影响。 其中一个预期的观测量，确定中微子-电子相关系数的相对不确定度为0.1%，将允许提取Vud，Cabbibo-Kobayashi-Maskawa（CKM）矩阵的左上角元素，以测试其酉性。 目前NSAC的长期计划发现，“改善Vud的唯一现实方法是通过中子衰变。“来自高能物理的新结果给CKM矩阵的幺正性蒙上了阴影，并重新开启了寻找新交换玻色子或第四代夸克的可能性。另一个可观察到的是Fierz项的确定，它将衰变电子光谱的已知形状的偏差参数化到大约0.001，允许以互补的方式找到新的粒子，这些粒子在低能量下介导标量或张量相互作用，并且灵敏度与LHC的预期竞争。

Research in Fundamental Nuclear and Particle Physics is being categorized in three areas: At the "Energy Frontier", physicists hope to discover new physics, e.g. new symmetries or particles, by colliding particles with increasingly high energy. At the "Cosmic Frontier", physicists use the cosmos as a laboratory that contains detectable information about new physics. And finally, at the "Precision Frontier", physicists attempt to measure quantities that are believed to be known very precisely, and to interpret deviations from the known physics predictions as a signal for new physics. This award supports a measurement of the third type, a precision measurement of several observables in free neutron beta decay which serve to better understand the fundamental interactions between elementary particles. The process of beta decay has been incorporated into the standard model of elementary particle physics, which gives us a successful description of elementary particle processes. A discovery of new physics would have a critical impact on Nuclear and High Energy Physics, and on Cosmology. Being on the forefront of scientific research and on the edge of what is technologically feasible, the project will serve as a training ground for future generations of scientists. Most of the research will be done by students at the undergraduate and graduate level. This grant supports the effort to design, construct, commission, and operate the Nab spectrometer. The spectrometer will use the state of the art neutron beamline (FNPB) at the new Spallation Neutron Source (SNS) in Oak Ridge, TN to determine the two-dimensional energy distribution of decay electron and protons in neutron beta decay. The redundancy inherent in the standard model description of the neutron beta decay process allows uniquely sensitive checks of the model's validity and limits, with strong implications in astrophysics. One of the expected observables, the determination of the neutrino-electron-correlation coefficient with a relative uncertainty of 0.1%, will allow an extraction of Vud, the upper left element of the Cabbibo-Kobayashi-Maskawa (CKM) matrix, in order to test its unitarity. The current NSAC long range plan finds that "The only realistic way to improve Vud is via neutron decay." New results from high energy physics cast a shadow on the unitarity of the CKM matrix, and re-open the possibility of finding hints for new exchange bosons, or a fourth quark generation. The other observable, the determination of the Fierz term, which parametrizes a deviation from the known shape of the spectrum of the decay electrons, to about 0.001, allows finding new particles that mediate scalar or tensor interactions at low energy, in a complementary way and with sensitivity competitive to what is expected from LHC.