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能量越来越高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”，物理学家将宇宙用作实验室，其中包含有关新物理的可检测信息。最后，在“精密前沿”，物理学家试图测量被认为是非常精确的已知数量，并将与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数据进行的精确测量，这些测量有助于更好地了解基本粒子之间的基本相互作用。在基本粒子物理的标准模型中引入了β衰变过程，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，处于技术上可行的边缘，将成为未来几代科学家的训练场。大部分研究将由本科生和研究生完成。这笔赠款支持NAB光谱仪的设计、建造、调试和操作。该光谱仪将在田纳西州橡树岭的新散裂中子源(SNS)使用最先进的中子束线(FNPB)来确定中子β衰变中衰变电子和质子的二维能量分布。中子贝塔衰变过程的标准模型描述中固有的冗余性允许对模型的有效性和限度进行独特而敏感的检查，这在天体物理学中具有重要意义。中微子-电子相关系数的测定是预期的观测之一，其相对不确定度为0.1%，这将允许提取Cabbibo-Kobayashi-Maskawa(CKM)矩阵的左上角元素VUD，以测试其统一性。目前NSAC的远程计划发现，“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矩阵的正则性蒙上了一层阴影，并重新开启了找到新交换玻色子或第四代夸克的线索的可能性。另一个可观察到的是确定费尔兹项，它将衰变电子光谱的已知形状的偏差参数化为约0.001，允许找到新的粒子，这些粒子以一种互补的方式在低能下调解标量或张量相互作用，并具有与大型强子对撞机预期的竞争的灵敏度。

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.