Precision Experiments with Cold and Ultracold Neutrons

基础核与粒子物理的研究分为三个领域：在“能量前沿”，物理学家希望发现新的物理现象，例如，通过高能粒子的碰撞，发现新的对称性或新的粒子。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探测信息。最后，在“精确前沿”，物理学家试图测量那些被认为是非常精确的量，并将偏离已知物理预测的偏差解释为新物理的信号。该奖项支持第三种类型的测量，即对自由中子衰变中几个可观测值的精确测量，这有助于更好地理解基本粒子之间的基本相互作用。衰变过程已被纳入基本粒子物理的标准模型，这给了我们一个基本粒子过程的成功描述。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。这笔拨款支持设计、建造、调试和操作Nab光谱仪的工作。该光谱仪将在田纳西州橡树岭的新散裂中子源（SNS）上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局限性进行独特的敏感检查，这在天体物理学中具有重要意义。其中一个预期的观测值，即相对不确定度为0.1%的中微子电子相关系数的测定，将允许提取Vud，即Cabbibo-Kobayashi-Maskawa （CKM）矩阵的左上角元素，以测试其统一性。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。”高能物理学的新结果给CKM矩阵的统一性投下了阴影，并重新开启了寻找新交换玻色子或第四代夸克的线索的可能性。另一个可观察到的结果是Fierz项的确定，该项参数化了衰变电子光谱已知形状的偏差，偏差约为0.001，这使得人们能够以互补的方式发现在低能量下调解标量或张量相互作用的新粒子，其灵敏度与LHC的预期相媲美。

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.