Fundamental Studies in Nuclear Physics

项目技术说明：我们的项目-核物理基础研究-在加州理工学院主要集中在实验研究解决自由中子的基本性质。特别是，我们将继续与UCNA实验的数据，同时也开发硬件建设一个新的，高灵敏度的实验，以寻找中子的电偶极矩。UCNA实验已经首次使用超冷中子对中子衰变中的β不对称性进行了精确测量。这个数量允许CKM夸克混合矩阵的幺正性的敏感测试，并有可能为超越粒子相互作用标准模型的新物理提供证据。随着最近对设备的改进，我们准备在此期间通过额外的数据采集和分析将不对称性的精度提高到~ 0.3%。新的电偶极矩实验-在橡树岭国家实验室的中子源进行的nEDM-的目标是将灵敏度比以前的测量提高100倍;电偶极矩的观测信号违反了时间反演和电荷宇称（CP）对称性，并可能提供有关宇宙物质-反物质不对称的起源和负责的新物理现象的重要信息。我们在加州理工学院的项目主要集中在研究自由中子的基本性质。中子是一种不稳定的粒子，半衰期为10分钟，衰变为质子、电子和反中微子。我们正在使用可以储存在瓶子里的非常慢的中子来精确测量中子衰变。这些测量应该使我们能够对自旋极化中子衰变产生的电子分布进行最精确的测量。这些数据可以将我们对中子衰变的理解推向理论理解的极限，并可能表明我们目前的图片是不完整的，必须进行修改。我们也正在开发一个新的实验来寻找自由中子的电偶极子。电偶极子是中性粒子中正负电荷的分离。这种偶极子的存在将标志着所谓的电荷宇称（CP）对称性的重大突破。这种对称性的一个重要突破被提出来解释为什么宇宙被标准粒子组成的物质所主导，而宇宙中几乎没有反粒子。这个谜题（更多的是物质而不是反物质）已经困扰了物理学家很多年，我们希望发现和测量中子的电偶极子，以帮助解释宇宙的这一基本性质。我们还注意到，博士后学者以及研究生和本科生的参与是研究计划的重要组成部分，为年轻研究人员提供了特殊的机会，以促进他们在核物理这一前沿领域的培训和教育。

Technical Description of Project:Our project - Fundamental Studies in Nuclear Physics - at California Institute of Technology is primarily focused on experimental research addressing fundamental properties of the free neutron. In particular we will continue taking data with the UCNA experiment while also developing hardware for construction of a new, highly sensitive experiment to search for the neutron's electric dipole moment. The UCNA experiment has already produced a precision measurement of the beta-asymmetry in neutron decay using Ultra-Cold Neutrons for the first time. This quantity allows a sensitive test of the unitarity of the CKM quark-mixing matrix and has the potential to provide evidence for new physics beyond the Standard Model of particle interactions. With recent improvements to the apparatus we are poised to improve the precision on the asymmetry down to ~ 0.3% with additional data-taking and analysis during the period of this grant. The goal of the new electric dipole moment experiment - nEDM at the Spallation Neutron Source at Oak Ridge National Laboratory - is to improve the sensitivity by a factor of 100 compared to previous measurements; observation of an electric dipole moment signals violation of time-reversal and charge-parity (CP) symmetry, and could provide important information related to the origin of the matter-antimatter asymmetry of the universe and the new physical phenomena that is responsible. Non-technical Description of ProjectOur project at Caltech is primarily focused on studying fundamental properties of the free neutron. The neutron is an unstable particle and decays with a ten minute half-life into a proton, an electron and an anti-neutrino. We are making precision measurements of the neutron decay using very slow neutrons that can be stored in bottles. These measurements should allow us to make the most precise measurements of the distribution of electrons from the decay of spin-polarized neutrons. These data can push our understanding of the neutron's decay to the limits of theoretical understanding and may indicate that our current picture is incomplete and must be revised. We are also developing a new experiment to search for an electric dipole for the free neutron. An electric dipole is a separation of the plus and minus charge within this neutral particle. The existence of this dipole would signal a significant breakdown of so-called Charge-Parity (CP) symmetry. A significant breakdown of this symmetry has been suggested to explain why the universe is dominated by material composed of standard particles with virtually no anti-particles present in the universe. This puzzle (more matter than anti-matter) has stumped physicists for many years and we hope to discover and measure the neutron's electric dipole to help explain this basic property of the universe. We also note that participation by postdoctoral scholars as well as graduate and undergraduate students is an essential part of the research program, affording young researchers exceptional opportunities to advance their training and education in this frontier area of nuclear physics.