RUI: A Search for Long-Range Spin-Spin Interactions and Thallium-Fluoride Investigations

基本粒子有一种叫做自旋的固有特性——它们就像陀螺一样不停地旋转。就像陀螺在重力作用下会进动一样，基本粒子的自旋在磁场中也会进动。这种进动是核磁共振的基础，核磁共振是医学诊断中使用的基础物理，称为磁共振成像（MRI）。最近发展的精密光学技术使研究与粒子自旋的相互作用具有前所未有的精度。研究人员将使用这些精密技术作为工具来研究自然界的基本力量和对称性。在最基本的层面上，我们目前对自然的理解可以用粒子物理学的“标准模型”来概括。这个模型需要四种基本力（引力、电磁力、强作用力和弱作用力）来描述目前已知的所有现实。在一个实验中，研究人员将在粒子自旋之间寻找一种标准模型无法描述的新的远程力。为了优化他们的搜索，他们将测量实验室自旋与地球内所有电子自旋之和的相互作用。在他们的另一个实验中，研究人员希望最终能看到自然的基本定律在时间上是否不对称。这种对“时间对称性”的破坏可以通过在电场中寻找核自旋的进动来研究。在这里，可以通过使用非常冷的分子束来提高实验灵敏度。额外的时间不对称（超出已经观察到的）被认为是解释我们宇宙存在的必要条件。如果没有违反时间反转，我们的宇宙将会产生等量的物质和反物质。它们的相互湮灭不可能形成星系、恒星、行星和生命。最近，研究人员绘制了地球内部电子自旋密度的首张地图。这些“地电子”构成了已知的最大的极化自旋源。在地球表面的实验室中，精确测量自旋进动频率作为其应用磁场方向的函数，允许人们寻找地电子和实验室自旋之间的远程自旋-自旋相互作用（LRSSI）。在第一个实验中，将构建一个精细的自旋进动装置，该装置校准良好，相对不受交流光效应的影响。这至少可以使这些测量对LRSSI的灵敏度提高一个数量级。如果观察到这种效果，就表明存在一种新的自然力。在目前的模型中，这种力可能与超轻矢量介子、“暗”光子、“非粒子”或扭转引力有关。在第二个实验中，将使用紫外激光和冷分子源研究确定氟化铊（TlF）中电偶极矩（edm）实验可行性的关键参数。具体来说，研究人员希望证明TlF中存在循环跃迁，并测量TlF从表面烧蚀的效率。如果这些测量结果是有利的，那么TlF将被提议作为冷束精确测量铊核edm的候选系统。永久核电火花的发现将意味着对时间对称性的破坏，并有助于解释我们物质主导的宇宙的存在。

Elementary particles have an intrinsic property called spin--they act as if they were constantly spinning around like tops. Just as a tops precess in the presence of gravity, the spins of fundamental particles precess in a magnetic field. This precession is the basis of nuclear magnetic resonance which is the underlying physics used in the medical diagnostic known as magnetic resonance imaging (MRI). Recently developed precision optical techniques have allowed the study of interactions with particle spins with unprecedented precision. The researchers will use these precision techniques as tools to investigate the fundamental forces and symmetries of nature. At the most basic level, our present understanding of nature is summarized in the "Standard Model' of particle physics. This model requires four fundamental forces (gravitational, electromagnetic, strong and weak) to describe all of reality as it is presently known. In one experiment, the investigators will look for a new long-range force between particle spins that can't be described by the Standard Model. To optimize their search, they will measure the interaction of their laboratory spins with the sum of all of the electron spins within the Earth. In their other experiment, the researchers hope eventually to see if the fundamental laws of nature might be asymmetric in time. This breaking of "time symmetry" can be studied by looking for the precession of a nuclear spin in an electric field. Here the experimental sensitivity can be increased by using a very cold beam of molecules. Additional time asymmetry (beyond that which has already been observed) is believed to be necessary to explain the existence of our universe. Without time-reversal violation, our universe would have produced equal amounts of matter and anti-matter. Their mutual annihilation would not have allowed for the formation of galaxies, stars, planets and life. Recently, the researchers created the first map of the electron-spin density within the Earth. These "geo-electrons" constitute the largest polarized spin source known. Precision measurement of spin-precession frequencies in laboratories at the surface of the Earth as a function of their applied magnetic-field direction, allows one to look for long-range spin-spin interactions (LRSSI) between the geo-electrons and the laboratory spins. In the first proposed experiment, a refined spin-precession apparatus will be constructed which is both well calibrated and relatively immune to AC light effects. This should allow at least an order of magnitude improvement in the sensitivity of these measurements to LRSSI. If an effect is seen it would suggest the existence of a new force of nature. In current models this force might be associated with an ultra-light vector meson, a "dark" photon, the "unparticle," or torsion gravity. In the second proposed experiment, critical parameters that determine the viability of an electric-dipole moment (edm) experiment in thallium fluoride (TlF) will be investigated using an ultraviolet laser and a cold molecular source. Specifically, the researchers hope to demonstrate the existence of a cycling transition in TlF and to measure the efficiency with which TlF can be ablated from a surface. If the results of these measurements are favorable, TlF will then be proposed as a candidate system for a cold-beam precision measurement of the edm of the thallium nucleus. The discovery of a permanent nuclear edm would imply a violation of time symmetry and could help explain the existence of our matter-dominated universe.