PM: RUI: Searching for Optical Cycling in TlF and Long-Range Spin-Spin Interactions

对基本粒子自旋相互作用的精确测量可以为自然界的基本定律提供新的见解。 基本粒子有一种内在的性质，叫做自旋--它们的行为就像陀螺一样不停地旋转。 就像陀螺在重力作用下的进动一样，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，核磁共振是用于医学诊断的基础物理学，称为磁共振成像（MRI）。 最近发展起来的精密光学技术使研究粒子自旋的相互作用具有前所未有的保真度。 研究人员将使用这些技术作为工具来研究自然界的基本力和对称性。 在最基本的层面上，我们目前对自然的理解可以概括为粒子物理学的“标准模型”。 这个模型需要四种基本力（引力、电磁力、强力和弱力）来描述目前已知的所有现实。 在一个实验中，研究人员将寻找标准模型无法描述的粒子自旋之间的新长程力。 为了优化他们的搜索，他们将测量他们的实验室自旋与地球内所有对齐的电子自旋的相互作用。 在他们的另一个实验中，研究人员希望最终看到自然的基本定律是否在时间上是不对称的。 这种"时间对称性"的破缺可以通过观察电场中原子核自旋的进动来研究。 在这里，通过使用非常冷的分子束来增加实验灵敏度。 额外的时间不对称性（超过已经观察到的时间不对称性）被认为是解释我们宇宙存在的必要条件。 如果没有时间反演违反，我们的宇宙将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。 对这些实验的追求将为许多本科生提供一个诱人的STEM介绍，并为下一代STEM研究人员提供持续的经验。2013年，研究人员创建了地球内部电子自旋密度的第一张地图。 这些“地球电子”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作为磁场方向的函数的自旋-进动频率，允许人们寻找地球电子和实验室自旋之间的长程自旋-自旋相互作用（LRSSI）。 在第一个提出的实验中，一个完善的自旋进动装置正在建设中，这是良好的校准和相对免疫AC光效应。 这应该允许这些LRSSI测量的灵敏度至少提高一个数量级。 如果看到一种效应，那就意味着一种新的自然力的存在。 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。 在第二个拟议的实验中，研究人员将继续研究关键参数，这些参数将最终确定氟化铊（TlF）核电偶极矩（nEDM）实验的灵敏度，该实验目前正在由CeNTREX合作在Argon国家实验室建造。具体来说，研究人员希望继续改进他们对TlF中光学循环的测量，并证明这种循环可用于对TlF施加光学力。 这些光学力将用于横向冷却TlF低温分子束。 如果成功的话，横向冷却可以将TIF nEDM实验的灵敏度提高大约一个数量级。 有了这种额外的敏感性，就有可能发现一个永久性的nEDM。这样的发现将意味着违反时间对称性，并可能有助于解释我们物质主导的宇宙的存在。这个奖项反映了NSF的法定使命，并已被认为是值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估的支持。

Precision measurements of the spin interactions of elementary particles can provide new insights into the fundamental laws of nature. Elementary particles have an intrinsic property called spin – they act as if they are constantly spinning around like tops. Just as 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 fidelity. The researchers will use these 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 cannot be described by the Standard Model. To optimize their search, they will measure the interaction of their laboratory spins with all of the aligned 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 is 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. The pursuit of these experiments will provide an enticing introduction to STEM for many undergraduate students and provide continue experiences for the next generation of STEM researchers.In 2013 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 the 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 is under construction 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 LRSSI measurements. 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, the researchers will continue their investigation of critical parameters that will ultimately determine the sensitivity of the thallium fluoride (TlF) nuclear electric-dipole moment (nEDM) experiment that is presently being constructed at Argon National Lab by the CeNTREX collaboration. Specifically, the researchers hope to continue to improve their measurements of optical cycling in TlF and to demonstrate that this cycling can be used to exert optical forces on TlF. These optical forces will be used to transverse cool a cryogenic molecular beam of TlF. If successful, transverse cooling could increase the sensitivity of the TlF nEDM experiment by about an order of magnitude. With this additional sensitivity it is possible that a permanent nEDM will be found. Such a discovery would imply a violation of time symmetry and could help explain the existence of our matter-dominated universe.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.