RUI: A Search for Long-Range Spin-Spin Interactions and Optical Forces in TlF

对基本粒子自旋的精确测量可以提供对自然基本定律的新见解。基本粒子有一种叫做自旋的固有特性，这使得它们像机械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保真度。这个项目将使用这些精密技术作为工具来研究自然的基本力量和对称性。在最基本的层面上，物理学家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，他们将测量他们的实验室自旋与地球内所有排列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，通过使用一束非常冷的分子来提高实验灵敏度。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。2013年，研究人员绘制了第一张地球内部电子自旋密度的地图。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，将构建一个精密的自旋进动装置，该装置校准良好，相对不受交流光效应的影响。这应该允许这些LRSSI测量的灵敏度至少提高一个数量级。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，研究人员将继续研究关键参数，这些参数将最终决定氟化铊（TlF）电偶极矩（edm）实验的灵敏度，该实验目前正在耶鲁大学由CeNTREX合作建造。具体来说，研究人员希望继续改进他们对TlF中光循环的测量，并证明这种循环可以用来对TlF施加光力。这些光力将被用来横向冷却低温的TlF分子束。这种横向冷却将使TlF电火花实验的灵敏度提高大约一个数量级。有了这种额外的灵敏度，就有可能发现一个永久性的核edm。如果这个edm被发现，它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。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Precision measurements of elementary particles' spins can provide new insight into the fundamental laws of nature. Elementary particles have an intrinsic property called spin which makes them act as if they are constantly rotating like mechanical 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. This project will use these precision techniques as tools to investigate the fundamental forces and symmetries of nature. At the most basic level, physicists' present understanding of nature is summarized by 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 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 beam of very cold 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. 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 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 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) electric-dipole moment (edm) experiment that is presently being constructed at Yale 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 transversely cool a cryogenic molecular beam of TlF. This transverse cooling should increase the sensitivity of the TlF edm experiment by about an order of magnitude. With this additional sensitivity it is possible that a permanent nuclear edm will be discovered. If this edm is found, it 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.