Searches for Exotic Spin-Dependent Forces using High-Frequency Mechanical Oscillators

该奖项支持在亚毫米尺度上对重力和电磁力之外的基本自然力进行实验研究。 力可能取决于所涉及的测试对象的几个属性，例如质量（如重力）或电荷（如电磁力）。 拟议中的实验集中在依赖于自旋的力上，自旋可以被认为是物体围绕其轴的旋转。大多数亚原子粒子（包括普通物质的质子、中子和电子）都有非零自旋，这决定了它们的磁性：磁力取决于自旋。 发现一种额外的依赖于自旋的宏观力，将对从亚原子到宇宙尺度的物理学产生巨大的影响。 标准模型（迄今为止最成功的粒子物理学理论，但人们普遍认为是不完整的）的各种扩展预测了短程自旋相关力。 这些力是由可能有助于“暗物质”的粒子介导的，“暗物质”是一个未知的量，假设它占宇宙质量的四分之一，并解释了星系的表观质量分布。在执行过程中，这个基础项目还将为所有参与者（包括对该提案做出贡献的本科生）提供机械设计，真空技术，低噪声电子学，半导体和磁性材料加工以及其他在广泛的科学和工程领域有用的实用技术方面的经验。 该实验也将成为印第安纳州大学时空对称中心（IUCSS）的一个重点，该中心由PI及其同事创立，旨在加强和宣传IU在时空结构研究方面不断增长的专业知识。该实验使用1千赫平面振荡器作为测试质量，它们之间有一个薄屏蔽来抑制背景，该技术已经证明了使用相对大的（平方厘米）质量探测微米级距离的能力，并且在室温下在仪器热噪声的极限下操作。 主要的挑战将是克服与使用极化试验质量相关的预期磁背景。 为此，我们已经开发了自旋极化的测试质量非常低的内在磁性。我们将在我们的实验中使用它们来进行初始搜索，以获得前所未有的灵敏度低于1毫米的几个速度无关的自旋耦合力，并搜索目前不受实验约束的几个速度相关的相互作用。我们还将研究极化材料，这些材料有可能将我们的实验灵敏度提高几个数量级。在奖励期结束时，我们预计，在没有发现的情况下，我们将在亚毫米范围内对多达11个以前不受约束的电子自旋耦合相互作用设定限制，并在灵敏度至少比目前的限制大一个数量级的情况下再设定4个限制。

This award supports an experimental search for fundamental forces of nature beyond gravity and electromagnetism at sub-millimeter length scales. Forces could depend on several properties of the test objects involved, such as mass (like gravity), or charge (like electromagnetism). The proposed experiments concentrate on forces that depend on spin, which can be thought of as the rotation of an object around its axis. Most subatomic particles (including the protons, neutrons, and electrons of ordinary matter) have non-zero spin, which determines their magnetic properties: magnetic forces depend on spin. The discovery of an additional macroscopic force that depends on spin would have enormous implications for physics from subatomic to cosmological scales. A variety of extensions to the Standard Model (the most successful theory of particle physics to date, but widely believed to be incomplete) predict short-range spin-dependent forces. These forces are mediated by particles that could contribute to "dark matter," the otherwise unknown quantity hypothesized to make up about one quarter of the mass of the universe and to explain the apparent mass distributions of galaxies. During its execution, this fundamental project will also provide all participants (including undergraduates who have contributed to this proposal) with experience in mechanical design, vacuum technology, low-noise electronics, semiconductor and magnetic materials processing, and other practical techniques useful in a wide range of science and engineering fields. The experiment will also be a key focus of the Indiana University Center for Spacetime Symmetries (IUCSS), which was founded by the PI and his colleagues to strengthen and publicize IU's growing expertise in the investigation of the structure of spacetime.The experiment uses 1-kilohertz planar oscillators as test masses with a thin shield between them to suppress backgrounds, a technique that has demonstrated the capability to probe micron-scale distances using relatively large (square-centimeter) masses, and to operate at the limit of instrumental thermal noise at room temperature. The principal challenge will be to overcome the anticipated magnetic backgrounds associated with the use of polarized test masses. To this end we have developed spin-polarized test masses with very low intrinsic magnetism. We will use them in our experiments to conduct initial searches for several velocity-independent spin-coupled forces with unprecedented sensitivity below one millimeter, and to search for several more velocity-dependent interactions that are presently unconstrained by experiment. We will also investigate polarized materials that have the potential to improve our experimental sensitivity by several additional orders of magnitude. At the completion of the award period, it is our expectation that, in the absence of a discovery, we will have set limits on as many as eleven previously unconstrained spin-coupled interactions of electrons in the sub-millimeter range, and up to four more limits with sensitivity at least order of magnitude greater than present constraints.