Pinpointing with a Broad Beam: h/m and the Fine Structure Constant

精细结构常数表征电磁相互作用的强度，在物理学中普遍存在。测量它的方法来自不同的物理学领域，这些方法的一致性有力地证实了理论和实验的一致性。利用精确测量的基本常数（里德伯常数和原子质量比），精细结构常数可以与散射光子的原子的反冲速度相关联。本项目将利用这种关系，以有史以来最高的精度测量精细结构常数。这将使研究小组能够测试标准模型的几个扩展，例如暗物质候选，并探测电子的子结构，以测试电子是否确实是基本粒子。反冲速度（与原子质量成反比）的测量也可以用作质量单位的超精确实现，从而成为修订的国际单位制的基石。这个实验是以原子干涉仪为基础的。它使用的是一束非常接近无限电磁波的理想情况的宽激光束。一个强大的脉冲激光将用于这个项目，以及容纳光束的原子喷泉。原子干涉仪将采用一种新的几何结构，垂直偏移同步共轭干涉仪（OSCIs）。这将消除振动、重力甚至重力梯度的影响。该项目将使用布拉格衍射和布洛赫振荡与铯原子的组合来测量h/m，精度为0.02 ppb。从h/m，将确定精细结构常数，目标精度为0.039 ppb，受mAt/me精度的限制。希望在铯或电子质量测量方面的改进将精细结构常数的误差降低到0.02 ppb。现在，这种精细结构常数的测量已经对几种提出的粒子设定了严格的限制。例如，标量和伪标量玻色子、矢量玻色子、暗光子、隐藏扇区光子和轴矢量玻色子都将被发现。这对粒子物理学的标准模型进行了广泛而深入的检验。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。

The fine structure constant characterizes the strength of the electromagnetic interaction and is ubiquitous in physics. It has been measured using methods from across physics, whose agreement is a powerful confirmation of the consistency of theory and experiment. Using well-measured fundamental constants (the Rydberg constant and atomic mass ratios), the fine structure constant can be related to the recoil velocity of an atom that has scattered a photon. This project will use this relationship in order to measure the fine structure constant with the highest precision ever. This will allow the research team to test several extensions of the standard model, such as dark matter candidates, and probe for a substructure of the electron, to test if the electron is indeed an elementary particle. The measurement of the recoil velocity (which is inversely proportional to the atomic mass) can also be used as an ultra-precise realization of the unit of mass and thus a cornerstone of the revised international system of units. This experiment is based on an atom interferometer. It uses a broad laser beam which comes extremely close to the ideal case of an infinite electromagnetic plane wave. A powerful pulsed laser will be available for this project, as well as an atomic fountain accommodating the beam. A new geometry for the atom interferometer will be used, vertically offset simultaneous conjugate interferometers (OSCIs). This will cancel the effects of vibrations, gravity, and even the gravity gradient. The project will measure h/m using a combination of Bragg diffraction and Bloch oscillations with cesium atoms at an accuracy of 0.02 ppb. From h/m, the fine structure constant will be determined with a target accuracy of 0.039 ppb, limited by the accuracy of mAt/me. It is hoped that that improvements in the cesium or electron mass measurement will reduce the error in the fine structure constant to 0.02 ppb. Already now, such measurements of the fine structure constant set stringent limits on several proposed particles. For example, scalar and pseudoscalar bosons, vector bosons, dark photons, hidden-sector photons, and axial-vector bosons would all be discoverable. This tests the standard model of particle physics broadly and deeply.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.