MRI: Development of an Instrument for Ultra-High Resolution 1S-2S Spectrosopy of Exotic Hydrogenic Atoms

我们关于原子结构和相互作用的最精确信息最终是基于对各种原子和分子发射和吸收的光的精确测量。这种光的颜色光谱(或“共振频率”)为计算和测量提供了基本输入，这些计算和测量用于化学鉴定、反应速率估计以及在从医学到国防和制造质量控制的广泛应用中预测新结构的稳定性。最近，对不同类型的氢原子(由一个中心重质子和一个类行星电子或µ子组成)的测量结果对质子的半径存在分歧。这种差异暗示了三种可能性之一：(1)被称为量子电动力学或QED的带电粒子如何相互作用的理论是不正确的；(2)我们对质子结构的了解是不正确的；或者(3)还没有发现某种新的相互作用或自然力。为了确定这一谜团是否源于QED的问题，来自加州大学河滨分校(UCR)的两名科学家提议开发一种新的仪器，该仪器将能够以比目前技术水平高1000倍的精度测量正电子的共振频率。由于正电子素是最简单的原子，由一个电子与一个反电子结合而成，它应该用QED理论来完美地描述。使用拟议的仪器进行的新测量将决定QED理论是否是问题所在，并对我们对质子结构的理解上的差异设定更高的限制。这一新仪器将改进精密光谱学领域中不稳定原子的光谱学。纯粹的轻子原子正电子(Ps)是唯一非常适合于测试束缚态量子电动力学(QED)的原子，它为我们提供了从较重的轻子和强子的精密原子测量中提取非QED物理的理解和背景。很少有人敢于尝试在万亿分之几的水平上对正电子进行测量，这些测量将允许深入了解物理，如质子电荷半径和更高水平的反冲效应修正，这可能会显示出轻轻子和重轻子之间的差异。对正电子进行精确测量的理想能级间距是在大约1 233 607 216 MHz处的1S-2S间隔。这个间隔的前10位的知识已经存在了20年，不确定度为±3 MHz。拟议的仪器将实施几项新技术，通过高精度的单个原子轨迹分析，极大地提高Ps原子光谱学和其他高分辨率光谱学实验的精度，使线中心的测量精度达到千赫精度。原子通过激光场的过程中的激光频率测量将通过记录与热稳定超低膨胀玻璃参考腔线中心的瞬时偏差来完成，相对和绝对精度将达到亚千赫，该参考腔是使用受GPS约束的射频参考频率梳校准的。通过提供超稳定的环境、机械和振动隔离，仪器的总体规格将足以产生窄线宽(~2 MHz)，减少系统误差以使精度提高1000倍，并将计数率提高10倍以在可管理的测量持续时间(月与年)内进行精确测量。

Our most precise information about the structure and interactions of atoms is ultimately based on precise measurements of the light that is emitted and absorbed by various atoms and molecules. The spectrum of colors (or "resonant frequencies") of this light provides the basic input to calculations and measurements that are used for chemical identification, for reaction rate estimates, and for predictions of stability of new structures in a broad range of applications from medicine to defense and quality control in manufacturing. Recently, measurements on different types of hydrogen atoms (consisting of a central heavy proton and a planet-like electron or muon) are in disagreement about the radius of the proton. The discrepancy suggests one of 3 possibilities: (1) the theory of how charged particles interact, known as quantum electrodynamics or QED, is incorrect; (2) our knowledge of the structure of the proton is incorrect; or (3) there is some new kind of interaction or force of nature yet to be discovered. To decide whether the mystery is due to a problem with QED or not, two scientists from the University of California Riverside (UCR) propose to develop a novel instrument that will enable a measurement of the resonant frequency of positronium at ultimately 1000X more accuracy than the current state of the art. Because positronium is the simplest possible atom, consisting of an electron bound to an anti-electron, it should be described perfectly by QED theory. New measurements with the proposed instrument will decide whether QED theory is the problem and set significantly higher limits on the discrepancy in our understanding of the structure of the proton. The new instrument will lead to an improvement in the spectroscopy of unstable atoms at the frontier of the field of precision spectroscopy. The purely leptonic atom positronium (Ps) is uniquely well-suited for testing bound-state quantum electrodynamics (QED) and provides the understanding and background by which we may extract non-QED physics out of precision atomic measurements on heavier leptons and hadrons. Few have dared to try measurements on positronium at the few parts per trillion level that would allow insight into physics such as the proton charge radius and higher level recoil effect corrections in muonium, and that might show differences between light and heavy leptons. The ideal level spacing for a precision measurement on positronium is the 1S-2S interval at approximately 1 233 607 216 MHz. Knowledge of the first 10 digits of this interval has stood for 20 years with an uncertainty of ±3 MHz. The proposed instrument will implement several new techniques that would dramatically improve the accuracy of Ps atom spectroscopy and potentially other high resolution spectroscopy experiments by high precision individual atom trajectory analysis allowing line-centers to be measured to kHz precision. The laser frequency metrology during the transit of the atoms through the laser field will be accomplished with sub-kHz relative and absolute accuracies by recording the instantaneous deviations from the line-center of a thermally-stabilized ultra-low expansion glass reference cavity that is calibrated using a GPS-disciplined RF-referenced frequency comb. By providing ultrastable environmental, mechanical, and vibrational isolation, the overall specifications of the instrument will be sufficient to produce a narrow linewidth (~2 MHz), a reduction in systematic errors to enable up to 1000X increase in accuracy, and a 10X higher count rate to enable precision measurements in manageable measurement durations (months vs years).