1S-2S Spectroscopy of Positronium

原子内部基本粒子的相互作用是原子物理学、化学和所有物质结构的基础。对这些相互作用的理解是基于对各种原子和分子发射和吸收的光的精确测量。这种光的精确颜色（或“共振频率”）是一种独特的指纹，用于从医学到国防和制造业质量控制的广泛应用中的化学识别。对指纹的精确测量还可以告诉我们组成原子的基本粒子的细节。最近，对氢（电子与质子结合）和介子氢（电子的重版本，介子与质子结合）的特定共振频率的测量揭示了一个谜团：质子的直径似乎因其所在的原子而异。如果人们假设电子和介子与质子的相互作用是根据公认的束缚态量子电动力学（QED）理论进行的，并且任何差异都必须归因于核结构，那么这个谜团就会出现。假设没有错误，有三种可能性：QED是不正确的，核结构是不正确的，或者有一些没有考虑到的新的相互作用。为了确定这个谜团是否是由于QED的问题，来自加州大学河滨分校（UCR）的两位科学家提议测量正电子的共振频率，正电子是最简单的原子（由一个与反电子结合的电子组成），它应该被束缚态QED理论完美地描述。为这些测量而开发的新技术将导致不稳定原子光谱学的改进，这是精密光谱学领域的前沿。参与该项目的科学家们在增加物理学的多样性方面有着良好的记录。PI在当地社区K-12体系中发挥着特殊作用，就增加高中物理学生数量、增加高等教育和教师培训中物理科学/工程专业的学生数量提供建议。联合项目建立了一个渠道，招募当地大专转校生到加州大学洛杉矶分校，让他们参与研究，并指导他们最终申请研究生院。纯轻子原子正电子是唯一适合于测试束缚态量子电动力学（QED）的，并提供了理解和背景，人们可以从较重的轻子和强子的精确原子测量中提取非QED物理。很少有人敢于尝试在10^12的水平上对正电子进行测量，这将使人们能够深入了解物理学，如质子电荷半径和介子中更高水平的反冲效应修正，这可能会显示出轻子和重子之间的差异。精确测量正电子的理想能级间距是1S-2S区间，约为1.234 PHz。最后一次测量是与S. Chu合作进行的，对这个间隔的前10位的了解已经持续了20年，不确定度为±3.2 MHz。提出的实验实现了一种新技术，该技术将极大地提高Ps原子光谱和其他高分辨率光谱实验的准确性，例如通过单个原子轨迹分析muonium。位置敏感的飞行时间探测器将记录每一个被探测到的原子的轨迹和速度，从而消除二阶多普勒频移，AC斯塔克频移，并更好地解释透射时间展宽。新的检测方法将允许降低迄今为止限制测量精度的激光强度和被检测原子的速度。分析的基本特征以及分析和减少不确定度的方法的发展将使用相对于超稳定参考腔的±2 MHz测量波长来完成。该方法将足以产生窄线宽（~1 MHz），并在提案的3年期间大大提高信噪比。通过获取激光频率梳基准，可以实现100倍的测量和1000倍精度的系统效应探索。

The interactions of the fundamental particles inside atoms underpins atomic physics, chemistry, and the structure of all matter. The understanding of these interactions is based on precise measurements of the light that is emitted and absorbed by various atoms and molecules. The precise set of colors (or "resonant frequencies") of this light is a unique fingerprint that is used for chemical identification in a broad range of applications from medicine to defense and quality control in manufacturing. The precise measurement of the fingerprint can also tell us details about the fundamental particles that make up atoms. Recently, a measurement of a particular resonant frequency in hydrogen (an electron bound to a proton) and muonic hydrogen (a heavy version of the electron, the muon, bound to a proton) has revealed a mystery: the diameter of the proton appears to be different depending on which atom it is in. This mystery arises if one assumes that the electron and muon interact with the proton by the well-accepted theory called bound-state quantum electrodynamics (QED) and that any differences must be attributed to the nuclear structure. Assuming that there have been no mistakes, there are 3 possibilities: QED is incorrect, the nuclear structure is incorrect, or there is some new kind of interaction which has not been taken to account. 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 measure the resonant frequencies of positronium, the simplest atom (consisting of an electron bound to an anti-electron) that should be described perfectly by bound-state QED theory. The new techniques developed for these measurements will lead to an improvement in the spectroscopy of unstable atoms which are at the frontier of the field of precision spectroscopy. The scientists involved with the project have a strong record in increasing diversity in physics. The PI plays a special role in the local community K-12 system, advising on increasing the number of students taking high school physics and increasing the pool of physical science/engineering majors in higher education and in teacher training. The co-PI has established a pipeline for recruiting local area junior college transfers to UCR, engaging them in research, and mentoring their eventual applications to graduate schools.The purely leptonic atom positronium is uniquely well-suited for testing bound-state quantum electrodynamics (QED) and provides the understanding and background by which one 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 in 10^12 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 1.234 PHz. Last measured in collaboration with S. Chu, knowledge of the first 10 digits of this interval has stood for 20 years with an uncertainty of ±3.2 MHz. The proposed experiment implements a new technique that would dramatically improve the accuracy of Ps atom spectroscopy and potentially other high resolution spectroscopy experiments, for example in muonium, by individual atom trajectory analysis. A position sensitive time-of-flight detector will record the trajectory and speed of every detected atom and thereby remove second-order Doppler shifts, AC Stark Shifts, and better account for transit-time broadening. The new detection method will allow for reducing the laser intensity and the speed of the detected atoms that heretofore have limited the measurement precision. The basic features of the analysis and development of the methods to analyze and reduce uncertainties will be accomplished using a wavemeter with ±2 MHz metrology relative to an ultrastable reference cavity. This method will be sufficient to produce narrow line widths (~1 MHz) and a vastly improved signal-to-noise ratio in the 3 year period of the proposal. A 100 fold better measurement and exploration of the systematic effects at 1000 fold better accuracy will be achieved with the acquisition of a laser frequency comb reference.