Precision Measurement of the 1S-2S Interval in Positronium

正电子素原子（Positronium atom，Ps）是一种由电子和它的反粒子正电子形成的奇特的短寿命原子。该项目将提高Ps结构测量的准确性，揭示电子和正电子之间基本相互作用的细节。 与传统的原子不同，Ps不包含任何质子或中子;因此Ps原子的光谱学是现代原子物理学的基石理论“束缚态量子电动力学”的一个很好的测试。 该项目推动了对短寿命原子和分子的超高精度测量的最新技术，并为更好地理解反应发生时产生的瞬态化学物质打开了大门。研究生将建造和操作专门为这个实验设计的仪器。本科生将协助实验，主要是在夏天。当地的高中教师将参加为期一周的加州大学滨江高中教师暑期学院，在那里教师接触到研究，参观实验室，并受到讲座和演示的启发，解释如何将研究纳入高中物理课程。PI将访问高中物理教师和教室，并与当地县教育科学项目委员会合作，根据新的下一代科学标准和国家科学标准推进初中和高中课程开发。因此，该项目将从初中物理教室开始，通过培养学生掌握尖端技术来扩大国家的高技术劳动力，并增加STEM领域参与者的多样性。技术观众摘要：纯轻子正电子素（Ps）原子是唯一适合测试束缚态量子电动力学（QED）的原子。万亿分之几的高精度Ps光谱学将提供背景信息，可用于从较重轻子和强子的精确原子测量中提取非QED物理。通过这种方式，Ps光谱可以揭示质子电荷半径之谜以及最近里德伯常数值的变化（大约是以前接受的实验不确定性的7倍）。每当一个被测量的常数被移动了这么大的量，那么就有机会进行更仔细的检查，以产生新的实验和新的物理理解如何测量常数。更高的精度也可以帮助限制更高级别的反冲效应修正μ。先前由Co-PI和S.朱棣文已经站了25年。测量中的不确定性来自正电子素原子在激光束中花费的时间太少（渡越时间加宽），原子移动太快（二阶多普勒频移），激光强度太高（AC斯塔克频移），以及计量限制在~1 MHz水平。该项目将采用几种新技术来提高准确度和精确度。位置敏感的飞行时间探测器将记录每个被探测原子通过较大激光束轮廓时的轨迹和速度，从而减少由于二阶多普勒频移、AC斯塔克频移和渡越时间加宽而引起的不确定性。采用频率梳和拍频技术，监测电子偶素原子通过激发光束时的瞬时激光频率。这些创新应将测量的不确定性降低300倍，目标是达到10 kHz的不确定性。这项工作将培养研究生在原子物理学，正电子科学，激光光谱学和计量学的组合。这项培训将有利于学生以及国家提供高技能的博士学位。是为科学工作者准备的加州滨江大学是教育部指定的西班牙裔服务机构（HSI），50%的物理专业和10%的国内研究生来自代表性不足的少数民族群体。几位本科生研究人员和一位高中教师也将参与这些实验，这将有助于吸引更多的参与者进入STEM学科。该奖项反映了NSF的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。

General audience abstract:The positronium atom (Ps) is an exotic short-lived atom formed by an electron and its anti-particle, the positron. This project will improve the accuracy of measurements of the structure of Ps, revealing detail about the fundamental interactions between the electron and positron. Unlike more conventional atoms Ps does not contain any protons or neutrons; thus spectroscopy of Ps atoms is an excellent test of “bound-state quantum electrodynamics,” the cornerstone theory for modern atomic physics. This project pushes the state-of-the-art in the ultra-high precision measurement of short lived atoms and molecules and opens the door to better understanding of transient chemical species that are produced as a reaction is taking place. Graduate students will build and operate the apparatus designed especially for this experiment. Undergraduate students will assist in the experiments, mostly during the summers. Local high school teachers will attend a week-long University of California Riverside Summer Academy for High School teachers at which teachers are exposed to research, visit the labs, and are inspired by lectures and demonstrations that explain how to incorporate research within the high school physics curriculum. The PI will visit high school Physics teachers and classrooms and work with local county Board of Education science projects to advance junior and senior high school curriculum development under the new Next Generation Science Standards and state science standards. This project will thus help expand the nation's high technology work force by training students in cutting edge techniques and increase the diversity of participants in STEM fields, starting with the pipeline from junior high school physical science classrooms.Technical audience abstract:The purely leptonic positronium (Ps) atom is uniquely well-suited for testing bound-state quantum electrodynamics (QED). High accuracy Ps spectroscopy at the few parts per trillion level will provide background information that can be used to extract non-QED physics out of precision atomic measurements with heavier leptons and hadrons. In this way, Ps spectroscopy can shed light on the proton charge radius puzzle as well as the recent shifting of the value of the Rydberg constant (by about 7 times the previously accepted experimental uncertainty). Whenever a measured constant is shifted by such a large amount, then there is an opportunity for closer examination to yield new experiments and new physical understanding in how to measure the constants. Higher accuracy can also help constrain higher level recoil effect corrections in muonium. The previous precision measurement of the Ps 1S-2S interval, 1,233,607,216.4 +/- 3.2 MHz, performed by the Co-PI and S. Chu has stood for 25 years. The uncertainty in this measurement came from positronium atoms spending too little time in the laser beam (transit time broadening), atoms moving too fast (second-order Doppler shift), laser intensity too high (AC Stark shift), and metrology limited at the ~1 MHz level. This project will implement several new techniques to improve accuracy and precision. A position-sensitive time-of-flight detector will record the trajectory and speed of every detected atom as it passes through a larger laser beam profile, and thereby reduce the uncertainty due to second-order Doppler shifts, AC Stark Shifts, and transit-time broadening. A frequency-comb and a frequency-beat technique will be used to monitor the instantaneous laser frequency as positronium atoms transit the excitation beam. These innovations should reduce the uncertainty in the measurement by a factor of 300, with the goal of reaching a 10 kHz uncertainty. This work will train graduate students in a combination of atomic physics, positron science, laser spectroscopy, and metrology. This training will benefit the students as well as the nation by providing highly skilled Ph.D.'s for the scientific workforce. The University of California Riverside is a Department of Education Designated Hispanic Serving Institution (HSI) with 50% of Physics majors and 10% of domestic graduate students from under-represented ethnic minority groups. Several undergraduate researchers and a high school teacher will also be involved in these experiments, and this will help to attract more participants to STEM disciplines.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.