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

摘要：正电子原子（Ps）是由一个电子和它的反粒子正电子形成的一种奇异的短寿命原子。这个项目将提高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。与更传统的原子不同，Ps不包含任何质子或中子；因此，Ps原子的光谱学是对“束缚态量子电动力学”的绝佳测试，而“束缚态量子电动力学”是现代原子物理学的基石理论。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。研究生将建造并操作专门为本实验设计的仪器。本科生将协助实验，主要在夏季进行。当地的高中教师将参加为期一周的加州大学河滨分校高中教师暑期学院，在那里教师们将接触到研究，参观实验室，并受到解释如何将研究纳入高中物理课程的讲座和示范的启发。PI将访问高中物理教师和教室，并与当地县教育委员会科学项目合作，根据新一代科学标准和国家科学标准推进初中和高中课程的开发。因此，该项目将通过培训学生掌握尖端技术，增加STEM领域参与者的多样性，从而帮助扩大美国的高科技劳动力，从初中物理科学教室的管道开始。摘要：纯轻子正电子原子（Ps）非常适合于测试束缚态量子电动力学（QED）。万亿分之几的高精度Ps光谱将提供背景信息，可用于从具有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光谱学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（大约是先前接受的实验不确定度的7倍）。每当测量的常数发生如此大的位移时，就有机会进行更仔细的检查，从而产生新的实验和对如何测量常数的新的物理理解。更高的精度也可以帮助约束更高水平的反冲效应修正。之前由Co-PI和S. Chu进行的Ps 1S-2S区间的精度测量为1,233,607,216.4 +/- 3.2 MHz，已经持续了25年。测量中的不确定度来自于正电子原子在激光束中停留的时间过短（传输时间展宽）、原子运动的速度过快（二阶多普勒频移）、激光强度过高（交流斯塔克频移）以及测量限制在~1 MHz水平。该项目将采用几种新技术来提高准确性和精密度。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在通过更大的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斯塔克频移和透射时间展宽造成的不确定性。频率梳和频率拍技术将用于监测正电子原子通过激发光束时的瞬时激光频率。这些创新应该将测量中的不确定度降低300倍，目标是达到10 kHz的不确定度。这项工作将培养原子物理学、正电子科学、激光光谱学和计量学的研究生。这种培训将为科学队伍提供高技能的博士，从而使学生和国家受益。加州大学河滨分校是教育部指定的西班牙裔服务机构（HSI），有50%的物理专业学生和10%的国内研究生来自代表性不足的少数民族。几名本科生研究人员和一名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加入STEM学科。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。

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.