1S-2S Spectroscopy of Positronium

正电子的 1S-2S 光谱

• 批准号: 1404576
• 负责人: Harry Tom
• 金额: $ 54.78万
• 依托单位: University of California-Riverside
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2014
• 资助国家: 美国
• 起止时间: 2014-09-01 至 2019-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1404576&HistoricalAwards=false
• 关键词: 1S; 2S; Spectroscopy; Positronium

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.

原子内部基本粒子的相互作用是原子物理、化学和所有物质结构的基础。对这些相互作用的理解建立在对各种原子和分子发射和吸收的光的精确测量的基础上。这种光的精确颜色集(或“共振频率”)是一种独特的指纹，用于从医学到国防和制造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(束缚在质子上的电子)和介子氢(束缚在质子上的电子的重版本)的特定共振频率的测量揭示了一个谜团：质子的直径似乎是不同的，这取决于它所在的原子。如果人们根据被广泛接受的束束态量子电动力学(QED)理论假设电子和µ子与质子相互作用，并且认为任何差异都必须归因于原子核结构，那么这个谜团就出现了。假设没有错误，有三种可能性：QED是不正确的，核结构不正确，或者有某种新的相互作用没有被考虑在内。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)的两名科学家提议测量正电子素的共振频率。正电子素是最简单的原子(由一个电子与一个反电子结合组成)，应该由束缚态QED理论完美地描述。为这些测量开发的新技术将导致不稳定原子光谱的改进，这些原子处于精密光谱领域的前沿。参与该项目的科学家在增加物理学多样性方面有着很好的记录。PI在当地社区K-12系统中发挥着特殊的作用，建议增加高中物理课程的学生人数，并在高等教育和教师培训中增加物理科学/工程专业的人数。联合国际已经建立了一条管道，招募当地的专科生到UCR，让他们参与研究，并指导他们最终在研究生院的应用。纯粹的轻子原子正电子素是唯一非常适合测试束缚态量子电动力学(QED)的原子，并提供了人们可以从对更重的轻子和强子的精确原子测量中提取非QED物理的理解和背景。很少有人敢于尝试在10^12能级的少数几个部分对正电子进行测量，这些测量将允许深入了解物理，如质子电荷半径和更高能级的反冲效应修正，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大约1.234 phz处的1S-2S间隔。上一次是与朱夏莲合作测量的，这个间隔的前10位的知识已经保持了20年，不确定度为±3.2 MHz。拟议的实验实施了一项新技术，该技术将通过单个原子轨迹分析，极大地提高Ps原子光谱学的准确性，并可能提高其他高分辨率光谱学实验的精度，例如在Muonium中。位置敏感的飞行时间探测器将记录每个探测到的原子的轨迹和速度，从而消除二阶多普勒频移、交流斯塔克频移，并更好地解释渡越时间展宽。新的探测方法将允许降低激光强度和被探测原子的速度，这些都是迄今为止限制测量精度的因素。分析和开发分析和减少不确定度的方法的基本特征将使用相对于超稳定参考腔的±2 MHz测量波长计来完成。这种方法将足以在提议的3年内产生窄线宽(~1 MHz)和极大地改善信噪比。通过获取激光频率梳基准，系统效应的测量和探测将以1000倍的精度提高100倍。