Precision spectroscopy of Rydberg Positronium

氢是量子物理学的试验场，一直是激烈实验的主题，为QED理论提供了非常严格的测试。这个基本系统的光谱学是如此先进，以至于现在它受到我们对质子结构的不完整知识的限制，这不是由QED控制的，并且不能以足够的精度计算。为了尝试和改进质子结构的测量，人们用μ子氢进行了一些引人注目的实验：这是一个氢原子中的电子被μ子取代的系统。虽然μ子本质上是不稳定的，在几微秒内就会衰变，但它们可以作为质子电荷半径的优秀探针，因为它们的质量是电子的200倍以上。这些实验[见R。Pohl等人，“The size of the proton”，Nature，466 213（2010）]已经非常成功，提供了质子尺寸的高度精确的测量。然而，这种改进的精度揭示了与以前的氢光谱和电子-质子散射测量的差异，尚未得到解释，被称为“质子半径之谜”。该项目旨在从不同的角度解决这个问题：而不是用重μ子取代氢电子，从而增加质子大小的影响，我们将通过用正电子取代质子来完全消除这种影响。这种正电子-电子结合态被称为正电子素，尽管它本质上是不稳定的，并且容易自我湮灭，但它可以通过微波和光谱学进行研究。为了以足够的精度做到这一点，我们需要使用激光将Ps原子置于高度激发的里德伯态，以防止自湮灭，从而显着延长其寿命。此外，激发里德堡态的运动可以通过外部电场控制，由于它们的大偶极矩，使得有可能产生一个缓慢，长寿命，并集中Ps原子束。有了一束寿命很长的原子，我们可以直接测量它们在地球引力场中的下落。这将有助于回答这个问题：反物质与物质的下落不同吗？如果答案不是“不”，那么对我们现有的物理理论将产生深远的影响。同样，如果质子半径之谜没有一个简单的解释，它也可能是一个令人兴奋的新物理学的标志。无论这些问题的答案是什么，这个项目都将使用涵盖原子和电子偶素物理学几个领域的尖端方法来解决它们，并且还需要沿着发展许多新技术。

Hydrogen is the testing ground for quantum physics and has been the subject of intense experimentation, providing very stringent tests of QED theory. Spectroscopy of this fundamental system is so advanced that it is now limited by our incomplete knowledge of the structure of the proton, which is not governed by QED and cannot be calculated with sufficient accuracy. In order to try and improve measurements of the proton structure some remarkable experiments were conducted using muonic hydrogen: this is a system in which the electron in a hydrogen atom is replaced with a muon. Although muons are intrinsically unstable and decay in a few microseconds, they can act as excellent probes of the charge radius of the proton because they are over 200 times more massive than electrons. These experiments [see R. Pohl, et al. "The size of the proton", Nature, 466 213 (2010)] have been very successful, providing a highly accurate measurement of the proton size. However, this improved precision has revealed a discrepancy with previous hydrogen spectroscopy and electron-proton scattering measurements that has not yet been explained, and is known as the "proton radius puzzle". This project aims to address the problem from a different perspective: instead of replacing the hydrogenic electron with a heavy muon, and thereby increasing the effect of the proton size, we will completely eliminate the effect by replacing the proton with a positron. This positron-electron bound state is known as positronium and, even though it is intrinsically unstable and prone to self-annihilation, it can be studied via microwave and optical spectroscopy. In order to do this with sufficient precision we will need to use lasers to put the Ps atoms into highly-excited Rydberg states which prevents self-annihilation, significantly extending their lifetimes. Moreover, the motion of excited Rydberg states can be controlled via external electric fields owing to their large dipole moments, making it possible to generate a slow, long-lived, and focused Ps atom beam. With a beam of atoms that live for a long time we can directly measure how they fall in the gravitational field of the earth. This will help answer the question: does antimatter fall differently to matter? If the answer is not "no" there will be profound implications for our existing physical theories. Similarly, if the proton radius puzzle does not have a simple explanation it too may be a sign of exciting new physics. Whatever the answers to these questions are, this project will address them using cutting edge methods covering several areas of atomic and positronium physics, and will also require the development of many new techniques along the way.