Precision spectroscopy of Rydberg Positronium

氢是量子物理的试验场，一直是密集实验的对象，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，以至于它现在受到我们对质子结构的不完全了解的限制，这些结构不受QED的支配，也不能足够准确地计算。为了尝试和改进对质子结构的测量，使用介子氢进行了一些引人注目的实验：这是一个用介子取代氢原子中的电子的系统。尽管µ子本质上是不稳定的，会在几微秒内衰变，但它们可以作为质子电荷半径的极佳探测器，因为它们的质量是电子的200多倍。这些实验[见R.Pohl等人。“质子的大小”，《自然》，466213(2010)]非常成功，为质子大小提供了高度准确的测量。然而，这种提高的精度揭示了与以前的氢谱和电子-质子散射测量的差异，这一差异尚未得到解释，因此被称为“质子半径之谜”。这个项目旨在从不同的角度解决这个问题：我们将用正电子取代质子，而不是用重缪子取代类氢电子，从而增加质子大小的影响，从而完全消除这种影响。这种正电子束缚状态被称为正电子，尽管它本质上是不稳定的，容易自湮灭，但它可以通过微波和光学光谱学来研究。为了以足够的精度做到这一点，我们需要使用激光将Ps原子置于高激发的里德堡态，这可以防止自湮灭，大大延长它们的寿命。此外，激发里德堡态的运动可以通过外加电场来控制，因为它们有很大的偶极矩，这使得产生慢的、长寿命的、聚焦的Ps原子束成为可能。用一束寿命很长的原子束，我们可以直接测量它们如何落在地球引力场中。这将有助于回答这个问题：反物质落在物质上的方式是否不同？如果答案不是“不”，将对我们现有的物理理论产生深远的影响。同样，如果质子半径之谜没有一个简单的解释，那么它也可能是一个令人兴奋的新物理学的迹象。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正电子物理几个领域的尖端方法来解决这些问题，同时还需要开发许多新技术。

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.