Precision Microwave Spectroscopy of Positronium

氢原子只由一个质子和一个电子组成：因此，它是你在元素周期表上能找到的最简单的原子系统，因此在原子物理学中扮演着特殊的角色。在过去，氢就像一种罗塞塔石头，让我们能够通过探测它的原子结构来破译量子物理的基本规则。氢原子的这种简单性意味着有可能获得薛定谔方程的解析解，它提供了对系统的完整描述，同时提供了对更复杂系统(即多电子原子和分子)中原子过程的基本量子本质的深刻见解，这些系统无法用解析方式描述。在氢中发现的同样的简单性也存在于其他单电子原子(称为类氢原子)中，例如只剩下一个电子的离子，或者在所谓的奇异原子中：在这些系统中，电子或质子被相同电荷的不同粒子所取代。奇异原子的例子有：介子氢(质子-介子)、介子(电子-反介子)和正电子(电子-正电子)。这些系统在原子物理学中发挥着独特的作用，因为与非奇异的系统相比，它们可能具有夸大(或抑制)的性质，但仍然是简单的“单电子”系统，符合理论。本提案中描述的工作涉及使用正电子(Ps)原子进行的光谱实验。在某些方面，PS甚至比氢更基本，因为质子的子结构由强大的核力决定，因此从根本上不能很好地理解它。正电子和电子都是轻子，就我们所知，它们是点粒子，没有内部结构。这使得Ps成为研究束缚态QED理论的一个很好的系统。湮没过程的存在以及相同的电子和正电子质量意味着，对于Ps和对于氢，必须考虑的QED过程是非常不同的。因此，Ps是一个“纯”的QED系统，它对各种量子效应都很敏感。由于Ps原则上可以完全用QED来描述(因为实际上没有强子效应)，所以有可能利用这个系统来寻找新的物理效应。如果我们认为我们用标准模型理解了关于Ps的一切，我们发现了一些意想不到的东西，这可能是某种新物理学的迹象。然而，为了做到这一点，有必要对Ps的性质进行至少与现有QED理论一样精确的测量，否则就无法判断是否存在任何新的物理效应。不幸的是，近几十年来，Ps的QED理论有了很大的进步，而实验却没有。我们的目标是通过在与理论相称的水平上对Ps精细结构进行光谱分析来改变这种情况。这将需要对实验结果进行数量级的改进，这反过来又需要开发一些新的测量技术。

The hydrogen atom is made of just one proton and one electron: it is therefore the simplest atomic system you can find on the periodic table, and as such plays a special role in atomic physics. Hydrogen has in the past acted as a kind of Rosetta Stone, allowing us to decipher the basic rules of quantum physics by probing its atomic structure. This simplicity of the hydrogen atom means that it is possible to obtain analytic solutions to the Schrödinger equation, which provide a complete description of the system, and at the same time offer deep insights into the fundamental quantum nature of atomic processes in more complex systems (i.e., multi-electron atoms and molecules) that cannot be described analytically. The same simplicity found in hydrogen is also present in other single electron atoms, (known, as hydrogenic atoms) such as ions with only 1 electron remaining, or in so-called exotic atoms: these are systems in which an electron or proton has been replaced with a different particle of the same charge. Examples of exotic atoms are muonic hydrogen (proton-muon), muonium (electron-antimuon) and positronium (electron-positron). These systems play a unique role in atomic physics as they may have exaggerated (or suppressed) properties compared to their non-exotic counterparts, but are nevertheless still simple "single electron" systems that are amenable to theory. The work described in this proposal relates to spectroscopic experiments conducted using positronium (Ps) atoms. Ps is in some ways even more basic than hydrogen, because the proton has a sub-structure that is determined by the strong nuclear force and thus is not well understood from a fundamental perspective. Positrons and electrons are leptons, and to the best of our knowledge they are point particles, with no internal structure. This makes Ps a good system to study bound state QED theory. The presence of annihilation processes and the equal electron and positron mass mean that the QED processes that have to be taken into account for Ps and for hydrogen are quite different. Thus Ps is a "pure" QED system with its own sensitivities to various quantum effects. Since Ps can in principle be fully described by QED (as there effectively are no hadronic effects) it is possible to use this system to search for new physics effects. If we think we understand everything about Ps using the Standard Model, and we discover something unexpected, this could be a sign of some New Physics. In order to do this, however, it is necessary to perform measurements of Ps properties that are at least as precise as the existing QED theory, otherwise one cannot tell if any new physics effects are present. Unfortunately QED theory of Ps has advanced significantly in recent decades, while experiments have not. Our goal is to change this situation by performing spectroscopy of the Ps fine structure at a level commensurate with theory. This will require an order of magnitude improvement in experimental results, which in turn will require the development of some new measurement techniques.