Precision Microwave Spectroscopy of Positronium

氢原子仅由一个质子和一个电子组成，因此它是你能在元素周期表上找到的最简单的原子系统，因此在原子物理学中起着特殊的作用。氢在过去扮演着罗塞塔石碑的角色，让我们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Schrödinger方程的解析解，它提供了系统的完整描述，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性质的深刻见解，这些系统无法解析描述。在氢中发现的同样简单性也存在于其他单电子原子（称为氢原子）中，例如只剩下一个电子的离子，或所谓的外来原子：这些系统中的电子或质子已被具有相同电荷的不同粒子所取代。外来原子的例子是介子氢（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扮演着独特的角色，因为它们与非奇异的对应物相比可能具有夸大（或抑制）的性质，但仍然是简单的“单电子”系统，可以适用于理论。本提案中描述的工作涉及使用正电子原子（Ps）进行的光谱实验。Ps在某些方面甚至比氢更基本，因为质子有一个由强核力决定的子结构，因此从基本的角度来看还不能很好地理解。正电子和电子都是轻子，据我们所知，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。湮灭过程的存在以及电子和正电子质量相等意味着必须考虑的Ps和氢的QED过程是完全不同的。因此，Ps是一个“纯”QED系统，对各种量子效应具有自己的灵敏度。由于Ps原则上可以用QED完全描述（因为实际上没有强子效应），因此可以使用该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p的一切，并且我们发现了一些意想不到的东西，这可能是一些新物理学的标志。然而，为了做到这一点，有必要对Ps的性质进行测量，至少要和现有的QED理论一样精确，否则就无法判断是否存在任何新的物理效应。不幸的是，Ps的QED理论在最近几十年里取得了重大进展，而实验却没有。我们的目标是通过在理论水平上对Ps精细结构进行光谱分析来改变这种情况。这将需要实验结果的一个数量级的改进，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.