Precision Microwave Spectroscopy of Positronium

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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.