Precision spectroscopy of positronium: Testing bound-state QED theory and the search for physics beyond the Standard Model

Positronium (Ps) is an exotic hydrogenic atom composed of an electron bound to a positron via the Coulomb force. Being composed of two low-mass leptons, positronium is, for all practical purposes, fully described by quantum electrodynamics (QED). The absence of hadronic components suggests that positronium energy levels and decay rates can be calculated to very high precision, limited only by the order of the corresponding perturbative expansion and the tiny effects of heavy or weakly interacting virtual particles and exotic decay modes. Moreover, as it is a low-mass particle-antiparticle system, the QED description of positronium is strongly affected by annihilation and recoil effects that are either weaker or not present in other atoms. As a result, sufficiently precise measurements of Ps energy levels and decay properties can serve as stringent tests of bound-state QED theory, and may be sensitive to processes not present in the theory, such as axion-like particles (beyond the QCD axion), or a fifth fundamental force. In addition, since positronium is an eigenstate of the fundamental symmetries C and P, various symmetry violating mechanisms can be probed through searches for anomalous decay modes. In the last three decades, there have been significant experimental advances in positron and positronium physics which open up the possibility to test QED bound-state theory with unprecedented precision. Here we present the current state-of-the-art in experimental positronium spectroscopy, and discuss explicitly how such measurements can be used to test bound-state QED theory, and how such tests may contribute to the search for physics beyond the Standard Model. (c) 2022 Elsevier B.V. All rights reserved.

正电子素（Ps）是一种奇异的类氢原子，由一个电子通过库仑力与一个正电子结合而成。由于正电子素由两个低质量的轻子组成，在所有实际应用中，它完全可以用量子电动力学（QED）来描述。由于没有强子成分，这表明正电子素的能级和衰变率可以计算到非常高的精度，其精度仅受相应微扰展开的阶数以及重的或弱相互作用的虚粒子和奇异衰变模式的微小影响所限制。此外，由于它是一个低质量的粒子 - 反粒子系统，正电子素的量子电动力学描述受到湮灭和反冲效应的强烈影响，而这些效应在其他原子中要么较弱，要么不存在。因此，对正电子素能级和衰变特性进行足够精确的测量可以作为对束缚态量子电动力学理论的严格检验，并且可能对该理论中未包含的过程敏感，例如类轴子粒子（超出量子色动力学轴子）或第五种基本力。此外，由于正电子素是基本对称性C和P的本征态，通过寻找异常衰变模式可以探测各种对称性破缺机制。在过去的三十年中，正电子和正电子素物理学在实验方面取得了重大进展，这为以前所未有的精度检验量子电动力学束缚态理论提供了可能性。在这里，我们介绍了实验正电子素光谱学的当前最新技术水平，并明确讨论了如何利用这些测量来检验束缚态量子电动力学理论，以及这些检验如何有助于寻找超出标准模型的物理学。（c）2022爱思唯尔公司。保留所有权利。