Fundamental Interactions and Beyond with X-ray Spectroscopy of Exotic Atoms

From dark matter and dark energy, to neutrino oscillations and the lack of antimatter in the universe, there is growing evidence that the Standard Model is incomplete. Tests of Quantum Electrodynamics (QED) with few-electron systems offer a promising avenue for looking for new physics, as QED is the best understood quantum field theory and extremely precise predictions can be obtained for few-electron systems. Unfortunately, despite decades of effort, QED is poorly tested in the regime of strong coulomb fields, precisely the region where new exotic physics may be most visible. This is due to a confluence of difficulties linked to experimental limitations in accelerator-based spectroscopy and nuclear uncertainties. I will present a new paradigm for probing higher-order QED effects using spectroscopy of Rydberg states in exotic atoms, where orders of magnitude stronger field strengths can be achieved while nuclear uncertainties may be neglected [1]. Such tests are now possible due to the advent of quantum sensing detectors and new facilities providing low-energy intense beams of exotic particles for precision physics. I will present first results from experiments with muonic atoms at J-PARC within the context of the HEATES collaboration [2], and discuss a new project for antiprotonic atom spectroscopy at CERN. Then, the paradigm can be flipped upside down, and the same methodology can be used specifically to look at fingerprint of nuclear properties in the atomic structure. The QUARTET collaboration at the Paul Scherrer Institute made first proof-of-principle tests for precision measurements of charge radii in light nuclei with muonic atoms in 2023 and may someday even be sensitive to beyond standard model physics [3]. QUARTET is the first application of Magnetic Microcalorimeter detectors (MMC) to muonic atom spectroscopy, and first results will be presented from the test beam measurements.

[1] N. Paul et al, Physical Review Letters 126, 173001 (2021)

[2] T. Okumura et al, Physical Review Letters 30, 173001 (2023)

[3] B. Ohayon et al, arXiv:2310.03846 (2023)