Precision X-ray Spectroscopy with Exotic Atoms to Probe QED and Nuclear Structure

Exotic atoms, in which electrons are replaced by a heavier negatively charged particle, provide a unique and sensitive laboratory for exploring the frontiers of fundamental physics. Because of the presence of heavier bound particles, such as a muon or antiproton, their electromagnetic interactions with the nucleus are significantly enhanced (due to the inverse dependence on the Bohr radius), allowing precision spectroscopy to probe effects beyond the reach of ordinary atoms. A key technological advancement enabling such measurements is the use of the novel cryogenic microcalorimeters. These detectors offer a great combination of energy resolution and detection efficiency, overcoming limitations of conventional semiconductors and crystal detectors and thus opening new paths for precision X-ray spectroscopy measurement.

In this context, I will present two complementary experimental initiatives, \textbf{QUARTET} and \textbf{PAX}. The QUARTET experiment, conducted at PSI, focuses on X-ray spectroscopy of muonic atoms to determine the nuclear charge radii of light elements (Z = 3–10), achieving an order of magnitude improvement in accuracy over previous results. In parallel, the newly launched PAX experiment aims to test strong-field bound-state quantum electrodynamics (QED) by measuring high-circular Rydberg transition energies in antiprotonic atoms (9 < Z < 83). These circular states are minimally affected by nuclear structure, allowing precise measurements that isolate higher-order QED effects.