PAX · antiProtonic Atom X-ray spectroscopy

Numerous experimental observations have shown that the Standard Model is not complete. Precision measurements in quantum systems are one of the privileged frontiers for searching for new physics, as new particles may couple to atoms, provoking tiny changes in atomic structure that can be measured with state-of-the-art methods. Such searches are founded on an accurate understanding of quantum electrodynamics (QED), the field theory that describes the interaction between light and charged particles. While QED is well understood for light systems like the hydrogen atom where agreement between theory and experiment have been achieved up to third-order interactions with the quantum vacuum, for high-Z atoms in the strong Coulomb field regime, the theory remains untested beyond first-order interactions. This is due to both experimental complications, and theoretical uncertainties linked to unknown nuclear properties. I propose a new approach for testing strong-field QED via the x-ray spectroscopy of antiprotonic atoms. In these systems, orders of magnitude higher Coulomb fields can be obtained, acting like a magnifying glass for QED effects that become easier to measure. Using transitions between Rydberg states, uncertainties from nuclear properties can be avoided and two orders of magnitude sensitivity can be gained with respect to the best current experiments, making testing strong-field QED finally possible for a broad range of atomic species.The realization of this project relies on the novel combination of two new technologies: slow antiproton beams at CERN, and quantum sensing x-ray detectors. The compatibility of these two requires new developments that will lead to a dedicated precision x-ray spectroscopy platform for antiprotonic atoms, with transverse applications beyond QED in nuclear and new physics searches.