Bound-state QED test above the Schwinger limit with kaonic fluorine

TL;DR

This study uses high-precision x-ray spectroscopy of kaonic fluorine to test bound-state QED in electromagnetic fields exceeding the Schwinger limit, providing a new way to explore extreme quantum electrodynamics effects.

Contribution

First experimental test of bound-state QED in electromagnetic fields above the Schwinger limit using kaonic atoms and high-precision spectroscopy.

Findings

Transition energies agree with Dirac-Fock calculations.

Observed transitions probe fields exceeding the Schwinger limit.

Residual measurement uncertainty on key transition is 5.8 ± 4.7 (stat.) ± 5.5 (syst.) eV.

Abstract

Kaonic atoms, formed when a negatively charged kaon replaces an electron in an atomic orbit, provide access to bound-state quantum electrodynamics (BSQED) in electromagnetic fields far stronger than in ordinary atoms. Here, we report an experimental test of BSQED in a regime where the mean Coulomb field exceeds the Schwinger limit. Using high-precision x-ray spectroscopy of kaonic fluorine with the SIDDHARTA-2 experiment at DA$\Phi$NE, corresponding to an integrated luminosity of 22.4 pb$^{-1}$, we observe transitions involving the 4f and 3d levels, probing field-to-Schwinger-limit ratios of 1.11 and 3.70, respectively. The measured transition energies agree with state-of-the-art Dirac-Fock calculations. In particular, the 5g-4f transition showing a residual of 5.8 $\pm$ 4.7 (stat.) $\pm$ 5.5 (syst.) eV and a $\sim$ 9$\sigma$ sensitivity to QED contributions. These results provide a direct test of BSQED in the strong-field regime of QED above the Schwinger limit, opening a new avenue for precision studies in extreme electromagnetic fields.