Bound-state QED test above the Schwinger limit with kaonic fluorine
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 DANE, corresponding to an integrated luminosity of 22.4 pb, 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 4.7 (stat.) 5.5 (syst.) eV and a 9 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.
Keywords
Cite
@article{arxiv.2604.19387,
title = {Bound-state QED test above the Schwinger limit with kaonic fluorine},
author = {F. Clozza and S. Manti and F. Sgaramella and L. Abbene and F. Artibani and M. Bazzi and G. Borghi and D. Bosnar and M. Bragadireanu and A. Buttacavoli and M. Carminati and A. Clozza and L. De Paolis and R. Del Grande and K. Dulski and C. Fiorini and I. Friščić and C. Guaraldo and M. A. Iliescu and P. Indelicato and M. Iwasaki and A. Khreptak and J. Marton and P. Moskal and H. Ohnishi and K. Piscicchia and F. Principato and A. Scordo and M. Silarski and D. Sirghi and F. Sirghi and M. Skurzok and J. Sommerfeldt and A. Spallone and K. Toho and O. Vazquez Doce and J. Zmeskal and C. Curceanu},
journal= {arXiv preprint arXiv:2604.19387},
year = {2026}
}