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

F. Clozza; S. Manti; F. Sgaramella; L. Abbene; F. Artibani; M. Bazzi; G. Borghi; D. Bosnar; M. Bragadireanu; A. Buttacavoli; M. Carminati; A. Clozza; L. De Paolis; R. Del Grande; K. Dulski; C. Fiorini; I. Friščić; C. Guaraldo; M. A. Iliescu; P. Indelicato; M. Iwasaki; A. Khreptak; J. Marton; P. Moskal; H. Ohnishi; K. Piscicchia; F. Principato; A. Scordo; M. Silarski; D. Sirghi; F. Sirghi; M. Skurzok; J. Sommerfeldt; A. Spallone; K. Toho; O. Vazquez Doce; J. Zmeskal; C. Curceanu

arXiv · 2604.19387

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

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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.

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F. Clozza, S. Manti, F. Sgaramella, L. Abbene, F. Artibani, M. Bazzi, G. Borghi, D. Bosnar, M. Bragadireanu, A. Buttacavoli, M. Carminati, A. Clozza, L. De Paolis, R. Del Grande, K. Dulski, C. Fiorini, I. Friščić, C. Guaraldo, M. A. Iliescu, P. Indelicato, M. Iwasaki, A. Khreptak, J. Marton, P. Moskal, H. Ohnishi, K. Piscicchia, F. Principato, A. Scordo, M. Silarski, D. Sirghi, F. Sirghi, M. Skurzok, J. Sommerfeldt, A. Spallone, K. Toho, O. Vazquez Doce, J. Zmeskal, C. Curceanu. 2026-04-21. Bound-state QED test above the Schwinger limit with kaonic fluorine. https://arxiv.org/abs/2604.19387

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