First Kaonic Boron Isotopes Measurements with SIDDHARTA-2 at DA$\Phi$NE

Abstract

A precision measurement of X-ray transitions in kaonic boron, performed by the SIDDHARTA-2 collaboration at the DA$\Phi$NE collider, is reported. The energies and yields of the $5g\to 4f$ and $4f\to 3d$ transitions were determined for both boron isotopes, kaonic ${}^{10}$B and kaonic ${}^{11}$B. For the $5g\to 4f$ transition, the measured energies are $7064.62\pm 16.93(stat.)\pm 2.00(sys.)$~eV for kaonic ${}^{11}$B and $6920.96\pm 58.23(stat.)\pm 2.00(sys.)$~eV for kaonic ${}^{10}$B. For the $4f\to 3d$ transition, the corresponding values are $15293.33\pm 4.80(stat.)\pm 5.30(sys.)$~eV and $15180.11\pm 20.86(stat.)\pm 5.30(sys.)$~eV, respectively. The yields for the $5g\to 4f$ transition are $0.076\pm 0.013(stat.{)}_{-0.011}^{+0.012}(sys.)$ for kaonic ${}^{11}$B and $0.079\pm 0.014(stat.){}_{-0.011}^{+0.013}(sys.)$ for kaonic ${}^{10}$B. For the $4f\to 3d$ transition, the corresponding yields are $0.115\pm 0.006(stat.){}_{-0.005}^{+0.002}(sys.)$ and $0.107\pm 0.007(stat.){}_{-0.005}^{+0.002}(sys.)$, respectively. No statistically significant deviation from pure electromagnetic (QED) calculations was observed in the measurement of the $4f\to 3d$ X-ray transition in kaonic ${}^{11}$B. Interpreted as upper limits, these results impose stringent constraints on the strong-interaction energy shift and width of the 3d level in light nuclei. Translating these limits into bounds on phenomenological kaon-nucleus optical potentials, and, within specific theoretical models, on the complex scattering amplitude, we constrain and disfavor scenarios predicting large shifts or widths in boron.