First Kaonic Boron Isotopes Measurements with SIDDHARTA-2 at DA$Φ$NE
A precision measurement of X-ray transitions in kaonic boron, performed by the SIDDHARTA-2 collaboration at the DA$Φ$NE collider, is reported. The energies and yields of the $5g\rightarrow4f$ and $4f\rightarrow3d$ transitions were determined for both boron isotopes, kaonic ${}^{10}$B and kaonic ${}^{11}$B. For the $5g\rightarrow4f$ transition, the measured energies are $7064.62 \pm 16.93~(\mathrm{stat.}) \pm 2.00~(\mathrm{sys.})$~eV for kaonic ${}^{11}$B and $6920.96 \pm 58.23~(\mathrm{stat.}) \pm 2.00~(\mathrm{sys.})$~eV for kaonic ${}^{10}$B. For the $4f\rightarrow3d$ transition, the corresponding values are $15293.33 \pm 4.80~(\mathrm{stat.}) \pm 5.30~(\mathrm{sys.})$~eV and $15180.11 \pm 20.86~(\mathrm{stat.}) \pm 5.30~(\mathrm{sys.})$~eV, respectively. The yields for the $5g\rightarrow4f$ transition are $0.076 \pm 0.013~(\mathrm{stat.})^{+0.012}_{-0.011}~(\mathrm{sys.})$ for kaonic ${}^{11}$B and $0.079 \pm 0.014~(\mathrm{stat.})~^{+0.013}_{-0.011}~(\mathrm{sys.})$ for kaonic ${}^{10}$B. For the $4f\rightarrow3d$ transition, the corresponding yields are $0.115 \pm 0.006~(\mathrm{stat.})~^{+0.002}_{-0.005}~(\mathrm{sys.})$ and $0.107\pm 0.007~(\mathrm{stat.})~^{+0.002}_{-0.005}~(\mathrm{sys.})$, respectively. No statistically significant deviation from pure electromagnetic (QED) calculations was observed in the measurement of the $4f\rightarrow3d$ 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.