Observation of Kaonic Hydrogen $K_{\alpha }$ X Rays

We have measured the shift and width of the kaonic hydrogen $1s$ state due to the $\overline{K}N$ strong interaction. We have observed, for the first time, distinct $K$-series kaonic hydrogen x rays with good signal-to-noise ratio in the energy spectrum. The measured energy shift and width were determined to be $\Delta E\left(1s\right)=-323\mathrm{}\pm 63\left(\mathrm{stat}\right)\pm 11\left(\mathrm{syst}\right)\mathrm{eV}$ (repulsive) and $\gamma \left(1s\right)=407\mathrm{}\pm 208\left(\mathrm{stat}\right)\pm 100\left(\mathrm{syst}\right)\mathrm{eV}$, respectively.