Observation of kaonic hydrogen atom x rays

We have measured the K-series x rays from kaonic hydrogen atoms and have succeeded for the first time in observing a distinct $K\alpha$ peak. The L x-ray peak has also been observed. The strong-interaction shift and width of the kaonic hydrogen atom $1s$ state were determined from the transition energy and the line width of the $K\alpha$ x rays. The sign of the strong-interaction shift was confirmed to be repulsive, with the results $\Delta E_{1s}=-323\pm 63\hspace{0.5em}\left(\mathrm{statistical}\right)\pm 11\hspace{0.5em}\left(\mathrm{systematic}\right)\hspace{0.5em}\mathrm{eV}$ and ${\Gamma }_{1s}=407\mathrm{}\pm 208\pm 100\hspace{0.5em}\mathrm{eV}.\mathrm{}$ The measured shift and width of the $1s$ state are consistent with those expected from other low energy $\overline{K}N$ data, and thus the long-standing kaonic hydrogen puzzle is resolved. The $K\alpha$ x-ray yield per stopped kaon was estimated to be $1.5\pm 0.5\%$ at a hydrogen target density of $10{\rho }_{\mathrm{STP}}.$