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 $K\ensuremath{\alpha}$ 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 $K\ensuremath{\alpha}$ x rays. The sign of the strong-interaction shift was confirmed to be repulsive, with the results $\ensuremath{\Delta}{E}_{1s}=\ensuremath{-}323\ifmmode\pm\else\textpm\fi{}63 (\mathrm{statistical})\ifmmode\pm\else\textpm\fi{}11 (\mathrm{systematic}) \mathrm{eV}$ and ${\ensuremath{\Gamma}}_{1s}=407\ifmmode\pm\else\textpm\fi{}208\ifmmode\pm\else\textpm\fi{}100 \mathrm{eV}.$ 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 $K\ensuremath{\alpha}$ x-ray yield per stopped kaon was estimated to be $1.5\ifmmode\pm\else\textpm\fi{}0.5%$ at a hydrogen target density of $10{\ensuremath{\rho}}_{\mathrm{STP}}.$