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 Kα 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 Kα x rays. The sign of the strong-interaction shift was confirmed to be repulsive, with the results ΔE₁ₛ=-323±63 (statistical)±11 (systematic) eV and Γ₁ₛ=407±208±100 eV. The measured shift and width of the $1s$ state are consistent with those expected from other low energy K̄N data, and thus the long-standing kaonic hydrogen puzzle is resolved. The Kα x-ray yield per stopped kaon was estimated to be 1.5±0.5% at a hydrogen target density of 10ρSTP.