Observation of kaonic hydrogen atom x rays

We have measured the {ital K}-series x rays from kaonic hydrogen atoms and have succeeded for the first time in observing a distinct K{alpha} peak. The {ital 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{sub 1s}={minus}323{plus_minus}63 (statistical){plus_minus}11 (systematic) eV and {Gamma}{sub 1s}=407{plus_minus}208{plus_minus}100 eV. The measured shift and width of the 1s state are consistent with those expected from other low energy {bar 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{plus_minus}0.5{percent} at a hydrogen target density of 10{rho}{sub STP}. {copyright} {ital 1998} {ital The American Physical Society}