Kaonic mass by critical absorption of kaonic-atom x rays

The energy of x rays from the transition 6h 5g in kaonic atoms of potassium falls on the K absorption edge of erbium. Measurement of the kaonic-x-ray attenuation in a precisely calibrated set of Er foils yields the x-ray energy 57 458.8 6.3 eV. The kaon mass is related to energy through the Klein-Gordon equation plus corrections for radiative effects, electron screening, and other effects. The negative-kaon mass was found to be 493.640 0.054 MeV/c2 in agreement with the currently accepted value 493.669 0.018 MeV/c2 which was determined from x rays emitted by high-Z atoms where the corrections were larger than for Z=19.