Observation of kaonic hydrogen atom X‐rays

Abstract A special liquid-hydrogen target has been used with a stopping kaon beam in order to search for the X-rays from kaonic hydrogen atoms. A peak at 6.52 ± 0.06 keV, consistent with the unshifted energy of 2p-1s X-rays from K − −p atoms, has been observed with a yield of about 0.1%. The energy shift would imply rather small values for the real K − −p scattering length at threshold.