Precision measurement of the 3d → 2p x-ray energy in kaonic 4He

Abstract We have measured the Balmer-series x-rays of kaonic 4 He atoms using novel large-area silicon drift x-ray detectors in order to study the low-energy K ¯ -nucleus strong interaction. The energy of the 3 d → 2 p transition was determined to be 6467 ± 3 ( stat ) ± 2 ( syst ) eV . The resulting strong-interaction energy-level shift is in agreement with theoretical calculations, thus eliminating a long-standing discrepancy between theory and experiment.