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

We have measured the Balmer-series x-rays of kaonic 4He 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 3d→2p 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.