Strong-interaction shifts and widths of kaonic helium isotopes

The kaonic ³He and ⁴He [Formula: see text] transitions in gaseous targets were observed by the SIDDHARTA experiment. The X-ray energies of these transitions were measured with large-area silicon-drift detectors using the timing information of the [Formula: see text] pairs produced by the DAΦNE [Formula: see text] collider. The strong-interaction shifts and widths both of the kaonic ³He and ⁴He 2p states were determined, which are much smaller than the results obtained by the previous experiments. The "kaonic helium puzzle" (a discrepancy between theory and experiment) was now resolved.