Precision spectroscopy of Kaonic helium‐3 and helium‐4 3d→2p X‐rays

Recently, the shift of the kaonic helium‐4 2p state was precisely determined by the E570 and SIDDHARTA experiments. Prior to the experiment by E570, the average of three earlier experimental results showed −43±8 eV, while most of the theoretical calculations give ∼0 eV. This five‐sigma discrepancy between theory and experiment was known as the “kaonic helium puzzle”. A recent theoretical model showed a possible resonance‐like shift of maximum 10 eV for a certain value of a deep antikaon‐nucleon interaction potential, which is different in helium 3 and helium 4.The E570 experiment determined the shift of the kaonic helium‐4 2p state as +2±2  (stat)±2 (sys) eV in 2007. The SIDDHARTA experiment determined the shift as 0±6(stat)±2(sys) eV in 2009. The results of these experiments resolved the long‐standing puzzle. A new experiment of the kaonic helium‐3 X‐ray measurement is being prepared by the J‐PARC E17 collaborators, and the kaonic helium‐3 X‐ray data taken very recently by the SIDDHARTA experiment are on...