Kaonic 3He and 4He X-ray measurements in SIDDHARTA

The SIDDHARTA experiment measured the kaonic 3He and 4He 3d →2p X-ray transitions using gaseous targets for the first time. The strong-interaction shift both of the kaonic 3He and 4He 2p states was determined with a precision of a few eV. The shift of kaonic 4He is much smaller than the values obtained in the experiments performed in 70’s and 80’s, while it is consistent with the recent result of the KEK E570 experiment, as well as the theoretical calculated values. Therefore, the problem on kaonic helium (the “kaonic helium puzzle”) was definitely solved. The first observation of the kaonic 3He X-rays was also achieved in the SIDDHARTA experiment. The shift both of kaonic 3He and 4He was found to be as small as a few eV.