The kaonic 3He and 4He X-rays emitted in the 3d-2p transitions were measured in the SIDDHARTA experiment. The widths of the kaonic 3He and 4He 2p states were determined to be Gamma_2p(3He) = 6 pm 6 (stat.) pm 7 (syst.) eV, and Gamma_2p(4He) = 14 pm 8
(stat.) pm 5 (syst.) eV, respectively. Both results are consistent with the theoretical predictions. The width of kaonic 4He is much smaller than the value of 55 pm 34 eV determined by the experiments performed in the 70s and 80s, while the width of kaonic 3He was determined for the first time.
The first observation of the kaonic 3He 3d - 2p transition was made using slow K- mesons stopped in a gaseous 3He target. The kaonic atom X-rays were detected with large-area silicon drift detectors using the timing information of the K+K- pairs of p
hi-meson decays produced by the DAFNE e+e- collider. The strong interaction shift of the kaonic 3He 2p state was determined to be -2+-2 (stat)+-4 (syst) eV.
