The strong-interaction shift of kaonic 3He and 4He 2p states was measured using gaseous targets for the first time in the SIDDHARTA experiment. The determined shift of kaonic 4He is much smaller than the values obtained in the experiments performed in 70's and 80's. Thus, the problems in kaonic helium (the "kaonic helium puzzle") was definitely solved by our measurements. The first observation of the kaonic 3He X-rays was also achieved. The shift both of kaonic 3He and 4He was found to be as small as a few eV.
@article{arxiv.1108.5612,
title = {Kaonic 3He and 4He X-ray measurements in SIDDHARTA},
author = {T. Ishiwatari and M. Bazzi and G. Beer and C. Berucci and L. Bombelli and A. M. Bragadireanu and M. Cargnelli and A. Clozza and G. Corradi and C. Curceanu and A. d'Uffizi and C. Fiorini and F. Ghio and B. Girolami and C. Guaraldo and R. S. Hayano and M. Iliescu and M. Iwasaki and P. Kienle and P. Levi Sandri and V. Lucherini and J. Marton and S. Okada and D. Pietreanu and K. Piscicchia and M. Poli Lener and T. Ponta and R. Quaglia and A. Rizzo and A. Romero Vidal and A. Scordo and H. Shi and D. L. Sirghi and F. Sirghi and H. Tatsuno and A. Tudorache and V. Tudorache and O. Vazquez Doce and E. Widmann and B. Wuenschek and J. Zmeskal},
journal= {arXiv preprint arXiv:1108.5612},
year = {2015}
}