The energy spacing between the ground-state spin doublet of 4 Λ He(1 + ,0 + ) was determined to be 1406 ± 2 ± 2 keV, by measuring γ rays for the 1 + → 0 + transition with a high efficiency germanium detector array in coincidence with the 4 He(K − , π − ) 4Λ He reaction at J-PARC. In comparison to the corresponding energy spacing in the mirror hypernucleus 4 Λ H, the present result clearly indicates the existence of charge symmetry breaking (CSB) in ΛN interaction. It is also found that the CSB effect is large in the 0 + ground state but is by one order of magnitude smaller in the 1 + excited state, demonstrating that the ΛN CSB interaction has spin dependence.

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1. Introduction. Whether there exist kaonic nuclei or not is a key issue to make our understandings on theKN interaction in vacuum and in nuclear medium. The information on theKN interaction has been obtained by analyzing low-energyKN scattering data and kaonic-atom X-ray data [1]. The recent measurement of the energy shift and width on kaonic-hydrogen X-ray in high precision by the SIDDHARTA group [2] has contributed a lot [3]. From the theoretical analyses by using these results, it is well known that theKN

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We report on the first observation of γ rays emitted from an sd-shell hypernucleus, _{Λ}^{19}F. The energy spacing between the ground state doublet, 1/2^{+} and 3/2^{+} states, of _{Λ}^{19}F is determined to be 315.5±0.4(stat)_{-0.5}^{+0.6}(syst)  keV by measuring the γ-ray energy of the M1(3/2^{+}→1/2^{+}) transition. In addition, three γ-ray peaks are observed and assigned as E2(5/2^{+}→1/2^{+}), E1(1/2^{-}→1/2^{+}), and E1(1/2^{-}→3/2^{+}) transitions. The excitation energies of the 5/2^{+} and 1/2^{-} states are determined to be 895.2±0.3(stat)±0.5(syst) and 1265.6±1.2(stat)_{-0.5}^{+0.7}(syst)  keV, respectively. It is found that the ground state doublet spacing is well described by theoretical models based on existing s- and p-shell hypernuclear data.