Spin-orbit splitting of 13ΛC hypernucleus studied by the 13C(K-, π-γ) reaction

We performed the 13C(K-,π-γ)13C experiment at 0.93 GeV/c at the AGS of BNL to obtain the ls-splitting of Λ single particle states in 13C with high precision. We succeeded in measuring gamma-rays from the 1/2- and 3/2- states, which have predominantly 12C(0+)xpΛ configuration, to the GS in 13ΛC by using NaI detectors. The splitting was conclusively found to be ΔE(1/2- -3/2-) = +152±54(stat)±36(syst) keV which was almost 1/30 to 1/20 times smaller than that of single particle states in nuclei around this mass region. The excitation energy of the 1/2- and 3/2- states was obtained as 10.982±0.031(stat)±0.056(syst) MeV and 10.830±0.031(stat)±0.056(syst)MeV, respectively. The jΛ=1Λ-1/2((p1/2)Λ) state appeared higher in energy as in normal nuclei, which is consistent with theoretical predictions. This result suggests that the ls-force between a Λ particle and a nucleus is attractive. (S.Y.)