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13LambdaC Hypernucleus Studied with the 13C(K-, pi- gamma) Reaction

The 13LambdaC hypernucleus was studied by measuring gamma rays in coincidence with the 13C(K-, pi-) reaction. gamma rays from the 1/2- and 3/2- states, which are the partners of the spin-orbit doublet states with a predominant configuration of [12C_g.s.(0+) x p_Lambda], to the ground state were measured. The splitting of the states was found to be DeltaE(1/2- - 3/2-) = +152+-54(stat)+-36(syst) keV. This value is 20-30 times smaller than that of single particle states in nuclei around this mass region. The j_Lambda=l_Lambda-1/2 ((p_1/2)_Lambda) state appeared higher in energy, as in normal nuclei. The value gives new insight into the YN interaction. The excitation energies of the 1/2- and 3/2- states were obtained as 10.982+-0.031(stat)+-0.056(syst) MeV and 10.830+-0.031(stat)+-0.056(syst) MeV, respectively. We also observed gamma rays from the 3/2+ state, which has a [12C(2+) x s_Lambda] configuration, to the ground state in 13LambdaC. The excitation energy of the 3/2+ state was obtained as 4.880+-0.010(stat)+-0.017(syst) MeV. Nuclear gamma rays with energies of 4.438 and 15.100 MeV had similar yields, which suggests that a quasi-free knockout of a Lambda particle is dominant in highly excited regions.