_Λ¹³C hypernucleus studied with the ¹³C(K^-,π^-γ) reaction

The _Λ¹³C hypernucleus was studied by measuring γ rays in coincidence with the ¹³C(K^-,π^-) reaction. γ 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⁺)⊗p_Λ], to the ground state were measured. The splitting of the states was found to be ΔE(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_Λ=l_Λ-1/2[(p_1/2)_Λ] 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) and 10.830±0.031(stat)±0.056(syst) MeV, respectively. We also observed γ rays from the 3/2⁺ state, which has a [¹²C(2⁺)⊗s_Λ] configuration, to the ground state in _Λ¹³C. The excitation energy of the 3/2⁺ state was obtained as 4.880±0.010(stat)±0.017(syst) MeV. Nuclear γ rays with energies of 4.438 and 15.100 MeV had similar yields, which suggests that a quasifree knockout of a Λ particle is dominant in highly excited regions.