Three- and four-body structure of light Λ hypernuclei

On the basis of the three- and four-body structure calculations of ⁴_ΛH, ⁴_ΛHe, ⁹_ΛBe, ¹³_ΛC and ⁷_ΛLi, it is emphasized what is interesting and important from view points of hypernuclear physics and few-body physics. By performing four-body calculation with both 3N + Λ and 3N + Σ channels with realistic NN and YN interactions, we show that the Σ-channel component plays an important role in making ⁴_ΛH and ⁴_ΛHe bound. In the microscopic 2α + Λ (3α + Λ) model for ⁹_ΛBe (¹³_ΛC), we predict the spin-orbit splittings of 5/2⁺₁ - 3/2⁺₁ in ⁹_ΛBe and 3/2^-₁ - 1/2^-₁ in ¹³_ΛC to be 0.08 - 0.16 MeV and 0.39 - 0.78 MeV, respectively, with the use of OBE-model NSC97a ∼ f. On the other hand quark-model ΛN spin-orbit force gives rise to half of the splittings of the smallest OBE-model prediction. On the basis of ⁷_ΛLi = ⁵_Λ He + N + N three-body model, it is suggested that a measurement of the 5/2⁺₁ → 1/2⁺₁ Ε2 transition rate in ⁷_ΛLi provides a unique opportunity to derive the hypernuclear size for the first time and hence to confirm the size contraction experimentally.