Structure of Λ20Ne hyper nucleus

Advanced studies on the hyper nucleus have revealed various new phenomena that are caused by hyperons. The present report describes some new phenomena found in the Λ20Ne (= 19Ne+Λ) system. The positive parity of 19Ne is associated with a shell like structure of (sd)3. A state of negative parity appears at a very low energy of 0.28 MeV. This is attributed to the development of the α + 15O cluster structure. If Λ is added to this system, the ground state is at the l-1 level though Λ enters the S1/2 orbit. The present approach is based on an orthogonal condition model that combines the two channels of (3He + 16O + Λ) and (α + 15O + Λ). The model space is represented by the product of 19Ne and Λ. Investigation shows that changes in the distance between 3He + 16O clusters are small at positive parity. Another investigation is made on experimental approaches to demonstrating the negative-parity ground state. In (K-, π-) reaction, strong excitation occurs at low parity because of small momentum transfer, while the momentum transfer is large in (π-, K+) reaction, leading to strong excitation at positive parity. Another approach uses the decay of Λ20Ne to study the stability of the ground state. The delayed γ-decay strongly reflects the parity of the ground state. (Nogami, K.)