γ transitions in A = 7 hypernuclei and a possible derivation of hypernuclear size

On the basis of the ⁵_ΛHe + N + N three-body model which has successfully been applied to a systematic study of the energy and nucleon halo structure of the A = 7 isotriplet hypernuclei, strengths of γ transitions in ⁷_ΛLi and ⁷_ΛHe are calculated. The new model confirms that the B(E2;5/2⁺→1/2⁺) value in ⁷_ΛLi is reduced remarkably in comparison with the corresponding B(E2;3⁺→1⁺) in the core nucleus ⁶Li. This is due to the gluelike role of the Λ particle which induces a contraction of the core nuclear size. It is suggested that a measurement of the 5/2⁺→1/2⁺E2 transition rate in ⁷_ΛLi (ongoing at KEK as E419) provides a unique opportunity to derive the hypernuclear size and hence to confirm the size contraction experimentally. The E2 and M1 transition strengths are also predicted for low-lying states in the hypernucleus ⁷_ΛHe whose core nucleus ⁶He is known to have a neutron halo. Another prediction is made of much enhanced E2 transitions in ⁷_ΛLi from the 5/2⁺ and 3/2⁺(T=1) states which are expected to have a proton halo structure.