Collective excitations of Λ hypernuclei

We discuss low-lying collective excitations of Λ hypernuclei using the self-consistent mean-field approaches. We first discuss the deformation properties of Λ hypernuclei in the sd-shell region. Based on the relativistic mean-field (RMF) approach, we show that the oblate deformation for 28Si may disappear when a Λ particle is added to this nucleus. We then discuss the rotational excitations of MgΛ25 using the three-dimensional potential energy surface in the deformation plane obtained with the Skyrme-Hartree-Fock method. The deformation of MgΛ25 is predicted to be slightly reduced due to an addition of a Λ particle. We demonstrate that this leads to a reduction of electromagnetic transition probability, B(E2), in the ground state rotational band. We also present an application of random phase approximation (RPA) to hypernuclei, and show that a new dipole mode, which we call a soft dipole Λ mode, appears in hypernuclei, which can be interpreted as an oscillation of the Λ particle against the core nucleus.