Beyond-mean-field approach to low-lying spectra of Λ hypernuclei

Taking the hypernucleus _Λ¹³C as an example, we illustrate the microscopic particle-rotor model for low-lying spectra of hypernuclei. This approach is based on the beyond-mean-field method, with the particle number and angular momentum projections. The quantum fluctuation of the mean-field is also taken into account for the core nucleus using the generator coordinate method. We show that the impurity effect of Λ hyperon, such as a change in B(E2), is well described with this model. Our calculation indicates that the most important impurity effect in sd-shell hypernuclei is a change in a deformation parameter rather than in a nuclear size. (author)