Nuclear models and the hypernuclear interaction

We present a first calculation of the potential and spin-orbit coupling for hypernuclei in the framework of a nuclear-potential model based on a spontaneous collective (superradiant) pion-nucleon-delta interaction. A perturbative evaluation of the propagation of a hyperon in such a coherent medium leads to an effective hypernuclear potential of the same shape and roughly half the depth (i.e -30 MeV) of the standard nuclear potential and a negligible spin-orbit coupling for both Lambda and Sigma hypernuclei. Such results are in complete agreement with existing experimental data. Moreover, in this framework, we are able to explain the surprisingly narrow decay widths of the Sigma hypernuclei.