On the production and spectroscopy of Σ hypernuclei

Abstract

The production mechanisms and the spectroscopy of Σ hypernuclei are critically examined, with a view towards understanding the data on the interactions of the Σ with nucleons (N) and nuclei in a consistent framework. Data from Σ − atoms and quasi-free Σ production in the (K −, π ±) reactions are used to derive information on the Σ-nucleus well depth. Starting from several parametrization of the ΣN effective interaction V ΣN in the nucleus, we perform a series of shell model calculations of the energy spectrum of the p-shell Σ hypernuclei. The models for V ΣN are motivated by successful one-boson exchange (OBE) descriptions of the free space ΣN scattering data. Certain levels of the Σ hypernucleaus reflect the coherent energy shifts due to the strong spin and isopin dependence of V ΣN. We isolate optimum experimental cases for the study of such coherent effects as well as other components of V ΣN, for instance the spin-orbit part. If the single-particle Σ-nucleus potential is indeed deep enough to support a spectrum of quasi-stable states, which is not clear experimentally, their width in some cases depends sensitively on the isospin purity and the degree of configuration mixing induced by V ΣN. The production cross sections of selected Σ hypernuclear states in the (K −, π ±) reactions are estimated in distorted wave Born approximation (DWBA); these are also sensitive to deviations of the wave functions from pure isospin and/or j-j coupling limits. On the basis of our results, we suggest several experiments which may resolve some of the open questions concerning the ΣN and Σ-nucleus interactions.