Hypernuclei and in-medium chiral dynamics

Abstract

A recently introduced relativistic nuclear energy density functional, constrained by features of low-energy QCD, is extended to describe the structure of hypernuclei. The density-dependent mean field and the spin-orbit potential of a $\Lambda$-hyperon in a nucleus, are consistently calculated using the SU(3) extension of in-medium chiral perturbation theory. The leading long-range $\Lambda N$ interaction arises from kaon-exchange and $2\pi$-exchange with a $\Sigma$-hyperon in the intermediate state. Scalar and vector mean fields, originating from in-medium changes of the quark condensates, produce a sizeable {\em short-range} spin-orbit interaction. The model, when applied to oxygen as a test case, provides a natural explanation for the smallness of the effective $\Lambda$ spin-orbit potential: an almost complete cancellation between the background contributions (scalar and vector) and the long-range terms generated by two-pion exchange.Comment: 8 pages and 6 figures. To appear in the proceedings of the Theoretical nuclear physics school "Exotic Nuclei: New Challenges", Les Houches (France) May 7-18, 200