Hypernuclear single particle spectra based on in-medium chiral SU(3) dynamics

Abstract

A previously derived relativistic energy density functional for nuclei, based on low-energy in-medium chiral dynamics, is generalized to implement constraints from chiral SU(3) effective field theory and applied to $Lambda$ hypernuclei. Density-dependent central and spin-orbit mean fields are calculated for a $Lambda$ hyperon using the SU(3) extension of in-medium chiral perturbation theory to two-loop order. Long range $Lambda N$ interactions arise from kaon-exchange and from two-pion-exchange with a $Sigma$ hyperon in the intermediate state. Short-distance dynamics is encoded in contact interactions. They include scalar and vector mean fields reflecting in-medium changes of quark condensates, constrained by QCD sum rules. The $Lambda$ single particle orbitals are computed for a series of hypernuclei from $^{13}_{Lambda}$C to $^{208}_{Lambda}$Pb. The role of a surface (derivative) term is studied. Its strength is found to be compatible with a corresponding estimate from in-medium chiral perturbation theory. Very good agreement with hypernuclear spectroscopic data is achieved. The smallness of the $Lambda$-nuclear spin-orbit interaction finds a natural explanation in terms of an almost complete cancellation between short-range scalar/vector contributions and longer range terms generated by two-pion exchange.