Mean-field theory of baryonic matter for QCD in the largeNcand heavy quark mass limits

Abstract

We discuss theoretical issues pertaining to baryonic matter in the combined heavy-quark and large ${N}_{c}$ limits of QCD. Witten's classic argument that baryons and interacting systems of baryons can be described in a mean-field approximation with each of the quarks moving in an average potential due to the remaining quarks is heuristic. It is important to justify this heuristic description for the case of baryonic matter since systems of interacting baryons are intrinsically more complicated than single baryons due to the possibility of hidden color states---states in which the subsystems making up the entire baryon crystal are not color-singlet nucleons but rather colorful states coupled together to make a color-singlet state. In this work, we provide a formal justification of this heuristic prescription. In order to do this, we start by taking the heavy quark limit, thus effectively reducing the problem to a many-body quantum mechanical system. This problem can be formulated in terms of integrals over coherent states, which for this problem are simple Slater determinants. We show that for the many-body problem, the support region for these integrals becomes narrow at large ${N}_{c}$, yielding an energy which is well approximated by a single coherent state---that is a mean-field description. Corrections to the energy are of relative order $1/{N}_{c}$. While hidden color states are present in the exact state of the heavy quark system, they only influence the interaction energy below leading order in $1/{N}_{c}$.