Heavy Quark Symmetry and Heavy-Flavor-Conserving Nonleptonic Weak Decays

Abstract

Recent years have seen intense activities in the field of heavy quark physics, and remarkable progress has been made.1–11 In the limit of infinite heavy quark mass, the strong interactions of a heavy quark become much simplified. Namely, the effective QCD Lagrangian becomes independent of the flavor and spin of the heavy quark. For N f heavy quark flavors, the new spin and flavor symmetries combine to form a SU(2N f ) symmetry group (heavy quark symmetry) which is not manifest in the original Lagrangian. Heavy quark symmetry allows us to predict many properties of heavy hadrons, which are taken to be particles containing a single heavy quark. Of course, even in this infinite heavy quark mass limit, low energy hadronic physics (confinement, etc.) remains non-perturbative and there is still no solution to it. What heavy quark symmetry can do for us is to provide simplifying relations among the static and transition properties of the heavy hadrons. For instance, the spin-flavor symmetry implies that the excitation spectrum and transition form factors of a heavy hadron do not depend on the spin and flavor of the heavy quark involved. Thus, in the symmetry limit, the heavy quark behaves like a static color source. This is analogous to the well known case of a hydrogen-like atom, whose excitation spectrum and transition form factors are independent of the mass and spin of its heavy nucleus, which acts as a static charge source.