The Heavy Quark Expansion of Qcd

These lectures contain an elementary introduction to heavy quark symmetry and the heavy quark expansion. Applications such as the expansion of heavy meson decay constants and the treatment of inclusive and exclusive semileptonic B decays are included. Heavy hadron production via nonperturbative fragmentation processes is also discussed. 1 Heavy Quark Symmetry In these lectures I will introduce the ideas of heavy quark symmetry and the heavy quark limit, which exploit the simpli cation of certain aspects of QCD for in nite quark mass, mQ ! 1. We will see that while these ideas are extraordinarily simple from a physical point of view, they are of enormous practical utility in the study of the phenomenology of bottom and charmed hadrons. One reason for this is the existence not just of an interesting new limit of QCD, but of a systematic expansion about this limit. The technology of this expansion is the Heavy Quark E ective Theory (HQET), which allows one to use heavy quark symmetry to make accurate predictions of the properties and behavior of heavy hadrons in which the theoretical errors are under control. While the emphasis in these lectures will be on the physical picture of heavy hadrons which emerges in the heavy quark limit, it will be important to introduce enough of the formalism of the HQET to reveal the structure of the heavy quark expansion as a simultaneous expansion in powers of QCD=mQ and s(mQ). However, what I hope to leave you with above all is an appreciation for the simplicity, elegance and coherence of the ideas which underlie the technical results which will be presented. The interested reader is also encouraged to consult a number of excellent reviews, which typically cover in more detail the material in the rst two sections of these lectures.