Heavy Hadron Spectroscopy

After having played a major role in the foundation of QCD, heavy hadron spectroscopy has witnessed in the last years a renewal of interest led by the many new data coming from the B factories, CLEO and the Tevatron and by the progress made in the theoretical methods. I will summarize the former and mostly focus on the latter. Much of the theoretical progress in the physics of heavy hadrons comes from effective field theories (EFTs) and lattice gauge theories. For most of these systems, they allow systematic treatments, which may be used to gain control over one of the most elusive sectors of the Standard Model, low-energy QCD, even in one of its most spectacular manifestations: the formation of exotic bound states. A systematic treatment of heavy-quark bound states is possible because the systems are characterized by at least two small parameters. One is the strong coupling constant at the heavy-quark mass scale, m, which is, by definition, larger than the typical hadronic scale, ΛQCD, and the other is the ratio ΛQCD/m. Expansions in these two parameters can be exploited in the description of systems made by one heavy quark, like heavy-light mesons or baryons. If the expansions are made manifest at the Lagrangian level, the resulting EFT is known as Heavy Quark Effective Theory, HQET [1] (for some reviews see [2]). Systems made by two heavy quarks are most frequently and successfully studied as non-relativistic bound states [3]. They are characterized by another small parameter, the heavy-quark velocity v, which comes with a hierarchy of energy scales: mv, mv, ... Making explicit at the Lagrangian level the expansions in mv/m and mv/m leads to an EFT known as non-relativistic QCD (NRQCD) [4, 5]. This EFT is similar to HQET, but with a different power counting. It also accounts for contact interactions between quarks and antiquarks (e.g. in decay processes) and hence has a wider set of operators. Making explicit at the Lagrangian level the expansion in mv/mv leads to another EFT known as potential NRQCD (pNRQCD) [6] (an alternative EFT is in [7]). pNRQCD is close to a Schrödinger-like description of the bound state and hence as simple. The bulk of the interaction is carried by potential-like terms, but non-potential interactions, associated with the propagation of low-energy degrees of freedom, are generally present as well. For a review on nonrelativistic EFTs we refer to [8]. It is important to establish when ΛQCD sets in, i.e. when we have to resort to nonperturbative methods. In the case of systems made by one heavy quark, ΛQCD becomes the most relevant scale once the heavy-quark mass has been integrated out: all HQET matrix elements are non-perturbative. The situ-