pi N --> Multi-pi N Scattering in the 1/N_c Expansion

We extend the 1/N_c expansion meson-baryon scattering formalism to cases in which the final state contains more than two particles. We first show that the leading-order large N_c processes proceed through resonant intermediate states (e.g., rho N or pi Delta). We then tabulate linear amplitude expressions for relevant processes and find that the pole structure of baryon resonances can be uniquely identified by their (non)appearance in eta N or mixed partial-wave pi Delta final states. We also show that quantitative predictions of pi N to pi Delta branching ratios predicted at leading order alone do not agree with measurements, but the inclusion of 1/N_c corrections is ample to explain the discrepancies.Comment: 23 pages, 3 eps figures, ReVTeX4, added reference and discussion, identical to PRD versio

The 1/N_c Approach for Baryon Resonances

I give a brief overview of the model-independent analysis of unstable baryon resonances in the 1/N_c QCD expansion. This approach produces numerous surprising semi-quantitative phenomenological predictions that are supported by available observations.Comment: 4 pages, LaTeX, style file included. Invited talk at International Conference on QCD and Hadronic Physics, Beijing, China, 16-20 Jun 200

Baryons, IN_c

Excited baryons may be analyzed in the 1/N_c expansion as true resonances in scattering amplitudes. The key idea making this program possible is a generalization of methods originally applied to chiral soliton models in the 1980's. One finds model-independent relations among amplitudes that impose mass and width degeneracies among resonances of various quantum numbers. Phenomenological evidence demonstrates that patterns of resonant decay predicted by 1/N_c agree with data. The analysis can be extended to subleading orders in 1/N_c, where again agreement with data is evident.Comment: 10 pages. Invited talk at "Continuous Advances in QCD 2004," Minneapolis, 13-16 Ma

QCD Constraints on Form Factor Shapes

This talk presents an introduction to the use of dispersion relations to constrain the shapes of hadronic form factors consistent with QCD. The applications described include methods for studying |V_{cb}| and |V_{ub}|, the strange quark mass, and the pion charge radius.Comment: 8 pages, 3 eps figures, presented at ``Exclusive & Semi-exclusive Processes at High Momentum Transfer,'' Jefferson Lab, Newport News, VA, May 20-22, 199

The Pentaquark Candidates in the Dynamical Diquark Picture

Starting with the dynamical picture of the exotic $c\bar c$-containing states $XY \! Z$ as the confinement-induced hadronization of a rapidly separating pair of a compact diquark and antidiquark, we describe the pentaquark candidates $P_c^+(4380)$ and $P_c^+(4450)$ in terms of a confined but rapidly separating color-antitriplet diquark $cu$ and color-triplet "triquark" $\bar c (ud)$. This separation explains the relatively small $P_c^+$ widths, despite these 5-quark systems lying far above both the $J/\psi \, p$ and $\Lambda_c \bar D^{(*)0}$ thresholds. The $P_c^+$ states are predicted to form isospin doublets with neutral partners $P_c^0$.Comment: 5 pages, 1 pdf figure, pdflatex. Version accepted by Phys. Lett.

N* Properties from the 1/N_c Expansion

The 1/N_c expansion (N_c being the number of QCD colors) has been applied in recent papers to the phenomenology of excited baryon resonances. This talk surveys the work done to date, and discusses its successes and remaining challenges.Comment: 10 pages, 1 figure. Invited plenary talk at NSTAR 2002, Pittsburgh, PA, 9-12 October 200

Explicit Quark-Hadron Duality in 1+1 Dimensions

Explicit quark-hadron duality in the limit of heavy quark mass is studied using the 't Hooft model, where both partonic and hadronic amplitudes may be computed exactly. Results for weak decays of heavy mesons are presented for both standard spectator decays, where the duality limit is convincingly approached, and annihilation decays of the valence quark-antiquark pair, where the approach to asymptotic duality is much less precise.Comment: 4 pages, Latex (worldsci.sty), 3 eps figures. Talk presented at the Third International Conference on Quark Confinement and the Hadron Spectrum, Jefferson Lab, Newport News, Virginia, June 7-12, 1998; to appear in Proceeding