The origins of quark-hadron duality: How does the square of the sum become the sum of the squares?

Bloom-Gilman duality demonstrates empirically that the electroproduction of $N^*$'s at low momentum transfers averages smoothly around the scaling curve measured at large momentum transfers. The latter is proportional to the sum of the squares of the constituent charges whereas the former involves the coherent excitation of resonances and is driven by the square of summed constituent charges. We determine the minimal necessary conditions for this equality to be realised so that duality can occur and consider the implications for a range of processes that may be studied soon at CEBAF.Comment: 9 page

Evidence that the Pomeron transforms as a non-conserved vector current

The detailed dependences of central meson production on the azimuthal angle phi, t and the meson J^P are shown to be consistent with the hypothesis that the soft Pomeron transforms as a non-conserved vector current. Further tests are proposed. This opens the way for a quantitative description of q-qbar and glueball production in p p -> p M p.Comment: 12 pages, latex, 4 figure

On baryon-antibaryon coupling to two photons

We discuss recent claims that $p\bar{p} \to \gamma \gamma$ may be described by a generalized parton picture. We propose that quark-hadron duality provides a justification for the effective dominance of the "handbag" diagram assumed in recent literature, and that handbag diagrams may dominate phenomena in kinematic regions far more extensive than that might be expected from pQCD alone.Comment: 11 pages, 2 eps figures, minor modifications made, version to appear on Phys. Lett.

Light Hadron Spectroscopy: Theory and Experiment

Rapporteur talk at the Lepton-Photon Conference, Rome, July 2001: reviewing the evidence and strategies for understanding scalar mesons, glueballs and hybrids, the gluonic Pomeron and the interplay of heavy flavours and light hadron dynamics. Dedicated to the memory of Nathan Isgur, long-time collaborator and friend, whose original ideas in hadron spectroscopy formed the basis for much of the talk.Comment: to be published in "Lepton Photon 2001 Conference Proceedings" (World Scientific Publishing), 19 pages with 6 figure

Hybrid meson properties in Lattice QCD and Flux Tube Models

Flux tube model predictions for hybrid meson decays are beginning to be confronted by Lattice QCD. We compare the two approaches for the $S$-wave decay of the exotic $1^{-+}$, and find excellent agreement. Results suggest that in Strong QCD, \qq~ creation occurs with S=1.Comment: 6 pages, 1 figure, v2: discussion of parameters and lattice uncertainties adde

Quarks, diquarks and QCD mixing in the N∗N^* resonance spectrum

We identify a "$\Lambda$ selection rule" for $N^*$ resonances in the presence of QCD mixing effects. We quantify these mixing effects from existing data and predict amplitudes for exciting {\bf 20} representations in SU(6), which are forbidden in strict diquark models. By classifying Particle-Data-Group (PDG) states at N=2, we show that $\gamma N\to K\Lambda$, $K^*\Lambda$, $K\Sigma$, $K^*\Sigma$, and $J/\psi \to \bar{p}N^*$ are ideal probes of baryon dynamics and for establishing whether strongly correlated diquarks survive for $L > 0$.Comment: Extended version accepted by PR

Scalar Glueball-qqbar Mixing above 1 GeV and implications for Lattice QCD

Lattice QCD predictions have motivated several recent studies of the mixing between the predicted JPC = 0++ glueball and a qqbar nonet in the 1.3 to 1.7 GeV region. We show that results from apparently different approaches have some common features, explain why this is so and abstract general conclusions. We place particular emphasis on the flavour dependent constraints imposed by decays of the f0(1370), f0(1500) and f0(1700) to all pairs of pseudoscalar mesons. From these results we identify a systematic correlation between glueball mass, mixing, and flavour symmetry breaking and conclude that the glueball may be rather lighter than some quenched lattice QCD computations have suggested. We identify experimental tests that can determine the dynamics of a glueball in this mass region and discuss quantitatively the feasibility of decoding glueball-qqbar mixing.Comment: 33 pages, Latex, 4 Figure

Symmetry breaking and quark-hadron duality in structure functions

We identify conditions under which a summation over nucleon resonances can yield, via quark-hadron duality, parton model results for electromagnetic and neutrino structure functions at large x. While a summation over the lowest even and odd parity multiplets is sufficient to achieve duality in the symmetric quark model, a suppression of transitions to specific final states is required for more realistic cases incorporating SU(6) breaking. We outline several scenarios consistent with duality, discuss their implications for the high Q^2 behavior of transition form factors, and illustrate how they can expose the patterns in the flavor-spin dependence of inter-quark forces.Comment: 30 pages, 9 figures, to appear in Phys. Rev.