Nathan Isgur

Strange hadronic loops of the proton: A quark model calculation

Nontrivial q{anti q} sea effects have their origin in the low-Q{sup 2} dynamics of strong QCD. The authors present here a quark model calculation of the contribution of s{anti s} pairs arising from a complete set of OZI-allowed strong Y{sup *}K{sup *} hadronic loops to the net spin of the proton, to its charge radius, and to its magnetic moment. The calculation is performed in an ``unquenched quark model which has been shown to preserve the spectroscopic successes of the naive quark model and to respect the OZI rule. They speculate that an extension of the calculation to the nonstrange sea will show that most of the ``missing spin of the proton is in orbital angular momenta.

Chiral loops and ghost states in the quenched scalar propagator

The scalar, isovector meson propagator is analyzed in quenched QCD, using the MQA pole-shifting ansatz to study the chiral limit. In addition to the expected short-range exponential falloff characteristic of a heavy scalar meson, the propagator also exhibits a longer-range, negative metric contribution which becomes pronounced for smaller quark masses. We show that this is a quenched chiral loop effect associated with the anomalous structure of the eta propagator in quenched QCD. Both the time dependence and the quark mass dependence of this effect are well-described by a chiral loop diagram corresponding to an eta-pi intermediate state, which is light and effectively of negative norm in the quenched approximation. The relevant parameters of the effective Lagrangian describing the scalar sector of the quenched theory are determined.

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 realized so that duality can occur and consider the implications for a range of processes that may be studied soon at CEBAF.

Valence quark spin distribution functions

The hyperfine interactions of the constituent quark model provide a natural explanation for many nucleon properties, including the {Delta} - N splitting, the charge radius of the neutron, and the observation that the protons quark distribution function ratio d(x)/u(x) {r_arrow} 0 as x {r_arrow} 1. The hyperfine-perturbed quark model also makes predictions for the nucleon spin-dependent distribution functions. Precision measurements of the resulting asymmetries A{sub 1}{sup p}(x) and A{sub 1}{sup n}(x) in the valence region can test this model and thereby the hypothesis that the valence quark spin distributions are normal.

Critique of a pion exchange model for interquark forces

I describe four serious defects of a widely discussed pion exchange model for interquark forces: it does not solve the spin-orbit problem as advertised, it fails to describe the internal structure of baryon resonances, it leads to disastrous conclusions when extended to mesons, and it is not reasonably connected to the physics of heavy-light systems. While extensions of the original pion exchange model may be able to correct these defects, this catalogue of criticisms defines some of the most formidable problems such elaborations must address. (c) 2000 The American Physical Society.

Local chirality of low lying Dirac eigenmodes and the instanton liquid model

The reasons for using low-lying Dirac eigenmodes to probe the local structure of topological charge fluctuations in QCD are discussed, and the interpretation of the local chiral orientation probability distribution in these modes (as compared with the instanton picture) is clarified. The results with overlap Dirac operator in Wilson gauge backgrounds at lattice spacings ranging from a{approx}0.04 fm to a{approx}0.12 fm are reported, and it is found that the size and density of local structures responsible for double-peaking of the distribution are in disagreement with the assumptions of the Instanton Liquid Model. We argue that our results suggest that vacuum fluctuations do not produce locally quantized (integer-valued) topological charge in QCD, contrary to the semiclassical picture.

Background enhancement of CPT reach at an asymmetric phi factory

Photoproduction of neutral-kaon pairs is studied from the perspective of CP and CPT studies. Interference of the P and S waves, with the former due to diffractive phi production and the latter to f{_}0/a{_}0 production, is shown to enhance the CPT reach. Results are presented of Monte Carlo studies based on rates expected in future experiments.

Meson-like baryons and the spin-orbit puzzle

The author describes a special class of meson-like {Lambda}{sub Q} excited states and present evidence supporting the similarity of their spin-independent spectra to those of mesons. He then examine spin-dependent forces in these baryons, showing that predicted effects of spin-orbit forces are small for them for the same reason they are small for the analogous mesons: a fortuitous cancellation between large spin-orbit forces due to one-gluon-exchange and equally large inverted spin-orbit forces due to Thomas precession in the confining potential. In addition to eliminating the baryon spin-orbit puzzle in these states, this solution provides a new perspective on spin-orbit forces in all baryons.

Beyond the adiabatic approximation: The impact of thresholds on the hadronic spectrum

In the adiabatic approximation, most of the effects of quark-antiquark loops on spectroscopy can be absorbed into a static interquark potential. The author develops a formalism which can be used to treat the residual nonadiabatic effects associated with the presence of nearby hadronic thresholds for heavy quarks. He then defines a potential which includes additional high energy corrections to the adiabatic limit which would be present for finite quark masses. This improved potential allows a systematic low energy expansion of the impact of thresholds on hadronic spectra.

Comment on ''Valence QCD: Connecting QCD to the quark model''

The author criticize certain conclusions about the physics of hadrons drawn from a ``valence QCD approximation to QCD. Lattice QCD is not just useful as a technique for calculating strong interaction observables like the proton mass: it can also be used to help understand QCD. This is the goal of the work described in reference 1. Its authors present a field theory which they call valence QCD (vQCD) which they hope can be identified with the valence quark model. The key feature built into vQCD is a form of suppression of Z-graphs, i.e., of quarks propagating backward in time. The authors make sound arguments for the importance of trying to capture the essence of the quark model in a field-theoretic framework, and present some interesting results (both theoretical and numerical) on vQCD. This comment is not directed at the goals of vQCD but rather at certain conclusions about the physics of hadrons which the authors have drawn from their work which the author considers unjustified. Foremost among these is the claim highlighted in their abstract that baryon hyperfine interactions are ``largely attributed to the Goldstone boson exchanges between the quarks and not to standard one-gluon-exchange (OGE) forces.