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Baryons and QCD
No full textThe author presents an idiosyncratic view of baryons which calls for a marriage between quark-based and hadronic models of QCD. He advocates a treatment based on valence quark plus glue dominance of hadron structure, with the sea of q pairs (in the form of virtual hadron pairs) as important corrections
Comment on ''Valence QCD: Connecting QCD to the quark model''
No full textThe 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
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Duality in inclusive semileptonic heavy quark decay
No full textThe author identifies a source of Lambda{sub QCD}/m{sub Q} corrections to the assumption of quark-hadron duality in the application of heavy quark methods to inclusive heavy quark semileptonic decays Q {r_arrow} ql{anti {nu}}{sub l}. These corrections could substantially affect the accuracy of such methods in practical applications and in particular compromise their utility for the extraction of the Cabibbo-Kobayashi-Maskawa matrix element V{sub cb}
Interpreting the neutron's electric form factor: Rest frame charge distribution or foldy term?
No full textThe neutron's electric form factor contains vital information on nucleon structure, but its interpretation within many models has been obscured by relativistic effects. The author demonstrates that, to leading order in the relativistic expansion of a constituent quark model, the Foldy term cancels exactly against a contribution to the Dirac form factor F{sub 1} to leave intact the naive interpretation of G{sup n}{sub E} as arising from the neutron's rest frame charge distribution
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Exclusive versus inclusive semileptonic {anti B} decays in the quark model
No full textSome emerging difficulties in the theoretical description of exclusive semileptonic {anti B} decays are discussed in the context of the quark model. While there are no unambiguous problems at this time, the author discusses physics beyond the valence quark model which should eventually be probed by precision measurements of B semileptonic decays
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Quark structure of matter
No full textThe author presents a overview of some central issues facing strong interaction physics today, with an emphasis on questions that will be addressed in concert by CEBAF at Jefferson Lab and by the new 50 GeV proton machine (JHP) being planned for KEK
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Why N*'s are Important
Get PDFThe study of N*'s can provide us with critical insights into the nature of QCD in the confinement domain. The keys to progress in this domain are the identification of its important degrees of freedom and the effective forces between them. The author reports on the growing evidence in support of the flux tube model, and comment on the connection between this model and spontaneous chiral symmetry breaking, on the spin-dependence of the long-range confining potential, on the evidence for short-range one gluon exchange, on instantons, and on the one pion exchange model
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Strangeness in the Nucleon or the quark model beyond the valence approximation
No full textSimple arguments based on unitarity indicate that meson loops diagrams, induced by an underlying qq pair creation process, should badly disturb the phenomenologically successful spectroscopy and dynamics of the valence quark model, including such simple but mysterious regularities as the OZI rule. The author discusses some recent progress in adding pair creation to the valence quark model in a way which provides rationale for the quark model's success
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Spin of the proton
No full textThe author argues that their response to the spin crisis should not be to abandon the naive quark model baby, but rather to allow it to mature. In particular, he advocates dressing the baby in qq pairs, first showing that this can be done without compromising the naive quark model's success with either spectroscopy or the OZI rule. Finally, he shows that despite their near invisibility elsewhere, pairs do play an important role in the proton's spin structure by creating an antipolarized qq sea. In the context of an explicit calculation he demonstrates that it is plausible that the entire ''spin crisis'' arises from this effect
Spin structure of the proton
No full textIn these lectures the author argues that their response to the spin crisis should not be to abandon the naive quark model baby, but rather to allow it to mature. He begin by recalling what a beautiful baby the quark model is via an overview of its successes in spectroscopy, dynamics, and valence spin structure. He also introduces the conservative hypothesis that dynamical q{anti q} pairs are its key missing ingredient. He then discusses dressing the baby. He first shows that it can be clothed in glue without changing its spectroscopic successes. In the process, several dynamical mysteries associated with quark model spectroscopy are potentially explained. Next, he dresses the baby in q{anti q} pairs, first showing that this can be done without compromising the naive quark model's success with either spectroscopy or the OZI rule. Finally, he shows that despite their near invisibility elsewhere, pairs do play an important role in the proton's spin structure by creating an antipolarized q{anti q} sea. In the context of an explicit calculation he demonstrate that it is plausible that the entire ''spin crisis'' arises from this effect
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