Object Type: Folder

In Folder: Nathan Isgur

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Understanding the underlying quark and gluon structure of the nucleons is not only a fundamental objective in its own right, but also an essential first step in providing a basis within QCD for nuclear physics. In this article I present a pedagogical introduction to a simple but very successful nonrelativistic constituent quark model for baryons.

10/20/21, 6:06 PM

I present an idiosyncratic view of baryone which calls for a marriage between quark-based and hadronic models of QCD. I advocate 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.

10/20/21, 6:06 PM

New symmetries of the strong interactions appear in heavy quark physics which can be used to predict many properties of hadrons containing a single heavy quark. In this talk, I discuss some of the implications of this heavy quark symmetry for the spectroscopy and structure of baryons, as well as some of the potential uses of the new predictive power it bestows.

10/20/21, 6:06 PM

An idiosyncratic view of Baryons '95 that calls for a marriage between quark-based and hadronic models of QCD is presented.A treatment based on valence quark plus glue dominance of hadron structure, with the sea of qq{bar} pairs (in the form of virtual hadron pairs) as important corrections is advocated.

10/20/21, 6:06 PM

In the adiabatic approximation, most of the effects of quark-antiquark loops on spectroscopy can be absorbed into a static interquark potential. I develop a formalism which can be used to treat the residual nonadiabatic effects associated with the presence of nearby hadronic thresholds for heavy quarks. I then define 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.

10/20/21, 6:06 PM

The scalar isovector meson propagator is studied in quenched QCD. For the lightest quark masses used, this propagator is dominated by a quenched chiral loop effect associated with the $\eta'$-$\pi$ two-meson intermediate state. Both the time dependence and the pion mass dependence of the effect are well-described by quenched chiral perturbation theory.

10/20/21, 6:06 PM

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 η′\eta 'η′ 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 η′−π\eta '- \piη′−π 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.

10/20/21, 6:06 PM

A Comment on the Letter by Nils A. Törnqvist and Matts Roos, Phys. Rev. Lett. 76, 1575 (1996). The authors of the Letter offer a Reply.

10/20/21, 6:06 PM

I criticize certain conclusions about the physics of hadrons drawn from a "valence QCD" approximation to QCD.

10/20/21, 6:06 PM

I describe four serious defects of a widely discussed pion exchange model for interquark forces: it doesn't 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.

10/20/21, 6:06 PM

I identify a source of Lambda_{QCD}/m_Q corrections to the assumption of quark-hadron duality in the application of heavy quark methods to inclusive heavy quark semileptonic decays Q->q l nu_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_{cb}.

10/20/21, 6:06 PM

I identify a source of Λ QCD / m Q corrections to the assumption of quark-hadron duality in the application of heavy quark methods to inclusive heavy quark decays. 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 cb . Note: An Abbreviated version of the original JLAB-THY-98-03 entitled 'Duality in Inclusive Semileptonic Heavy Quark Decay' • 16 pages, 5 figures, an abbreviated version of hep-ph/9809279, the original JLAB-THY-98-03 "Duality in Inclusive Semileptonic Heavy Quark Decay"

10/20/21, 6:06 PM

The low-lying eigenmodes of the Dirac operator associated with typical gauge field configurations in QCD encode, among other low-energy properties, the physics behind the solution to the $U_A(1)$ problem (i.e. the origin of the $\eta'$ mass), the nature of spontaneous chiral symmetry breaking, and the physics of string-breaking, quark-antiquark pair production, and the OZI rule. Moreover, the space-time chiral structure of these eigenmodes reflects the space-time topological structure of the underlying gauge field. We present evidence from lattice QCD on the local chiral structure of low Dirac eigenmodes leading to the conclusion that topological charge fluctuations of the QCD vacuum are not instanton-dominated. The result supports Witten's arguments that topological charge is produced by confinement-related gauge fluctuations rather than instantons.

10/20/21, 6:06 PM

Some emerging difficulties in the theoretical description of exclusive semileptonic $\bar B$ decays are discussed in the context of the quark model. While there are no unambiguous problems at this time, I discuss physics beyond the valence quark model which should eventually be probed by precision measurements of $\bar B$ semileptonic decays.

10/20/21, 6:06 PM

Flux tube zero-point motion produces quark displacements transverse to the flux tube which make significant contributions to hadronic charge radii. In heavy quark systems, these contributions can be related by Bjorken's sum rule to the rates for semileptonic decay to hybrid mesons. This connection can be generalized to other leptoproduction processes, where transverse contributions to elastic form factor slopes are related to the cross sections for the production of the associated hybrid states. I identify the flux tube overlap integral responsible for these effects as the strong QCD analogue of the Sudakov form factor of perturbative QCD.

10/20/21, 6:06 PM

This review summarizes contributions to the Parallel Workshop Session on Hadron Spectroscopy at the PANIC XII conference.

10/20/21, 6:06 PM

In this talk I review and comment upon recent developments in hadron spectroscopy and structure. The talk is organized into three main sections dealing with heavy quarkonia (QQ̄), hadrons containing a single heavy quark (Qq̄ and Qqq), and hadrons containing only light quarks and glue, although I will emphasize a surprising unity of the phenomena characterizing these systems.

10/20/21, 6:06 PM

It has recently been shown that hadrons containing a single heavy quark exhibit a new flavor-spin symmetry of QCD, and that this symmetry leads to relations between operator matrix elements involving such hadrons. On examining mechanisms which might break the symmetry at large recoil, I conclude that these relations are probably valid over the full kinematic ranges available in transitions involving b->c, b->s, b->u, c->s, and c->d currents.

10/20/21, 6:06 PM

New symmetries of the strong interactions appear in heavy quark physics. They can be used to predict many properties of hadrons containing a single heavy quark. Some of these predictions are expected to play an important role in determining the values of elements of the Cabibbo-Kobayashi-Maskawa matrix.

10/20/21, 6:06 PM

Heavy-quark symmetry relates the D-wave amplitude for D1(2420)→D*π to the amplitudes for the decays D2*(2460)→D*π and D2*(2460)→D*π. We discuss the extraction of the S and D partial wave amplitudes for D1→D*π and their significance as tests of the applicability of heavy-quark symmetry for charm quarks.

10/20/21, 6:06 PM

The neutron's electric form factor contains vital information on nucleon structure, but its interpretation within many models has been obscured by relativistic effects. I demonstrate 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_1 to leave intact the naive interpretation of G^n_E as arising from the neutron's rest frame charge distribution.

10/20/21, 6:06 PM