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Quark confinement in a constituent quark model

Description: On the level of an effective quark theory, we define confinement by the absence of quark anti-quark thresholds in correlation function. We then propose a confining Nambu-Jona-Lasinio-type model. The confinement is implemented in analogy to Anderson localization in condensed matter systems. We study the model`s phase structure as well as its behavior under extreme conditions, i.e. high temperature and/or high density.
Date: July 1, 1995
Creator: Langfeld, K. & Rho, M.
Partner: UNT Libraries Government Documents Department

Quasi two-body decays of nonstrange baryons.

Description: We examine the decays of nonstrange baryons to the final states Delta Pi, NRho, NEta, NEta', Nw, N.5 (1440)Pi, and Delta 3/2+(1600)Pi, in a relativized pair-creation (^3Po) model which has been developed in a previous study of the NPi decays of the same baryon states.As it is our goal to provide a guide for the possible discovery of new baryon states at CEBAF and elsewhere, we examine the decays of resonances which have already been seen in the partial-wave analyses, along with those of states which are predicted by the quark model but which remain undiscovered.The level of agreement between our calculation and the available widths from the partial-wave analyses in encouraging.
Date: August 1, 1993
Creator: Roberts, Winston & Capstick, Simon
Partner: UNT Libraries Government Documents Department

Physics of low-lying hadrons in quark model and effective hadronic approaches. Final report, September 1, 1996 - March 31, 2000

Description: There were basically three theoretical projects supported by this grant: (1) Use of confined quark models to study low energy hadronic processes; (2) Production of strangeness by Electromagnetic Probes; and (3) Diffractive dissociative production of vector mesons by virtual photons on nucleons. Each of them is summarized in the paper.
Date: June 19, 2000
Creator: Mizutani, T.
Partner: UNT Libraries Government Documents Department

Spin of the proton

Description: 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. 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.
Date: December 1, 1996
Creator: Isgur, Nathan
Partner: UNT Libraries Government Documents Department

Nucleon electromagnetic form factors and electroexcitation of low lying nucleon resonances in a light-front relativistic quark model

Description: We utilize a light-front relativistic quark model (LF RQM) to predict the 3q core contribution to the electroexcitation amplitudes for the {Delta}(1232)P{sub 33}, N(1440)P{sub 11}, N(1520)D{sub 13}, and N(1535)S{sub 11} up to Q{sup 2} = 12 GeV{sup 2}. The parameters of the model have been specified via description of the nucleon electromagnetic form factors in the approach that combines 3q and pion-cloud contributions in the LF dynamics.
Date: May 1, 2012
Creator: Aznauryan Inna, Burkert Volker
Partner: UNT Libraries Government Documents Department

Naive time-reversal odd phenomena in semi-inclusive deep-inelastic scattering from light-cone constituent quark models

Description: We present results for leading-twist azimuthal asymmetries in semi-inclusive lepton-nucleon deep-inelastic scattering due to naively time-reversal odd transverse-momentum dependent parton distribution functions from the light-cone constituent quark model. We carefully discuss the range of applicability of the model, especially with regard to positivity constraints and evolution effects. We find good agreement with available experimental data from COMPASS and HERMES, and present predictions to be tested in forthcoming experiments at Jefferson Lab.
Date: June 1, 2011
Creator: Barbara Pasquini, Peter Schweitzer
Partner: UNT Libraries Government Documents Department

A simple relation between the gamma N -> N(1535) helicity amplitudes

Description: It is shown that the helicity amplitudes A{sub 1/2} and S{sub 1/2} in the {gamma}N {yields} N(1535) reaction, can be well related by S{sub 1/2} = {radical}1+{tau}/{radical}2 M{sub S}{sup 2}-M{sup 2}/2M{sub S}Q A{sub 1/2} in the region Q{sup 2} > 2 GeV{sup 2}, where M and M{sub S} are the nucleon and N(1535) masses, q{sup 2} = -Q{sup 2} the four-momentum transfer squared, and {tau} = Q{sup 2}/(M{sub S} + M){sup 2}. This follows from the fact that the Pauli-type transition form factor F*{sub 2} extracted from the experimental data, turns up to show F*{sub 2} {approx_equal} 0 for Q{sup 2} > 1.5 GeV{sup 2}. The observed relation is tested by the experimental data and the MAID parametrization. A direct consequence of the relation is that the assumption,|A{sub 1/2}| >> |S{sub 1/2}|, is not valid for high Q{sup 2}. Instead, both amplitudes A{sub 1/2} and S{sub 1/2} have the same Q{sup 2} dependence in the high Q{sup 2} region, aside from that S{sub 1/2} has an extra factor, - 1/{radical} M{sub S}-M/2M{sub s}. The origin of this relation is interpreted in a perspective of a quark model.
Date: September 1, 2011
Creator: Gilberto Ramalho, Kazuo Tsushima
Partner: UNT Libraries Government Documents Department

Perturbative Gluon Exchange in a Covariant Quark Model of the Pion

Description: A covariant pion wave function, which reproduces the low energy data, is used to calculate the perturbative gluon exchange contributions to the pion charge form factor. It is found that the perturbative process dominates at q > 3.5 GeV/c. The dependence on the quark mass and the asymptotic behavior of the form factor are explicitly displayed.
Date: September 1, 1990
Creator: Gross, Franz & Ito, Hiroshi
Partner: UNT Libraries Government Documents Department

Electroproduction of the Roper Resonance as a Hybrid State

Description: The Q^2 dependence of the helicity amplitudes for the Roper resonance as a hybrid state is presented.Our study shows that the magnitude of the transverse helicity amplitude decreases rapidly as Q^2 increases, while the longitudinal helicity amplitude vanishes for the hybrid state.This feature is quite different from the prediction of the q^3 potential quark model, in which the Roper resonance is assumed to be an orbitally excited state.The comparison with data shows that the hybrid interpretation of the Roper is favoured.Future experiments at CEBAF can provide the information needed to definitely determine the spin-flavour content of this resonance.
Date: August 1, 1991
Creator: Burkert, Volker; Li, Zhenping & Li, Zhujun
Partner: UNT Libraries Government Documents Department

The anomalous suppression of [pi][sub 2](1670)->b[sub 1](1235)[pi]

Description: We show that current experimental data indicate that the strong decay mode {pi}{sub 2} {yields} b{sub 1}{pi} is anomalously small (more than 3 times smaller than all other decay modes of the {pi}{sub 2}). This acts as a powerful discriminator for and against various decay models. Nonrelativistic quark models with spin-1 pair creation, e.g. {sup 3}P{sub 0} (flux-tube breaking) and {sup 3}S{sub 1} and {sup 3}D{sub 1} (chromo-electric string-breaking) models, as well as lowest order one-boson (in this case {pi}) emission models, can accommodate the experimental data because of a quark-spin selection rule. Models that violate the selection rule, e.g. higher order one-boson emission decay mechanisms, as well as mixing with other Fock states and relativistic effects, may be constrained by the small {pi}{sub 2} {yields} b{sub 1}{pi} decay.
Date: January 1, 2002
Creator: Page, P. R. (Philip R.)
Partner: UNT Libraries Government Documents Department

Photoproduction amplitudes of P(sub)11 and P(sub)33 baryon resonances in the quark model

Description: Predictions are made for the photoproduction amplitudes of low-lying N^(1/2)+ (P(sub)(11)) and delta^(3/2)+ (P(sub)(33)) resonances, using a quark model with relativistic corrections to the transition operator, and mixed nonrelativistic wavefunctions which are correctly orthogonal to the ground states.These amplitudes are also calculated using relativized model wavefunctions. The results for the Roper resonance N(1440) are in marked disagreement with the data.
Date: January 1, 1992
Creator: Capstick, Simon
Partner: UNT Libraries Government Documents Department

Covariant Quark Model of Pion Structure

Description: A relativistic quark model wave function of the pion is obtained from a fit to the experimental data, including the charge radius, charge form factor and weak decay constant.The results are quite stable for different choices of quark mass.The two-photon decay width of the neutral pion and the asymptotic form factor for the gamma*+ pi^0 -> gamma transition are estimated.
Date: December 1, 1990
Creator: Gross, Franz & Ito, Hiroshi
Partner: UNT Libraries Government Documents Department

Parton distributions from SMC and SLAC data

Description: We have extracted spin-weighted parton distributions in a proton from recent data at CERN and SLAC. The valence, sea quark and Antiquark spin-weighted distributions are determined separately. The data are all consistent with a small to moderate polarized gluon distribution, so that the anomaly term is not significant in the determination of the constituent contributions to the spin of the proton. We have analyzed the consistency of the results obtained from various sets of data and the Biorken sum rule. Although all data are consistent with the sum rule, the polarized distributions from different experiments vary, even with higher order QCD corrections taken into account. Results split into two models, one set implying a large polarized strange sea which violates the positivity bound, and the other set yielding a smaller polarized strange sea. Only further experiments which extract information about the polarized sea will reconcile these differences. We suggest specific experiments which can be performed to determine the size of the polarized sea and gluons.
Date: January 4, 1996
Creator: Ramsey, G. P. & Goshtasbpour, M.
Partner: UNT Libraries Government Documents Department

Radial and orbital q{anti q} excitations: ``Higher quarkonia``

Description: Identification of the spectrum of non-{ital q{anti q}} exotica such as glueballs, hybrids and molecules will require a detailed understanding of radially and orbitally excited quarkonia, since these states are a background to non-{ital q{anti q}} states. The data likely to become available experimentally on these higher-mass resonances are their total widths and strong branching fractions to dominant modes. We advocate the use of these branching fractions to distinguish quarkonia from exotica; in this contribution we summarize our recent detailed {sup 3}P{sub 0} decay model calculations of the 374 two-body open-flavor decay modes of the 32 lightest {ital n{anti n}} states above 1S and 1P, which should be useful for this program.
Date: December 31, 1996
Creator: Barnes, T.
Partner: UNT Libraries Government Documents Department

Light-cone quantization and QCD phenomenology

Description: In principle, quantum chromodynamics provides a fundamental description of hadronic and nuclear structure and dynamics in terms of their elementary quark and gluon degrees of freedom. In practice, the direct application of QCD to reactions involving the structure of hadrons is extremely complex because of the interplay of nonperturbative effects such as color confinement and multi-quark coherence. A crucial tool in analyzing such phenomena is the use of relativistic light-cone quantum mechanics and Fock state methods to provide tractable and consistent treatments of relativistic many-body systems. In this article we present an overview of this formalism applied to QCD, focusing in particular on applications to the final states in deep inelastic lepton scattering that will be relevant for the proposed European Laboratory for Electrons (ELFE), HERMES, HERA, SLAC, and CEBAF. We begin with a brief introduction to light-cone field theory, stressing how it many allow the derivation of a constituent picture, analogous to the constituent quark model, from QCD. We then discuss several applications of the light-cone Fock state formalism to QCD phenomenology. The Fock state representation includes all quantum fluctuations of the hadron wavefunction, including far off-shell configurations such as intrinsic charm and, in the case of nuclei, hidden color. In some applications, such as exclusive processes at large momentum transfer, one can make first-principle predictions using factorization theorems which separate the hard perturbative dynamics from the nonpertubative physics associated with hadron binding. The Fock state components of the hadron with small transverse size, which dominate hard exclusive reactions, have small color dipole moments and thus diminished hadronic interactions. Thus QCD predicts minimal absorptive corrections, i.e., color transparency for quasi-elastic exclusive reactions in nuclear targets at large momentum transfer.
Date: December 1995
Creator: Brodsky, S. J. & Robertson, D. G.
Partner: UNT Libraries Government Documents Department

Discovery mass reach for excited quarks at hadron colliders

Description: If quarks are composite particles then excited states are expected. We estimate the discovery mass reach as a function of integrated luminosity for excited quarks decaying to dijets at the Tevatron the mass reach is 0.94 TeV for Run 11 (2 fb{sup -1}) and 1. 1 TeV for TeV33 (30 fb{sup -1}). At the LHC the mass reach is 6.3 TeV for 100 fb{sup -1}. At a VLHC with a center of mass energy {radical}s, of 50 TeV (200 TeV) the mass reach is 25 TeV (78 TeV) for an integrated luminosity of 10{sup 4} fb{sup -1}. However, an excited quark with a mass of 25 TeV would be discovered at a hadron collider with {radical}s = 100 TeV and an integrated luminosity of 13 fb{sup -1}, illustrating a physics example where a factor of 2 in machine energy is worth a factor of 1000 in luminosity.
Date: September 10, 1996
Creator: Harris, R.M.
Partner: UNT Libraries Government Documents Department

Theoretical nuclear physics. Final report

Description: As the three-year period FY93-FY96 ended, there were six senior investigators on the grant full-time: Bulgac, Henley, Miller, Savage, van Kolck and Wilets. This represents an increase of two members from the previous three-year period, achieved with only a two percent increase over the budget for FY90-FY93. In addition, the permanent staff of the Institute for Nuclear Theory (George Bertsch, Wick Haxton, and David Kaplan) continued to be intimately associated with our physics research efforts. Aurel Bulgac joined the Group in September, 1993 as an assistant professor, with promotion requested by the Department and College of Arts and Sciences by September, 1997. Martin Savage, who was at Carnegie-Mellon University, jointed the Physics Department in September, 1996. U. van Kolck continued as research assistant professor, and we were supporting one postdoctoral research associate, Vesteinn Thorssen, who joined us in September, 1995. Seven graduate students were being supported by the Grant (Chuan-Tsung Chan, Michael Fosmire, William Hazelton, Jon Karakowski, Jeffrey Thompson, James Walden and Mitchell Watrous).
Date: May 1, 1997
Partner: UNT Libraries Government Documents Department

Reaction theories for N* excitations in {pi}N and {gamma}N reactions

Description: The importance of developing reaction theories for investigating N* physics is illustrated in an analysis of pion photoproduction on the nucleon. It is shown that the {gamma}N {leftrightarrow} {Delta} transition amplitudes predicted by the constituent quark model are in agreement with the values extracted from the {gamma}N {r_arrow} {pi}N data only when the contributions from the reaction mechanisms calculated using a dynamical approach are taken into account in the analysis.
Date: December 31, 1996
Creator: Lee, T. S. H.
Partner: UNT Libraries Government Documents Department

Meson decays in a quark model

Description: A recent model of hadron states is extended to include meson decays. The authors find that the overall success of the model is quite good. Possible improvements to the model are suggested.
Date: August 1, 1997
Creator: Roberts, W. & Silvestre-Brac, B.
Partner: UNT Libraries Government Documents Department

The stability of the spectator, Dirac, and Salpeter equations for mesons

Description: Mesons are made of quark-antiquark pairs held together by the strong force. The one channel spectator, Dirac, and Salpeter equations can each be used to model this pairing. The authors look at cases where the relativistic kernel of these equations corresponds to a time-like vector exchange, a scalar exchange, or a linear combination of the two. Since the model used in this paper describes mesons which cannot decay physically, the equations must describe stable states. They find that this requirement is not always satisfied, and give a complete discussion of the conditions under which the various equations give unphysical, unstable solutions.
Date: August 1, 1998
Creator: Uzzo, Michael & Gross, Franz
Partner: UNT Libraries Government Documents Department

Leptonic production of baryon resonances

Description: In these lectures, the author focuses on the electromagnetic transition between non-strange baryon states. This sector received much attention in the early 1970's after the development of the first dynamical quark models. However, experimental progress was slow, partly because of the low rates associated with electromagnetic interactions, and partly because of the lack of guidance by theoretical models that went beyond the simplest quark models. It was also difficult for experiments to achieve the precision needed for a detailed analysis of the entire resonance region in terms of the fundamental photocoupling amplitudes over a large range in momentum transfer.
Date: March 1, 1994
Creator: Burkert, V.
Partner: UNT Libraries Government Documents Department

Strangeness in the Nucleon or the quark model beyond the valence approximation

Description: Simple 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.
Date: January 1, 1999
Creator: Isgur, Nathan
Partner: UNT Libraries Government Documents Department