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Local duality predictions for x {approx} 1 structure functions

Description: Recent data on the proton F{sub 2} structure function in the resonance region suggest that local quark-hadron duality works remarkably well for each of the low-lying resonances, including the elastic, to rather low values of Q{sup 2}. The authors discuss model-independent relations between structure functions x{approx}1 and elastic electromagnetic form factors. The x{yields}1 behavior of the nucleon polarization asymmetries and the neutron to proton structure function ratios is studied as a function of Q-squared using available data on the electric and magnetic form factors.
Date: March 24, 2000
Creator: Melnitchouk, W.
Partner: UNT Libraries Government Documents Department

Asymmetric quarks in the proton

Description: Asymmetries in the quark momentum distributions in the proton reveal fundamental aspects of strong interaction physics. Differences between {anti u} and {anti d} quarks in the proton sea provide insight into the dynamics of the pion cloud around the nucleon and the nature of chiral symmetry breaking. Polarized flavor asymmetries allow the effects of pion clouds to be disentangled from those of antisymmetrization. Asymmetries between s and {anti s} quark distributions in the nucleon are also predicted from the chiral properties of QCD.
Date: March 1, 2000
Creator: Melnitchouk, W.
Partner: UNT Libraries Government Documents Department

Flavor asymmetries in the proton and semi-inclusive processes

Description: Semi-inclusive electron scattering provides a powerful tool with which to study the spin and flavor distributions in the proton. Greater kinematic coverage at the proposed Electron-Polarized Ion Collider facility will enable the valence d/u ratio to be determined at large x through {pi}{sup {+-}} production. At small x, {pi} production can be used to extract the d/u ratio, complementing existing semi-inclusive measurements by HERMES, and Drell-Yan data from Fermilab. Asymmetries in heavier quark flavors can also be probed by tagging strange and charm hadrons in the final state.
Date: April 1, 1999
Creator: Melnitchouk, W.
Partner: UNT Libraries Government Documents Department

Flavor decomposition of the nucleon

Description: The author reviews some recent developments in the study of quark flavor distribution in the nucleon, including (1) valence quark distributions and quark-hadron duality prediction for the x {yields} 1 F{sup n}{sub 2}/F{sup p}{sub 2} ration (2) sea quark asymmetries and electromagnetic form factors (3) strange quarks in the nucleon.
Date: June 1, 1999
Creator: Melnitchouk, W.
Partner: UNT Libraries Government Documents Department

Quark-Hadron Duality: Resonances and the Onset of Scaling

Description: We discuss the origin of Bloom-Gilman duality and the relationship between resonances and scaling in deep-inelastic scattering. We present a simple quantum mechanical model which reproduces the essential features of Bloom-Gilman duality at low Q{sup 2}, and describe applications of local duality relating structure functions at x{approximately}1 and elastic electromagnetic form factors.
Date: March 2001
Creator: Melnitchouk, W.
Partner: UNT Libraries Government Documents Department

Asymmetric quarks in the proton

Description: Asymmetries in the quark momentum distributions in the proton reveal fundamental aspects of strong interaction physics. Differences between {anti u} and {anti d} quarks in the proton sea provide insight into the dynamics of the pion cloud around the nucleon and the nature of chiral symmetry breaking. Polarized flavor asymmetries allow the effects of pion clouds to be disentangled from those of antisymmetrization. Asymmetries between s and {anti s} quark distributions in the nucleon are also predicted from the chiral properties of QCD.
Date: June 1, 2000
Creator: Melnitchouk, W.
Partner: UNT Libraries Government Documents Department

QCD and the structure of the nucleon in electron scattering

Description: The internal structure of the nucleon is discussed within the context of QCD. Recent progress in understanding the distribution of flavor and spin in the nucleon is reviewed, and prospects for extending the author's knowledge of nucleon structure in electron scattering experiments at modern facilities such as Jefferson Lab are outlined.
Date: June 1, 2000
Creator: Melnitchouk, W.
Partner: UNT Libraries Government Documents Department

Quark-Hadron Duality in Electron Scattering

Description: Quark-hadron duality addresses some of the most fundamental issues in strong interaction physics, in particular the nature of the transition from the perturbative to non-perturbative regions of QCD. I summarize recent developments in quark-hadron duality in lepton-hadron scattering, and outline how duality can be studied at future high-luminosity facilities such as Jefferson Lab at 12 GeV, or an electron-hadron collider such as EPIC.
Date: September 1, 2000
Creator: Melnitchouk, W.
Partner: UNT Libraries Government Documents Department

A NEW METHOD FOR EXTRACTING SPIN-DEPENDENT NEUTRON STRUCTURE FUNCTIONS FROM NUCLEAR DATA

Description: High-energy electrons are currently the best probes of the internal structure of nucleons (protons and neutrons). By collecting data on electrons scattering off light nuclei, such as deuterium and helium, one can extract structure functions (SFs), which encode information about the quarks that make up the nucleon. Spin-dependent SFs, which depend on the relative polarization of the electron beam and the target nucleus, encode quark spins. Proton SFs can be measured directly from electron-proton scattering, but those of the neutron must be extracted from proton data and deuterium or helium-3 data because free neutron targets do not exist. At present, there is no reliable method for accurately determining spin-dependent neutron SFs in the low-momentum-transfer regime, where nucleon resonances are prominent and the functions are not smooth. The focus of this study was to develop a new method for extracting spin-dependent neutron SFs from nuclear data. An approximate convolution formula for nuclear SFs reduces the problem to an integral equation, for which a recursive solution method was designed. The method was then applied to recent data from proton and deuterium scattering experiments to perform a preliminary extraction of spin-dependent neutron SFs in the resonance region. The extraction method was found to reliably converge for arbitrary test functions, and the validity of the extraction from data was verifi ed using a Bjorken integral, which relates integrals of SFs to a known quantity. This new information on neutron structure could be used to assess quark-hadron duality for the neutron, which requires detailed knowledge of SFs in all kinematic regimes.
Date: January 1, 2009
Creator: Kahn, Y.F. & Melnitchouk, W.
Partner: UNT Libraries Government Documents Department

Parton distributions from lattice QCD

Description: We extract the x dependence of the nonsinglet u-d distribution function in the nucleon from the lowest few moments calculated on the lattice, using an extrapolation formula which ensures the correct behavior in the chiral and heavy quark limits. We discuss the implications for the quark mass dependence of meson masses lying on the J(PC)=1(--) Regge trajectory.
Date: August 1, 2001
Creator: Detmold, W.; Melnitchouk, W. & Thomas, A.W.
Partner: UNT Libraries Government Documents Department

Quark-hadron duality and the nuclear EMC effect

Description: Recent data on polarized proton knockout reactions off He-4 nuclei suggest a small but nonzero modification of proton electromagnetic form factors in medium. Using model independent relations derived on the basis of quark-hadron duality, we relate the medium modification of the form factors to the modification at large x of the deep-inelastic structure function of a bound proton. This places strong constraints on models of the nuclear EMC effect which assume a large deformation of the intrinsic structure of the nucleon in medium.
Date: November 1, 2001
Creator: Melnitchouk, W.; Tsushima, K. & Thomas, A.W.
Partner: UNT Libraries Government Documents Department

Connecting Structure Functions on the Lattice with Phenomenology

Description: We examine the extraction of moments of parton distributions from lattice data, focusing in particular on the chiral extrapolation as a function of the quark mass. Inclusion of the correct chiral behavior of the spin-averaged isovector distribution resolves a long-standing discrepancy between the lattice moments and experiment. We extract the x-dependence of the valence u-d distribution from the lowest few lattice moments, and discuss the implications for the quark mass dependence of meson masses lying on the a{sub 2} Regge trajectory. The role of chiral symmetry in spin-dependent distributions, and in particular the lattice axial vector charge, g{sub A}, is discussed.
Date: December 1, 2001
Creator: Detmold, W.; Melnitchouk, W. & Thomas, A.W.
Partner: UNT Libraries Government Documents Department

Dynamical symmetry breaking in the sea of the nucleon

Description: The authors derive the non-analytic chiral behavior of the flavor asymmetry dbar - ubar. Such behavior is a unique characteristic of Goldstone boson loops in chiral theories, including QCD, and establishes the unambiguous role played by the Goldstone boson cloud in the sea of the proton. Generalizing the results to the SU(3) sector, they show that strange chiral loops require that the s - sbar distribution be non-zero.
Date: May 1, 2000
Creator: Thomas, A.W.; Melnitchouk, W. & Steffens, F.M.
Partner: UNT Libraries Government Documents Department

A meson exchange model for the YN interaction

Description: We present a new model for the hyperon-nucleon (Lambda N, Sigma N) interaction, derived within the meson exchange framework. The model incorporates the standard one boson exchange contributions of the lowest pseudoscalar and vector meson multiplets with coupling constants fixed by SU(6) symmetry relations. In addition - as the main feature of the new model - the exchange of two correlated pions or kaons, both in the scalar-isoscalar (sigma) and vector-isovector (rho) channels, is included.
Date: September 1, 2001
Creator: Haidenbauer, J.; Melnitchouk, W. & Speth, J.
Partner: UNT Libraries Government Documents Department

Bound Nucleon Form Factors, Quark-Hadron Duality, and the Nuclear EMC Effect

Description: We discuss the electromagnetic form factors, axial form factors, and structure functions of a nucleon bound in the quark-meson coupling (QMC) model. Free space nucleon form factors are calculated using the improved cloudy bag model (ICBM). After describing finite nuclei and nuclear matter in the quark-based (EMC) model, the in-medium modification of the bound nucleon form factors is calculated in the same model. Finally, the bound nucleon structure function, F2, is extracted using the calculated in-medium electromagnetic form factors and Bloom-Gilman (quark-hadron) duality.
Date: September 13, 2002
Creator: Tsushima, K.; Lu, D.H.; Melnitchouk, W.; Saito, K. & Thomas, A.W.
Partner: UNT Libraries Government Documents Department

A New Slant on Hadron Structure

Description: Rather than regarding the restriction of current lattice QCD simulations to quark masses that are 5--10 times larger than those observed, we note that this presents a wonderful opportunity to deepen our understanding of QCD. Just as it has been possible to learn a great deal about QCD by treating N{sub c} as a variable, so the study of hadron properties as a function of quark mass is leading us to a much deeper appreciation of hadron structure. As examples we cite recent progress in using the chiral properties of QCD to connect hadron masses, magnetic moments, charge radii and structure functions calculated at large quark masses within lattice QCD with the values observed physically.
Date: December 1, 2000
Creator: Detmold, W.; Melnitchouk, W.; Leinweber, D. B.; Thomas, A. W. & Wright, S.V.
Partner: UNT Libraries Government Documents Department

Light quark simulations with FLIC fermions

Description: Hadron masses are calculated in quenched lattice QCD in order to probe the scaling behavior of a novel fat-link clover fermion action in which only the irrelevant operators of the fermion action are constructed using APE-smeared links. Light quark masses corresponding to an m{sub pi}/m{sub p} ratio of 0.35 are considered to assess the exceptional configuration problem of clover-fermion actions. This Fat-Link Irrelevant Clover (FLIC) fermion action provides scaling which is superior to mean-field improvement and offers advantages over nonperturbative improvement, including reduced exceptional configurations.
Date: June 1, 2002
Creator: Zanotti, J.M.; Leinweber, D.B.; Melnitchouk, W.; Williams, A.G. & Zhang, J.B.
Partner: UNT Libraries Government Documents Department

Neutron structure function and A=3 mirror nuclei

Description: The authors demonstrate that the free neutron structure function can be extracted in deep-inelastic scattering from A=3 mirror nuclei, with nuclear effects canceling to within 2% for x {approx_lt} 0.85.
Date: June 1, 2000
Creator: Afnan, I.R.; Bissey, F.; Gomez, J.; Katramatou, A.T.; Melnitchouk, W.; Petratos, G.G. et al.
Partner: UNT Libraries Government Documents Department

Spin-3/2 baryons in lattice QCD

Description: We present first results for masses of spin-3/2 baryons in lattice QCD, using a novel fat-link clover fermion action in which only the irrelevant operators are constructed using fat links. In the isospin-1/2 sector, we observe, after appropriate spin and parity projection, a strong signal for the J{sup P} = 3/2{sup -} state, and find good agreement between the 1/2{sup +} mass and earlier nucleon mass simulations with a spin-1/2 interpolating field. For the isospin-3/2 Delta states, clear mass splittings are observed between the various 1/2{sup +/-} and the 3/2{sup +/-} channels, with the calculated level orderings in good agreement with those observed empirically.
Date: June 1, 2002
Creator: Zanotti, J.M.; Choe, S.; Leinweber, D.B.; Melnitchouk, W.; Williams, A.G. & Zhang, J.B.
Partner: UNT Libraries Government Documents Department

Excited Baryons in Lattice QCD

Description: We present first results of calculations of masses of positive and negative parity excited baryons in lattice QCD using an O(a{sup 2}) improved gluon action and a fat-link clover fermion action in which only the irrelevant operators are constructed with fat links. The results are in agreement with earlier calculations of N* resonances using improved actions and exhibit a clear mass splitting between the nucleon and its chiral partner. The results also indicate a splitting between the lowest J{sup P}=(1/2){sup -} states for the two standard nucleon interpolating fields. The study of different Lambda interpolating fields suggests a similar splitting between the lowest two Lambda{sup 1/2{sup -}} octet states. However, the empirical mass suppression of the Lambda*(1405) is not evident in these quenched QCD simulations, suggesting an important role for the meson cloud of the Lambda*(1405) or a need for more exotic interpolating fields.
Date: March 1, 2002
Creator: Melnitchouk, W.; Bilson-Thompson, S.; Bonnet, F.; F.X.Lee; D.B.Leinweber; A.G.Williams et al.
Partner: UNT Libraries Government Documents Department

Baryon Resonances from a Novel Fat-Link Fermion Action

Description: We present first results for masses of positive and negative parity excited baryons in lattice QCD using an O(a{sup 2}) improved gluon action and a Fat Link Irrelevant Clover (FLIC) fermion action in which only the irrelevant operators are constructed with fat links. The results are in agreement with earlier calculations of N* resonances using improved actions and exhibit a clear mass splitting between the nucleon and its chiral partner, even for the Wilson fermion action. The results also indicate a splitting between the lowest J{sup P}=1/2{sup -} states for the standard nucleon interpolating fields.
Date: July 1, 2001
Creator: Melnitchouk, W.; Bilson-Thompson, S.; Bonnet, F. D. R.; Coddington, P. D.; Lee, F. X.; Leinweber, D. B. et al.
Partner: UNT Libraries Government Documents Department

Hadron Masses From Novel Fat-Link Fermion Actions

Description: The hadron mass spectrum is calculated in lattice QCD using a novel fat-link clover fermion action in which only the irrelevant operators in the fermion action are constructed using smeared links. The simulations are performed on a 16{sup 3} x 32 lattice with a lattice spacing of a=0.125 fm. We compare actions with n=4 and 12 smearing sweeps with a smearing fraction of 0.7. The n=4 Fat-Link Irrelevant Clover (FLIC) action provides scaling which is superior to mean-field improvement, and offers advantages over nonperturbative 0(a) improvement, including a reduced exceptional configuration problem.
Date: November 1, 2001
Creator: Zanotti, J. M.; Bilson-Thompson, S.; Bonnet, F. D. R.; Coddington, P. D.; Leinweber, D. B.; Williams, A. G. et al.
Partner: UNT Libraries Government Documents Department

Novel Fat-Link Fermion Actions

Description: The hadron mass spectrum is calculated in lattice QCD using a novel fat-link clover fermion action in which only the irrelevant operators in the fermion action are constructed using smeared links. The simulations are performed on a 16{sup 3} x 32 lattice with a lattice spacing of a=0.125 fm. We compare actions with n=4 and 12 smearing sweeps with a smearing fraction of 0.7. The n=4 Fat-Link Irrelevant Clover (FLIC) action provides scaling which is superior to mean-field improvement, and offers advantages over nonperturbative 0(a) improvement.
Date: July 1, 2001
Creator: Zanotti, J. M.; Bilson-Thompson, S.; Bonnet, F. D. R.; Coddington, P. D.; Leinweber, D. B.; Williams, A. G. et al.
Partner: UNT Libraries Government Documents Department