Showing papers by "Anthony W. Thomas published in 1998"
TL;DR: In this paper, the authors investigated theoretically whether it is feasible to detect η - and ω -nucleus bound states in closed shell nuclei, 16 O, 40 Ca, 90 Zr and 208 Pb.
84 citations
TL;DR: In this paper, a set of nucleon electromagnetic form factors were studied in the cloudy bag model (CBM) with center-of-mass and recoil corrections and the results showed that the shapes of these form factors are significantly improved with respect to the experimental data if the Lorentz contraction of the internal structure of the baryon is also appropriately taken into account.
Abstract: Nucleon electromagnetic form factors are studied in the cloudy bag model (CBM) with center-of-mass and recoil corrections. This is the first presentation of a full set of nucleon form factors using the CBM. The center-of-mass motion is eliminated via several different momentum projection techniques and the results are compared. It is found that the shapes of these form factors are significantly improved with respect to the experimental data if the Lorentz contraction of the internal structure of the baryon is also appropriately taken into account.
82 citations
TL;DR: In this paper, the properties of the kaon, K, and antikaon in nuclear medium are studied in the quark-meson coupling (QMC) model, where the scalar and vector mesons are assumed to couple directly to the nonstrange quarks and antiquarks in the K and K mesons.
79 citations
TL;DR: In-medium nucleon electromagnetic form factors are calculated in the quark meson coupling model as discussed by the authors, and they are typically found to be suppressed as the density increases, for example, at normal nuclear density and Q 2 ∼ 0.3 GeV 2.
72 citations
TL;DR: In this article, the authors investigate theoretically whether it is feasible to detect $\eta$-and $\omega$-nucleus bound states in closed shell and closed shell nuclei.
Abstract: We investigate theoretically whether it is feasible to detect $\eta$- and $\omega$-nucleus bound states. As well as the closed shell nuclei, $^{16}$O, $^{40}$Ca, $^{90}$Zr and $^{208}$Pb, we also investigate $^6$He, $^{11}$B and $^{26}$Mg, which are the final nuclei in the proposed experiment involving the (d,$^3$He) reaction at GSI. Potentials for the $\eta$ and $\omega$ mesons in these nuclei are calculated in local density approximation, embedding the mesons in the nucleus described by solving the mean-field equations of motion in the QMC model. Our results suggest that one should expect to find $\eta$- and $\omega$-nucleus bound states in all these nuclei.
69 citations
TL;DR: In this paper, the authors present a comprehensive analysis of the recent data from the E866 experiment at Fermilab on Drell-Yan production in pD and pP collisions, which indicates a non-trivial $x$ dependence for the asymmetry between ifmmode \bar{u}\else \={u}\fi{}$ and the quark distributions in the proton.
Abstract: We present a comprehensive analysis of the recent data from the E866 experiment at Fermilab on Drell-Yan production in $\mathrm{pD}$ and $\mathrm{pp}$ collisions, which indicates a non-trivial $x$ dependence for the asymmetry between $\ifmmode \bar{u}\else \={u}\fi{}$ and $\overline{d}$ quark distributions in the proton. The relatively fast decrease of the asymmetry at large $x$ suggests the important role played by the chiral structure of the nucleon, in particular the $\ensuremath{\pi}N$ and \ensuremath{\pi}\ensuremath{\Delta} components of the nucleon wave function. At small $x$ the data require an additional non-chiral component, which may be attributed to the Pauli exclusion principle, as first suggested by Field and Feynman.
64 citations
TL;DR: In this paper, the chemical-potential dependence of the dressed-quark propagator is explored, which provides a means of determining the behaviour of the chiral and deconfinement order parameters, and low-energy pion observables.
58 citations
45 citations
TL;DR: In this article, the authors examine the possibility that charge symmetry is violated in parton distributions, and propose a series of possible experimental tests of charge symmetry violation for both valence and sea quark distributions.
45 citations
TL;DR: In this article, the dispersion relation and spectral functions of the scalar and vector mesons in nuclear matter were studied in detail using the Walecka model over a wide range of densities and including the effects of a finite $\sigma$ width through the inclusion of a two-pion loop.
Abstract: The propagation of the scalar ($\sigma$) and vector ($\omega$) mesons in nuclear matter is studied in detail using the Walecka model over a wide range of densities and including the effects of a finite $\sigma$ width through the inclusion of a two-pion loop. We calculate the dispersion relation and spectral functions of the $\sigma$ and (transverse and longitudinal) $\omega$ mesons, including the effect of $\sigma$-$\omega$ mixing in matter. It is shown that the mixing effect is quite important in the propagation of the (longitudinal) $\omega$ and $\sigma$ mesons above normal nuclear matter density. We find that there is a two-peak structure in the spectral function of the $\sigma$ channel, caused by $\sigma$-$\omega$ mixing.
38 citations
TL;DR: In this article, the authors show that the electric and magnetic radii of the bound nucleon are almost linearly correlated with the bag constant of the quark-meson coupling (QMC) model.
TL;DR: In this paper, it was shown that a measurement of the rate asymmetry, which is CP violating, in B{sup {plus_minus}}{r_arrow}{rho}{sup { plus_minus}{rho 1 + ϵ 2 ϵ 0, ϵ 1, ϵ ∈ {1, 2}.
Abstract: The extraction of Cabibbo-Kobayashi-Maskawa matrix element information from hadronic B decays generally suffers from discrete ambiguities, hampering the diagnosis of physics beyond the standard model. We show that a measurement of the rate asymmetry, which is CP violating, in B{sup {plus_minus}}{r_arrow}{rho}{sup {plus_minus}}{rho}{sup 0}({omega}){r_arrow}{rho}{sup {plus_minus}}{pi}{sup +}{pi}{sup {minus}}, where the invariant mass of the {pi}{sup +}{pi}{sup {minus}} pair is in the vicinity of the {omega} resonance, can remove the mod({pi}) uncertainty in {alpha}{equivalent_to}arg[{minus}V{sub td}V{sup *}{sub tb}/(V{sub ud}V{sup *}{sub ub})] present in standard analyses. {copyright} {ital 1998} {ital The American Physical Society}
TL;DR: In this article, the authors examine two effects which might account for this experimental discrepancy: nuclear shadowing corrections for neutrinos, and strange quark contributions s(x) 6 ¯(x), and show that neither of these two corrections removes the experimental discrepancy between the structure functions.
Abstract: In principle one can test the validity of charge symmetry for parton distributions by comparing structure functions measured in neutrino and charged lepton deep inelastic scattering New experiments make such tests possible; they provide rather tight upper limits on parton charge symmetry violation [CSV] for intermediate Bjorken x, but appear to show evidence for CSV effects at small x We examine two effects which might account for this experimental discrepancy: nuclear shadowing corrections for neutrinos, and strange quark contributions s(x) 6 ¯(x) We show that neither of these two corrections removes the experimental discrepancy between the structure functions We are therefore forced to consider the possibility of a surprisingly large CSV effect in the nucleon sea quark distributions PACS: 1360Hb, 1315+g, 1240Vv, 1140Ha
TL;DR: In this article, the invariant mass distribution and the Dalitz plot for the $NN{\to}N{\Sigma}K$ reaction were used to provide direct information about the resonance excitations and meson exchange contributions.
Abstract: Using the resonance model, which was successfully applied for the study of the $pp{\to}p{\Lambda}K^+$ reaction, we investigate $NN{\to}N{\Sigma}K$ reactions that should provide cleaner information about resonance excitations and meson exchange contributions. For this purpose we demonstrate that the invariant mass distribution for the ${\Sigma}K$ system, as well as the Dalitz plot for the $NN{\to}N{\Sigma}K$ reaction, provide direct information about the ${\Sigma}K$ production mechanism, which can be tested in the near future by experiments at COSY.
TL;DR: In this article, the change in the form factor of the free case with respect to the quark meson coupling model was investigated and it was found that the free form factor may be suppressed by as much as 12% at Q2 = 0.5 GeV2.
TL;DR: In this article, the authors studied the effect of the invariant mass of the bottom baryon pair in the vicinity of the ϵ resonance on the branching ratio of the hadronic wave function of the neutron.
Abstract: We study direct $\mathrm{CP}$ violation in the bottom baryon decays ${\ensuremath{\Lambda}}_{b}\ensuremath{\rightarrow}f{\ensuremath{\rho}}^{0}(\ensuremath{\omega})\ensuremath{\rightarrow}f{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}} (f=n$ or $\ensuremath{\Lambda}).$ It is found that in these decays via $\ensuremath{\rho}\ensuremath{-}\ensuremath{\omega}$ mixing the $\mathrm{CP}$ violation could be very large when the invariant mass of the ${\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$ pair is in the vicinity of the $\ensuremath{\omega}$ resonance. With a typical value ${N}_{c}=2$ in the factorization approach, the maximum $\mathrm{CP}$-violating asymmetries are more than 50% and 68% for ${\ensuremath{\Lambda}}_{b}\ensuremath{\rightarrow}n{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$ and ${\ensuremath{\Lambda}}_{b}\ensuremath{\rightarrow}\ensuremath{\Lambda}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}},$ respectively. With the aid of heavy quark symmetry and phenomenological models for the hadronic wave functions of ${\ensuremath{\Lambda}}_{b},$ $\ensuremath{\Lambda}$ and the neutron, we estimate the branching ratios of ${\ensuremath{\Lambda}}_{b}\ensuremath{\rightarrow}n(\ensuremath{\Lambda}){\ensuremath{\rho}}^{0}.$
TL;DR: In this paper, shadowing corrections to the structure function of neutrino and muon deep inelastic scattering on heavy nuclear targets were discussed, and the role played by shadowing in the comparison of the structure functions was examined.
Abstract: We discuss shadowing corrections to the structure function ${F}_{2}$ in neutrino deep-inelastic scattering on heavy nuclear targets. In particular, we examine the role played by shadowing in the comparison of the structure functions ${F}_{2}$ measured in neutrino and muon deep inelastic scattering. The importance of shadowing corrections in the determination of the strange quark distributions is explained.
TL;DR: In this article, the density dependence of the quark and gluon condensates in nuclear matter was studied using a quark-meson coupling (QMC) model.
Abstract: Using the quark–meson coupling (QMC) model, we study the density dependence of the quark and gluon condensates in nuclear matter. We show that the change of the quark condensate is mainly driven by the scalar field in the medium and that the reduction of the quark condensate is suppressed at high density, even in the mean-field approximation. The gluon condensate decreases by 4–6% at nuclear saturation density. We also give a simple relationship between the change of the quark condensate and that of a hadron mass in the medium.
TL;DR: In this article, the changes induced in the spin-dependent, valence parton distributions of the proton and neutron bound in $3$He and $6$Li were estimated.
Abstract: The determination of the spin distribution functions for the neutron requires an understanding of the changes induced by a nuclear medium. We present the first estimates of the changes induced in the spin-dependent, valence parton distributions of the proton and neutron bound in $^3$He and $^6$Li. For $^3$He the result is quite reassuring, in that the off-shell correction to $g_{1n}$ is very small. On the other hand, in $^6$Li the corrections to the spin distributions in the bound proton are sufficiently large that they should be taken into account if one aims for a precision of better than 10%. We provide a simple parametrization of the off-shell changes in the valence spin distributions of $^3$He and $^6$Li, which can be used with any conventional estimate of nuclear binding and Fermi motion corrections.
Posted Content•
[...]
06 Oct 1998
TL;DR: In this article, it was shown that the D^- meson will form narrow bound states with ∆ ∆ Pb, where ∆ is the number of particles in the quark-meson coupling (QMC) model.
Abstract: We show that the D^- meson will form narrow bound states with ^{208}Pb. Mean field potentials for the D^0, D^0-bar and D^- in ^{208}Pb are calculated self-consistently using the quark-meson coupling (QMC) model in local density approximation. The meson-^{208}Pb bound state energies are then calculated by solving the Klein-Gordon equation with these potentials. The experimental confirmation and comparison with the D^0-bar and D^0 will provide distinctive information on the nature of the interaction between the charmed meson and matter.
Book•
01 Jan 1998
TL;DR: Chiral symmetry and symmetry breaking QCD at finite temperatures and densities lattice and functional methods phenomenology and phenomenological approaches strong-coupling QED and other field theories.
Abstract: Chiral symmetry and symmetry breaking QCD QCD at finite temperatures and densities lattice and functional methods phenomenology and phenomenological approaches strong-coupling QED and other field theories.
TL;DR: In this paper, the authors explore the feasibility of directly extracting the large x valence d / u ratio through a measurement of pions in the current fragmentation region of semi-inclusive deep-inelastic scattering from protons.
01 Aug 1998
TL;DR: In this paper, an elementary introduction to some of the quark models used to understand the properties of light mesons and baryons is presented for both theoretical and experimental graduate students beginning their study of the strong interaction.
Abstract: We present an elementary introduction to some of the quark models used to understand the properties of light mesons and baryons. These lectures are intended for both theoretical and experimental graduate students beginning their study of the strong interaction.
01 Jul 1998
TL;DR: In this paper, the quark meson coupling model was used to calculate the change in the proton electric and magnetic form factors when it is bound in a specific shell model orbit.
Abstract: We review recent calculations using the quark meson cou-pling model which should be of particular interest at Jefferson Lab.In particular, we discuss the change in the proton electric and mag-netic form factors when it is bound in a specific shell model orbit.Modern quasi-elastic electron scattering experiments should be ableto detect effects of the size predicted. We also examine the meanfield potential felt by an ω in a finite nucleus, concluding that theω should be bound by between 50 and 100 MeV.I. INTRODUCTION
01 Dec 1998-Nuclear Instruments & Methods in Physics Research Section A-accelerators Spectrometers Detectors and Associated Equipment
TL;DR: In this article, a superconducting wire is wound on the inner cooling shield of the vertical dilution refrigerator of the Bonn Frozen Spin Target (BoFroST), which is used for a measurement of the target asymmetry in the η-photoproduction γp↑→ηp with the PHOENICS-detector at the BER.
Abstract: A new concept of a small superconducting ‘holding magnet’, placed inside a polarization refrigerator, has been developed for frozen spin targets. The superconducting wire is wound on the inner cooling shield of the vertical dilution refrigerator of the Bonn Frozen Spin Target (BoFroST). The maximum field of the magnet is 0.38 tesla. Based on this concept, the frozen spin target was used for a measurement of the target asymmetry in the η-photoproduction γp↑→ηp with the PHOENICS-detector at the Bonn electron stretcher accelerator ELSA.
Posted Content•
TL;DR: In this paper, the quark meson coupling model was used to calculate the change in the proton electric and magnetic form factors when it is bound in a specific shell model orbit.
Abstract: We review recent calculations using the quark meson coupling model which should be of particular interest at Jefferson Lab. In particular, we discuss the change in the proton electric and magnetic form factors when it is bound in a specific shell model orbit. Modern quasi-elastic electron scattering experiments should be able to detect effects of the size predicted. We also examine the mean field potential felt by an $\omega$ in a finite nucleus, concluding that the $\omega$ should be bound by between 50 and 100 MeV.
TL;DR: In this article, a comparison of e ± D deep inelastic scattering through the charged current, for example at HERA, could provide a direct test of the effect of minority valence quark distributions in the nucleon.
Posted Content•
07 Dec 1998
TL;DR: In this article, it was shown that pion exchange can not only account for leading neutron but also for a large fraction of the leading proton production in the H1 data.
Abstract: We point out that the $\Delta\pi$ component of the nucleon wave function is vital to the interpretation of the recent H1 data for leading baryon production. While the $n/p$ ratio is equal to two with the $N\pi$ component alone, the inclusion of the $\Delta\pi$ component brings this ratio very near to unity, as observed in the experiment. This result demonstrates that pion exchange can not only account for leading neutron but also for a large fraction of the leading proton production.