Showing papers by "Anthony W. Thomas published in 1997"

Variation of hadron masses in finite nuclei

Abstract: The quark-meson coupling model, based on a mean-field description of nonoverlapping nucleon bags bound by the self-consistent exchange of $\ensuremath{\sigma}$, $\ensuremath{\omega}$, and $\ensuremath{\rho}$ mesons, is extended to investigate the change of hadron properties in finite nuclei. Relativistic Hartree equations for spherical nuclei have been derived from a relativistic quark model of the structure of bound nucleons and mesons. Using this unified, self-consistent description of both infinite nuclear matter and finite nuclei, we investigate the properties of some closed-shell nuclei and study the changes in the hadron masses of the nonstrange vector mesons, the hyperons, and the nucleon in those nuclei. We find a new, simple scaling relation for the changes of the hadron masses, which can be described in terms of the number of nonstrange quarks in the hadron and the value of the scalar mean field in a nucleus.

Chiral bag model approach to {Delta} electroproduction

Abstract: Helicity amplitudes for the {gamma}{sup {asterisk}}N{r_arrow}{Delta} transition are calculated using the cloudy bag model. A correction for center-of-mass motion is carried out using a modified Peierls-Thouless projection method. This reduces the magnitude of the transition amplitudes at small momentum transfers and enhances them at modest momentum transfers. Our calculation shows that the pion cloud contributes substantially to the transition helicity amplitudes, with the final result giving reasonable agreement with the corresponding experimental values. {copyright} {ital 1997} {ital The American Physical Society}

ρ-meson mass in light nuclei

Abstract: The quark-meson coupling (QMC) model is applied to a study of the mass of the $\ensuremath{\rho}$-meson in helium and carbon nuclei. The average mass of a $\ensuremath{\rho}$-meson formed in ${}^{3,4}$He and ${}^{12}$C is expected to be around 730, 690, and 720 MeV, respectively.

Flavor asymmetry of the nucleon sea

Abstract: We reexamine the effects of antisymmetry on the antiquarks in the nucleon sea arising from gluon exchange and pion exchange between confined quarks. While the effect is primarily to suppress down relative to up antiquarks, this is numerically insignificant for the pion terms. {copyright} {ital 1997} {ital The American Physical Society}

Deep-inelastic structure functions in a covariant spectator model

Abstract: Deep-inelastic structure functions are studied within a covariant scalar diquark spectator model of the nucleon. Treating the target as a two-body bound state of a quark and a scalar diquark, the Bethe-Salpeter equation (BSE) for the bound state vertex function is solved in the ladder approximation. The valence quark distribution is discussed in terms of the solutions of the BSE. {copyright} {ital 1997} {ital The American Physical Society}

Quarks in finite nuclei

Abstract: We describe the development of a theoretical description of the structure of finite nuclei based on a relativistic quark model of the structure of the bound nucleons which interact through the (self-consistent) exchange of scalar and vector mesons.

Electromagnetic gauge invariance of the cloudy bag model

Abstract: We examine the question of the gauge invariance of electromagnetic form factors calculated within the cloudy bag model. One of the assumptions of the model is that electromagnetic form factors are most accurately evaluated in the Breit frame. This feature is used to show that gauge invariance is respected in this frame.

Near-threshold Isospin Violation in the Pion Form Factor from Chiral Perturbation Theory

Abstract: We examine the isospin violation in the timelike pion form factor near threshold at next-to-leading order in the chiral expansion using the techniques of Chiral Perturbation Theory. This next-to-leading order contribution contains the first nonvanishing isospin violation. This isospin violation is found to be very small near threshold. In particular, the isospin violation at threshold is found to be of order 10 4 , which should be compared with the few percent level seen in the vector

HERA Anomaly and Hard Charm in the Nucleon

Abstract: We explore the possibility that the excess neutral and charged current events seen by the H1 and ZEUS Collaborations at HERA at large x and Q^2 could arise from a hard charm component of the nucleon. While the symmetric intrinsic charm hypothesis is unable to account for the HERA anomaly, a non-symmetric charm distribution generated non-perturbatively, for which c-bar is much harder than c, can produce significant enhancement of cross sections at the HERA kinematics.

On extracting the rho-omega mixing amplitude from the pion form-factor

Abstract: In this paper we improve and extend a recent analysis which showed that the rho-omega mixing amplitude cannot be unambiguously extracted from the pion electromagnetic form-factor in a model independent way. In particular, we focus on the argument that the extraction is sensitive to the presence of any intrinsic omega_I -> pi pi coupling. Our extended analysis confirms the original conclusion, with only minor, quantitative differences. The extracted mixing amplitude is shown to be sensitive to both the intrinsic coupling omega_I -> pi pi and to the value assumed for the mass of the rho^0-meson.

Processing of mercurous chloride in space

Abstract: Mercurous chloride is an acoustical optical material with an unusually low acoustic velocity and high acousto-optical figure of merit, which makes it an interesting candidate for optical delay lines and Bragg cells for optical signal processors. It also has a broad range of spectral transmissivity which makes it an ideal candidate for wide band acoustically tuned optical filter (ATOF) applications. Single crystals of this material can be readily grown in normal gravity by closed-tube physical vapor transport, but the crystals appear to contain structural inhomogeneities which degrade the optical performance. The nature of these defects is not known, but their degree appears to correlate with the Rayleigh number that characterizes their growth; hence, it is suspected that uncontrolled convection may play a role in the defect structure. This prompted a space flight experiment to determine if these defects could be further reduced by virtually eliminating the buoyancy-driven convective flows which are always present to a degree in normal gravity. Single crystals of mercurous chloride (Hg2Cl2) were grown in the Space Experiment Facility (SEF) transparent furnace developed by the University of Alabama in Huntsville, Consortium for Materials Development in Space. The Northrop- Grumman Science and Technology Center provided the highly purified starting material and analyzed the crystals that were grown. This experiment was flown on Spacehab 4 (STS-77) in May 1996. The SEF is a transparent furnace which allowed the progress of the growth to be recorded by video. Extensive furnace profiling and modeling has been carried out to relate the growth front location to the thermal environment and to the crystal quality. The results of the flight experiment as well as the ground control experiments are presented.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

In-medium kaon and antikaon properties in the quark-meson coupling model

Abstract: The properties of the kaon, $K$, and antikaon, $\kbar$, in nuclear medium are studied in the quark-meson coupling (QMC) model. Employing a constituent quark-antiquark (MIT bag model) picture, their excitation energies in a nuclear medium at zero momentum are calculated within mean field approximation. The scalar, and the vector mesons are assumed to couple directly to the nonstrange quarks and antiquarks in the $K$ and $\kbar$ mesons. It is demonstrated that the $\rho$ meson induces different mean field potentials for each member of the isodoublets, $K$ and $\kbar$, when they are embedded in asymmetric nuclear matter. Furthermore, it is also shown that this $\rho$ meson potential is repulsive for the $K^-$ meson in matter with a neutron excess, and renders $K^-$ condensation less likely to occur.

Self-consistent description of $\Lambda$ hypernuclei in the quark-meson coupling model

Abstract: The quark-meson coupling (QMC) model, which has been successfully used to describe the properties of both finite nuclei and infinite nuclear matter, is applied to a study of $\Lambda$ hypernuclei. With the assumption that the (self-consistent) exchanged scalar, and vector, mesons couple only to the u and d quarks, a very weak spin-orbit force in the $\Lambda$-nucleus interaction is achieved automatically. This is a direct consequence of the SU(6) quark model wave function of the $\Lambda$ used in the QMC model. Possible implications and extensions of the present investigation are also discussed.

Direct CP Violation in $B^\pm\to\rho^\pm\pi^+\pi^-$ in the $\rho^0$-$\omega$ Interference Region

Abstract: We study direct CP violation in $B^\pm \to \rho^\pm\rho^0(\omega) \to \rho^\pm\pi^+\pi^-$ and focus specifically on the rate asymmetry in the $\rho^0$-$\omega$ interference region. Here the strong phase is dominated by isospin violation, so that it can be essentially determined by $e^+e^- \to \rho^0 (\omega) \to \pi^+\pi^-$ data. We find the CP-violating asymmetry to be of the order of 20% at the $\omega$ invariant mass. Moreover, it is robust with respect to the estimable strong-phase uncertainties, permitting the extraction of $\sin\alpha$ from this channel.

Non-Perturbative Structure of the Nucleon

Abstract: While much attention has been focussed on the successes of perturbative QCD in describing the $Q^2$-dependence of deep-inelastic structure functions, the starting distributions themselves contain important, non-perturbative information on the structure of the nucleon, which has been somewhat neglected We review some of the most important, recent discoveries resulting from studies of deep-inelastic scattering There are important connections between these discoveries and low energy properties of the nucleon and wherever possible we shall make these clear In particular, we shall see that well known features of QCD, such as dynamical symmetry breaking, are reflected in the properties of the measured parton distributions

Measuring anomalous ``spin'' in elastic e-p or ν-p and deep inelastic e-p scattering

Abstract: We obtain a general rule that the O(1/log m_h) term due to the current \bar{h}\gamma_\mu\gamma_5h of a mass-m_h quark h in f-flavour theory is -3\bar{g}^2_f(m_h)/{2\pi^2(33-2f)} times the flavour singlet current of the residual (f-1)-flavour theory, where \bar{g}_f is the f-flavour running coupling constant in a mass-independent renormalization scheme. The rule is applied to the Ellis-Jaffe moment below and well above charm threshold, and to low-energy Z^0-exchange amplitudes. The singlet axial charge of the proton common to these experiments is both scale and gauge invariant, but is related to the axial anomaly and the ``gluon spin'' by a non-perturbative renormalization factor.

Deuteron Structure Functions in the Context of Few-Body Physics

Abstract: Few-body systems offer a unique challenge to those interested in deep inelastic scattering. Using the deuteron as the simplest and most easily solvable nuclear system we review the main physics issues in the valence and sea quark regimes. For the former the key issue is the understanding of Fermi motion and binding corrections, and our ability to extract the ratio F_2n/F_2p as x -> 1. The most recent analysis of deuteron data for F_2n/F_2p is consistent with perturbative QCD, but inconsistent with the assumptions common to all standard parton distribution fits. This should be taken into account in calculating event rates at HERA --- especially at large x and Q^2. The extraction of g_1n also presents an opportunity to refine the description of nuclear effects in spin-dependent structure functions. In the region of small x one must correct for shadowing and meson exchange current effects in order to explore the flavor structure of sea, which is, in turn, deeply related to the chiral structure of the nucleon.

Testing Parton Charge Symmetry at HERA

Abstract: There are strong theoretical indications that the minority valence quark distributions in the nucleon may break charge symmetry by as much as 3-5%. We show that a comparison of $e^{\pm} D$ deep inelastic scattering through the charged current, for example at HERA, could provide a direct test of this effect. This measurement is also be sensitive to an intrinsic component of the strange quark sea which leads to $s eq \bar{s}$.

Quark and gluon condensates in the quark-meson coupling 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.