arXiv subjects

A. W. Thomas

Publications and source records attributed to A. W. Thomas.

At least 397 records · Page 22Linked to original sources

arXiv1997

Hadrons in Dense Matter

There is currently enormous interest in the investigation of how hadron properties may be altered by immersion in matter. There is strong evidence of a reduction in the mass of the rho meson from relativistic heavy ion collisions as well as a hint from a recent experiment on photoproduction in light nuclei. We briefly review the main theoretical ideas which lead one to expect the mass of a hadron to change in matter, including the various QCD-based methods, notably the QCD sum rules, as well as mean-field, quark based models like QMC and conventional nuclear approaches such as QHD.

nucl-th↗

arXiv1997

In-medium electron-nucleon scattering

In-medium nucleon electromagnetic form factors are calculated in the quark meson coupling model. The form factors are typically found to be suppressed as the density increases. For example, at normal nuclear density and $Q^2 \sim 0.3 { GeV}^2$, the nucleon electric form factors are reduced by approximately 8% while the magnetic form factors are reduced by only 1 - 2%. These variations are consistent with current experimental limits but should be tested by more precise experiments in the near future.

nucl-th↗

arXiv1997

Naturalness in the quark-meson coupling model

The quark-meson coupling (QMC) model is examined using Georgi's ``naive dimensional analysis''. We argue that the QMC model is quite natural as an effective field theory for nuclei.

nucl-th↗

arXiv1997

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

We obtain a general rule that the O(1/log m_h) term due to the current \bar{h}γ_μγ_5h of a mass-m_h quark h in f-flavour theory is -3\bar{g}^2_f(m_h)/{2π^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.

hep-ph↗

arXiv1997

Rho-meson mass in light nuclei

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

nucl-th↗

arXiv1996

The Flavor Asymmetry of the Nucleon Sea

We re-examine the effects of anti-symmetry on the anti-quarks in the nucleon sea arising from gluon exchange and pion exchange between confined quarks. While the effect is primarily to suppress anti-down relative to anti-up quarks, this is numerically insignificant for the pion terms.

nucl-th↗

arXiv1996

Delta electroproduction in a chiral bag model approach

We study the γ^* N -> Δtransition amplitudes in a recoil corrected cloudy bag model approach. A modified Peierls-Thouless projection method is used to construct the Galilean invariant baryon states. The pionic contribution is found to be significant. The effect of the recoil correction is to reduce the magnitude of the transition amplitudes at small momentum transfer and to enhance them at modest momentum transfers.

nucl-th↗

arXiv1996

A chiral bag model approach to delta electroproduction

Helicity amplitudes for the $γ^* N \to Δ$ 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 transfer 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.

nucl-th↗

arXiv1996

Variation of hadron masses in finite nuclei

The quark-meson coupling model, based on a mean-field description of non-overlapping nucleon bags bound by the self-consistent exchange of $σ$, $ω$ and $ρ$ 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 non-strange 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 non-strange quarks in the hadron and the value of the scalar mean-field in a nucleus.

nucl-th↗

arXiv1996

Resonance model study of strangeness production in pp collisions

Results for the energy dependence of the elementary kaon production cross sections in proton-proton collisions are reported. Calculations are performed within an extended version of the resonance model which was used for the previous studies of elementary kaon production in pion-nucleon and pion-$Δ$ collisions. Although the model treatment is within the {\it empirical} tree level (observed widths for the resonances are used), it is fully relativistic, and includes all relevant baryon resonances up to 2 GeV. One of the purposes of this study is to provide the results for the simulation codes of subthreshold kaon production in heavy ion collisions. This is the first, consistent study of the elementary kaon production reactions including both $πB$ and $B B$ ($B=N, Δ$) collisions on the same footing. Comparisons are made between the calculated results and the existing semi-empirical parametrizations which are widely used for the simulation codes, as well as the experimental data.

nucl-th↗

arXiv1996

On the Interpretation of the NA51 Experiment

We study the $p-n$ Drell-Yan asymmetry, recently measured by the NA51 collaboration, and conclude that the value quoted by their experiment only sets a lower limit on the asymmetry of the proton sea. In particular, we notice that charge symmetry breaking between the proton and the neutron may produce corrections which should be taken into account.

hep-ph↗

arXiv1996

Determination of flavor asymmetry for $Σ^{\pm}$ by the Drell-Yan process

Flavor asymmetries for the valence and sea quarks of the $Σ^{\pm}$ can be obtained from Drell-Yan experiments using charged hyperon beams on proton and deuteron targets. A large, measurable difference in sea quark asymmetries is predicted between SU(3) and pseudoscalar meson models. The latter predict that in $Σ^{+}$, $\bar{u}/\bar{d} \leq 1/2$, whereas the former predict $\bar{u}/\bar{d} \approx 4/3$. Estimates of valence quark asymmetries based on quark models also show large deviations from SU(3) predictions, which should be measurable.

hep-ph↗

arXiv1996

CVC in Particle Physics

We review the hypothesis of the conserved vector current (CVC) within the Standard Model. In addition to the classic tests, such as pion beta decay and neutrino scattering, we mention recent tests involving LEP data. As well as providing a clear indication that the isovector current is not conserved, rho-omega mixing offers a fascinating opportunity to study CP violation at B-factories and we outline these ideas. Finally we briefly touch on a new approach to mass generation in the Standard Model which, for example, leads to the up-down mass difference which breaks CVC.

nucl-th↗

arXiv1996

Deep Inelastic Structure Functions in a Covariant Spectator Model

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.

hep-ph↗

arXiv1996

Polarized Deep-Inelastic Scattering from Nuclei -- A Relativistic Approach

A covariant formulation of spin-dependent deep-inelastic scattering from nuclei is presented. In the relativistic impulse approximation the inclusive nuclear cross section is described in terms of the amplitude for forward, virtual-photon scattering from an off-mass-shell nucleon. The general structure of the off-shell nucleon hadronic tensor is derived, and the leading behavior of the off-shell nucleon structure functions computed in the Bjorken limit. The formalism, which is valid for nucleons bound inside nuclei with spin 1/2 or 1, is applied to the case of the deuteron.

nucl-th↗

arXiv1996

Virtual Compton Scattering from the Proton and the Properties of Nucleon Excited States

We calculate the $N^*$ contributions to the generalized polarizabilities of the proton in virtual Compton scattering. The following nucleon excitations are included: $N^*(1535)$, $N^*(1650)$, $N^*(1520)$, $N^*(1700)$, $Δ(1232)$, $Δ^*(1620)$ and $Δ^*(1700)$. The relationship between nucleon structure parameters, $N^*$ properties and the generalized polarizabilities of the proton is illustrated.

nucl-th↗

arXiv1996

Virtual Compton Scattering and Generalized Polarizabilities of the Proton

Threshold photon electroproduction off the proton allows one to measure new electromagnetic observables which generalise the usual polarisabilities. There are -- a priori -- ten "generalised polarisabilities", functions of the virtual photon mass. The purpose of this paper is to lay down the appropriate formalism to extract these quantities from the photon electroproduction cross sections. We also give a first estimate of the generalised polarisabilities in the non relativistic quark model.

nucl-th↗

arXiv1996

Kaon-Nucleus Drell-Yan Processes and Kaon Structure Functions

We investigate the information which could be obtained from Drell-Yan processes with sufficiently intense beams of charged kaons on isoscalar targets. It is found that combinations of $K^+$ and $K^-$ Drell-Yan measurements on isoscalar nuclear targets would allow one to extract the kaon sea quark distributions. These cross sections are also sensitive to the strange valence quark distribution in the kaon, although one would need a significant increase over the presently available kaon fluxes in order to extract this quantity with sufficient accuracy.

hep-ph↗