arXiv subjects
Publications and source records attributed to A. W. Thomas.
An explicit pionic component of the nucleon may be identified by measuring polarized $Δ^{++}$ fragments produced in deep-inelastic scattering off polarized protons. The pion-exchange model predicts highly correlated polarizations of the $Δ^{++}$ and target proton, in marked contrast with the competing diquark fragmentation process.
We calculate the unpolarised deep inelastic structure function of a relativistic deuteron within a covariant framework. An exact treatment of nucleon off-shell effects is shown to give corrections to the widely-used convolution model, even in impulse approximation. Neglecting off-shell effects in the extraction of the neutron structure function from deuterium data introduces errors of order $1-2\%$.
The quark-meson coupling model which we have developed previously is extended to incorporate the $δ$ meson. It is then used to study the Nolen-Schiffer anomaly and isospin symmetry breaking in nuclear matter. We find that, in combination with the {\it u-d\/} mass difference, the difference between quark scalar densities in protons and neutrons generates an effective neutron-proton mass difference of the right magnitude. Finally we find that isospin symmetry breaking in the quark condensates can be directly related to the $δ$ meson field.
An explicit quark 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 used to investigate the properties of both nuclear and neutron matter. We establish a clear understanding of the relationship between this model, which incorporates the internal structure of the nucleon, and QHD. Finally, we use the model to study the density dependence of the quark condensate in-medium.
We derive the general structure of the hadronic tensor required to describe deep-inelastic scattering from an off-shell nucleon within a covariant formalism. Of the large number of possible off-shell structure functions we find that only three contribute in the Bjorken limit. In our approach the usual ambiguities encountered when discussing problems related to off-shellness in deep-inelastic scattering are not present. The formulation therefore provides a clear framework within which one can discuss the various approximations and assumptions which have been used in earlier work. As examples, we investigate scattering from the deuteron, nuclear matter and dressed nucleons. The results of the full calculation are compared with those where various aspects of the off-shell structure are neglected, as well as with those of the convolution model.
We provide an introduction to the ideas of spin-dependent deep-inelastic scattering. Recent experimental results are summarised and possible explanations related to the axial anomaly presented. Further experiments that could greatly clarify the situation are also described.
Shadowing corrections to structure functions of heavy nuclei are calculated at very low values of Bjorken-$x$ and at values of the momentum transfer relevant to recent experiments. Good agreement is obtained with data from the E665 Collaboration for Xe/D and Pb/D, and with the NMC data on Ca/D and C/D structure function ratios. Corrections to the deuteron structure function are also estimated down to $x \sim 10^{-5}$, and found to be less than about $3\%$ over the range of $x$ covered by the E665 data.
The momentum-dependence of the $ρ^0$-$ω$ mixing contribution to charge-symmetry breaking (CSB) in the nucleon-nucleon interaction is compared in a variety of models. We focus in particular on the role that the structure of the quark propagator plays in the predicted behaviour of the $ρ^0$-$ω$ mixing amplitude. We present new results for a confining (entire) quark propagator and for typical propagators arising from explicit numerical solutions of quark Dyson-Schwinger equations. We compare these to hadronic and free quark calculations. The implications for our current understanding of CSB experiments is discussed.
We discuss the role of the U(1) axial anomaly in the spin structure functions of the nucleon, with particular emphasis on how one might determine its x dependence in present and future deep inelastic scattering experiments. We focus on the C-odd spin structure function g3 and also the deuteron structure function g1^d.
The possible role played by vector mesons in inclusive deep inelastic lepton-nucleon scattering is investigated. In the context of the convolution model, we calculate self-consistently the scaling contribution to the nucleon structure function using the formalism of time-ordered perturbation theory in the infinite momentum frame. Our results indicate potentially significant effects only when the vector meson---nucleon form factor is very hard. Agreement with the experimental antiquark distributions, however, requires relatively soft form factors for the $πN$, $ρN$ and $ωN$ vertices.
We calculate nuclear shadowing in lepton-deuteron deep inelastic scattering, which arises from the double scattering of the virtual photon from both nucleons in the deuteron. The total correction to the deuteron structure function is found to be less than about 1\% at small Bjorken-x, but dependent on the model deuteron wavefunction. The resulting increase in the corrected neutron structure function is between 1-2\% for x near 0.004, which leads to a 4-10\% decrease in the value of the Gottfried sum obtained recently by the New Muon Collaboration.