arXiv subjects

A. W. Thomas

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

At least 361 records · Page 20Linked to original sources

arXiv1998

Spin Dependent Parton Distributions in a Bound Nucleon

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.

nucl-th↗

arXiv1998

Precise strength of th piNN coupling constant

We report here a preliminary value for the piNN coupling constant deduced from the GMO sumrule for forward piN scattering. As in our previous determination from np backward differential scattering cross sections we give a critical discussion of the analysis with careful attention not only to the statistical, but also to the systematic uncertainties. Our preliminary evaluation gives $g^2_c$(GMO) = 13.99(24).

hep-ph↗

arXiv1998

Dynamics of Light Antiquarks in the Proton

We 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 u-bar and d-bar 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 pi-N and pi-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.

hep-ph↗

arXiv1998

Are eta- and omega-nuclear states bound ?

We investigate theoretically whether it is feasible to detect $η$- and $ω$-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 $η$ and $ω$ 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 $η$- and $ω$-nucleus bound states in all these nuclei.

nucl-th↗

arXiv1998

Evidence for Charge Symmetry Violation in Parton Distributions

By comparing structure functions measured in neutrino and charged lepton deep inelastic scattering, one can test the validity of parton charge symmetry. New experiments allow us to make such tests, which set rather tight upper limits on parton charge symmetry violation [CSV] for intermediate Bjorken $x$, but which appear to show sizable CSV effects at small $x$. We show that neither nuclear shadowing nor contributions from strange and antistrange quark distributions can account for the experimentally observed difference between the two structure functions. We are therefore forced to consider the possibility of a large CSV effect in the nucleon sea quark distributions. We discuss the consequences of this effect for other observables, and we propose an experiment which could detect a large CSV component in the nucleon sea.

hep-ph↗

arXiv1998

Comparison of Neutrino and Muon Structure Functions, Shadowing Corrections and Charge Symmetry Violation

Comparison of structure functions measured in neutrino and charged lepton deep inelastic scattering can be used to test basic properties of parton distribution functions such as the validity of charge symmetry. Recent experiments indicate a substantial discrepancy between $F_2^ν$ and $F_2^μ$ in the region of small Bjorken-x. We discuss nuclear corrections and strange and anti strange quark effects and show that none of them can account for the observed discrepancy. These results suggest surprisingly large CSV effects in nucleon sea distributions.

hep-ph↗

arXiv1998

Classical Quark Models: An Introduction

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.

nucl-th↗

arXiv1998

Electromagnetic form factors of the bound nucleon

We calculate electromagnetic form factors of the proton bound in specified orbits for several closed shell nuclei. The quark structure of the nucleon and the shell structure of the finite nuclei are given by the QMC model. We find that orbital electromagnetic form factors of the bound nucleon deviate significantly from those of the free nucleon.

nucl-th↗

arXiv1998

Recent results from QMC relevant to TJNAF

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 $ω$ in a finite nucleus, concluding that the $ω$ should be bound by between 50 and 100 MeV.

nucl-th↗

arXiv1998

omega-nucleus bound states in the Walecka model

Using the Walecka model, we investigate theoretically whether an omega meson is bound to finite nuclei. We study several nuclei from ^{6}He to ^{208}Pb, and compare the results with those in the quark-meson coupling (QMC) model. Our calculation shows that deeper omega-nucleus bound states are predicted in the Walecka model than in QMC. One can expect to detect such bound states in the proposed experiment involving the (d,^3He) reaction at GSI.

nucl-th↗

arXiv1998

Extracting the rho-omega mixing amplitude from the pion form-factor

We re-examine 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 -> 2pi 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 -> 2pi and to the value assumed for the mass of the neutral rho meson.

hep-ph↗

arXiv1998

The Validity of Charge Symmetry for Parton Distributions

Recent measurements of the Gottfried sum rule have focused attention on the possibility of substantial flavor symmetry breaking in the proton sea. This is confirmed by pp and pD Drell-Yan processes measured at FNAL. Theoretical models used to `prove' flavor symmetry breaking rely on the assumption of charge symmetric parton distributions. Substantial charge symmetry violation [CSV] could affect current tests of flavor symmetry. In this review we examine the possibility of CSV for parton distributions. We first give definitions for structure functions without making the usual assumption of charge symmetry. Next we make estimates of CSV for both valence and sea quark distributions. We list a set of relations which must hold if charge symmetry is valid, and review current experimental limits on CSV. We propose a series of experimental tests of charge symmetry. These tests could either detect CSV, or they could provide stronger upper limits on CSV effects. We discuss CSV contributions to sum rules, and we propose sum rules which could differentiate between flavor symmetry, and charge symmetry violation in nuclear systems.

hep-ph↗

arXiv1998

Semi-inclusive Pion Production and the d/u Ratio

We 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.

hep-ph↗

arXiv1998

Evidence for Substantial Charge Symmetry Violation in Parton Distributions

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) unequal to sbar(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.

hep-ph↗

arXiv1998

Rho-omega Mixing and the Pion Electromagnetic Form-Factor

The suggestion of momentum dependence in the amplitude for rho-omega mixing has generated concern over related implications for vector meson dominance and the photon-rho coupling. We discuss two established representations of vector meson dominance and show that one of these is completely consistent with such a coupling. We then apply it to a calculation of the pion electromagnetic form-factor. Our analysis leads to a new value for the on-shell rho-omega mixing amplitude of (-3800 +/- 370) MeV^2.

hep-ph↗

arXiv1998

Constraints on the momentum dependence of rho-omega mixing

Within a broad class of models we show that the amplitude for rho^0-omega mixing must vanish at the transition from timelike to spacelike four momentum. Hence in such models the mixing is either zero everywhere or is necessarily momentum-dependent. This lends support to the conclusions of other studies of rho-omega mixing and calls into question standard assumptions about the role of rho-omega mixing in the theoretical understanding of charge-symmetry breaking in nuclear systems.

hep-ph↗

arXiv1998

Direct CP Violation in Λ_b -> n (Λ)π^+π^- Decays via ρ-ωMixing

We study direct CP violation in the bottom baryon decays Λ_b -> f ρ^0 (ω) -> fπ^+π^- (f=n or Λ). It is found that in these decays via ρ-ωmixing the CP violation could be very large when the invariant mass of the π^+π^- pair is in the vicinity of the ωresonance. With a typical value N_c =2 in the factorization approach, the maximum CP-violating asymmetries are more than 50% and 68% for Λ_b -> n π^+π^- and Λ_b -> Λπ^+π^-, respectively. With the aid of heavy quark symmetry and phenomenological models for the hadronic wave functions of Λ_b, Λand the neutron, we estimate the branching ratios of Λ_b -> n (Λ)ρ^0.

hep-ph↗

arXiv1998

Shadowing in neutrino deep inelastic scattering and the determination of the strange quark distribution

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.

hep-ph↗