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A. W. Thomas

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

At least 163 records · Page 9Linked to original sources
arXiv2010

Reassessment of the NuTeV determination of the Weinberg angle

In light of the recent discovery of the importance of the isovector EMC effect for the interpretation of the NuTeV determination of sin^2 theta_W, it seems timely to reassess the central value and the errors on this fundamental Standard Model parameter derived from the NuTeV data. We also include earlier work on charge symmetry violation and the recent limits on a possible asymmetry between s and \bar{s} quarks. With these corrections we find a revised NuTeV result of sin^2 theta_W = 0.2232 \pm 0.0013(stat) \pm 0.0024(syst), which is in excellent agreement with the running of sin^2 theta_W predicted by the Standard Model.

nucl-th↗
arXiv2010

gamma-Z corrections to forward-angle parity-violating e-p scattering

We use dispersion relations to evaluate the gamma-Z box contribution to parity-violating electron scattering in the forward limit arising from the axial-vector coupling at the electron vertex. The calculation makes full use of the critical constraints from recent JLab data on electroproduction in the resonance region as well as high energy data from HERA. At the kinematics of the Qweak experiment, this gives a correction of 0.0047{+0.0011}{-0.0004}$ to the Standard Model value 0.0713(8) of the proton weak charge. While the magnitude of the correction is highly significant, the uncertainty is within the anticipated experimental uncertainty of +- 0.003.

hep-ph↗
arXiv2010

The First Moments of Nucleon Generalized Parton Distributions

We extrapolate the first moments of the generalized parton distributions using heavy baryon chiral perturbation theory. The calculation is based on the one loop level with the finite range regularization. The description of the lattice data is satisfactory and the extrapolated moments at physical pion mass are consistent with the results obtained with dimensional regularization, although the extrapolation in the momentum transfer to $t=0$ does show sensitivity to form factor effects which lie outside the realm of chiral perturbation theory. We discuss the significance of the results in the light of modern experiments as well as QCD inspired models.

hep-ph↗
arXiv2010

An analysis of the nucleon spectrum from lattice partially-quenched QCD

The chiral extrapolation of the nucleon mass, M_n, is investigated using data coming from 2-flavour partially-quenched lattice simulations. A large sample of lattice results from the CP-PACS Collaboration is analysed using the leading one-loop corrections, with explicit corrections for finite lattice spacing artifacts. The extrapolation is studied using finite range regularised chiral perturbation theory. The analysis also provides a quantitative estimate of the leading finite volume corrections. It is found that the discretisation, finite-volume and partial quenching effects can all be very well described in this framework, producing an extrapolated value of M_n in agreement with experiment. Furthermore, determinations of the low energy constants of the nucleon mass's chiral expansion are in agreement with previous methods, but with significantly reduced errors. This procedure is also compared with extrapolations based on polynomial forms, where the results are less encouraging.

hep-lat↗
arXiv2009

Understanding the proton's spin structure

We discuss the tremendous progress that has been towards an understanding of how the spin of the proton is distributed on its quark and gluon constituents. This is a problem that began in earnest twenty years ago with the discovery of the proton ``spin crisis'' by the European Muon Collaboration. The discoveries prompted by that original work have given us unprecedented insight into the amount of spin carried by polarized gluons and the orbital angular momentum of the quarks.

hep-ph↗
arXiv2009

Octet baryon masses and sigma terms from an SU(3) chiral extrapolation

We report an analysis of the impressive new lattice simulation results for octet baryon masses in 2+1-flavor QCD. The analysis is based on a low order expansion about the chiral SU(3) limit in which the symmetry breaking arises from terms linear in the quark masses plus the variation of the Goldstone boson masses in the leading chiral loops. The baryon masses evaluated at the physical light quark masses are in remarkable agreement with the experimental values, with a model dependence considerably smaller than the rather small statistical uncertainty. From the mass formulae one can evaluate the sigma commutators for all octet baryons. This yields an accurate value for the pion-nucleon sigma commutator. It also yields the first determination of the strangeness sigma term based on 2+1-flavor lattice QCD and, in general, the sigma commutators provide a resolution to the difficult issue of fine-tuning the strange quark mass.

hep-lat↗
arXiv2009

Overview of Issues Surrounding Strangeness in the Nucleon

The calculation of the strangeness content of the nucleon and its experimental verification is a fundamental step in establishing non-perturbative QCD as the correct theory describing the structure of hadrons. It holds a role in QCD analogous to the correct calculation of the Lamb shift in QED. We review the latest developments in the vector and scalar matrix elements of the strange quarks in the proton, where there has recently been considerable progress.

nucl-th↗
arXiv2009

Positrons at Jefferson Laboratory

We review the compelling case for establishing a capability to accelerate positrons at Jefferson Lab. The potential appplications range from the study of two-photon exchange and deeply-virtual Compton scattering to exploiting the charge current weak interaction to probe the flavor structure of hadrons and nuclei. There are also fascinating ideas for using such a capability to discover new physics beyond the Standard Model of nuclear and particle physics.

nucl-ex↗
arXiv2009

Charge Symmetry at the Partonic Level

This review article discusses the experimental and theoretical status of partonic charge symmetry. It is shown how the partonic content of various structure functions gets redefined when the assumption of charge symmetry is relaxed. We review various theoretical and phenomenological models for charge symmetry violation in parton distribution functions. We summarize the current experimental upper limits on charge symmetry violation in parton distributions. A series of experiments are presented, which might reveal partonic charge symmetry violation, or alternatively might lower the current upper limits on parton charge symmetry violation.

hep-ph↗
arXiv2009

Medium modifications of the bound nucleon GPDs and the quark contribution to the spin sum rule

We estimate the nuclear medium modifications of the quark contribution to the bound nucleon spin sum rule, J^{q*}, as well as the separate helicity, ΔΣ*, and the angular momentum, L^{q*}, contributions to J^{q*}. For the calculation of the bound nucleon generalized parton distributions (GPDs), we use as input the bound nucleon elastic form factors predicted in the quark-meson coupling model. Our model for the bound nucleon GPDs is relevant for incoherent deeply virtual Compton scattering (DVCS) with nuclear targets. We find that the m5Aedium modifications increase J^{q*} and L^{q*} and decrease ΔΣ* compared to the free nucleon case. The effect is large and increases with increasing nuclear density ρ. For instance, at ρ=ρ_0=0.15 fm^{-3}, J^{q*} increases by 7%, L^{q*} increases by 20%, and ΔΣ* decreases by 17%. These in-medium modifications of the bound nucleon spin properties may be understood qualitatively in terms of the enhancement of the lower component of the quark Dirac spinor in the nuclear medium.

hep-ph↗
arXiv2009

The NJL-jet model for quark fragmentation functions

A description of fragmentation functions which satisfy the momentum and isospin sum rules is presented in an effective quark theory. Concentrating on the pion fragmentation function, we first explain why the elementary (lowest order) fragmentation process q --> q πis completely inadequate to describe the empirical data, although the "crossed" process π--> q \bar{q} describes the quark distribution functions in the pion reasonably well. Taking into account cascade-like processes in a generalized jet-model approach, we then show that the momentum and isospin sum rules can be satisfied naturally, without the introduction of ad hoc parameters. We present results for the Nambu--Jona-Lasinio (NJL) model in the invariant mass regularization scheme and compare them with the empirical parametrizations. We argue that the NJL-jet model, developed herein, provides a useful framework with which to calculate the fragmentation functions in an effective chiral quark theory.

nucl-th↗
arXiv2009

Strange magnetic form factor of the proton at $Q^2 = 0.23$ GeV$^2$

We determine the $u$ and $d$ quark contributions to the proton magnetic form factor at finite momentum transfer by applying chiral corrections to quenched lattice data. Heavy baryon chiral perturbation theory is applied at next to leading order in the quenched, and full QCD cases for the valence sector using finite range regularization. Under the assumption of charge symmetry these values can be combined with the experimental values of the proton and neutron magnetic form factors to deduce a relatively accurate value for the strange magnetic form factor at $Q^2=0.23$ GeV$^2$, namely $G_M^s=-0.034 \pm 0.021$ $μ_N$.

hep-ph↗
arXiv2009

Chiral extrapolation of octet-baryon charge radii

The charge radii of octet-baryons obtained in quenched lattice-QCD calculations are extrapolated within heavy-baryon chiral perturbation theory. Finite-range regularization (FRR) is applied to improve the convergence of the chiral expansion and to provide estimates of quenching artifacts. Lattice values of quark distribution radii and baryon charge radii for $\mpi^2$ in the range (0.1, 0.7)$\gev^2$ are described very well with FRR. Upon estimating corrections for both finite-volume and quenching effects, the obtained charge radii of the proton, neutron and $Σ^-$ are in good agreement with experimental measurements. The predicted charge radii of the remaining octet-baryons have not yet been measured and present a challenge to future experiments.

hep-ph↗
arXiv2009

Phase transition from quark-meson coupling hyperonic matter to deconfined quark matter

We investigate the possibility and consequences of phase transitions from an equation of state (EOS) describing nucleons and hyperons interacting via mean fields of sigma, omega, and rho mesons in the recently improved quark-meson coupling (QMC) model to an EOS describing a Fermi gas of quarks in an MIT bag. The transition to a mixed phase of baryons and deconfined quarks, and subsequently to a pure deconfined quark phase, is described using the method of Glendenning. The overall EOS for the three phases is calculated for various scenarios and used to calculate stellar solutions using the Tolman-Oppenheimer-Volkoff equations. The results are compared with recent experimental data, and the validity of each case is discussed with consequences for determining the species content of the interior of neutron stars.

nucl-th↗
arXiv2009

Photoproduction of hypernuclei within the quark-meson coupling model

We study the photoproduction of the ^12{_Lambda}B hypernucleus within a fully covariant effective Lagrangian based model, employing Lambda bound state spinors derived from the latest quark-meson coupling model. The kaon production vertex is described via creation, propagation and decay of N*(1650), N*(1710), and N*(1720) intermediate baryonic resonant states in the initial collision of the photon with a target proton in the incident channel. The parameters of the resonance vertices are fixed by describing the total and differential cross section data on the elementary gamma (p, K+) Lambda reaction in the energy regime relevant to the hypernuclear production. It is found that the hypernuclear production cross sections calculated with the quark model based hyperon bound state spinors differ significantly from those obtained with the phenomenological Dirac single particle wave functions.

nucl-th↗
arXiv2009

Spin and orbital angular momentum of the proton

Since the announcement of the proton spin crisis by the European Muon Collaboration there has been considerable progress in unravelling the distribution of spin and orbital angular momentum within the proton. We review the current status of the problem, showing that not only have strong upper limits have been placed on the amount of polarized glue in the proton but that the experimental determination of the spin content has become much more precise. It is now clear that the origin of the discrepancy between experiment and the naive expectation of the fraction of spin carried by the quarks and anti-quarks in the proton lies in the non-perturbative structure of the proton. We explain how the features expected in a modern, relativistic and chirally symmetric description of nucleon structure naturally explain the current data. The consequences of this explanation for the presence of orbital angular momentum on quarks and gluons is reviewed and comparison made with recent results from lattice QCD and experimental data.

hep-ph↗
arXiv2009

Neutral pion photoproduction at high energies

A Regge model with absorptive corrections is employed in a global analysis of the world data on the reactions gamma p -> pi0 p and gamma n -> pi0 n for photon energies from 3 to 18 GeV. In this region resonance contributions are expected to be negligible so that the available experimental information on differential cross sections and single- and double polarization observables at -t < 2 GeV^2 allows us to determine the non-resonant part of the reaction amplitude reliably. The model amplitude is then used to predict observables for photon energies below 3 GeV. A detailed comparison with recent data from the CLAS and CB-ELSA Collaborations in that energy region is presented. Furthermore, the prospects for determining the pi0 radiative decay width via the Primakoff effect from the reaction gamma p -> pi0 p are explored.

hep-ph↗