Showing papers by "Anthony W. Thomas published in 2010"
TL;DR: In this paper, a low-order expansion about the SU(3) chiral limit is proposed to improve the description of the lattice results by allowing the regularization scale of the effective field theory to be determined by the lattices data itself.
Abstract: We analyze the consequences of the remarkable new results for octet baryon masses calculated in 2+1- avour lattice QCD using a low-order expansion about the SU(3) chiral limit. We demonstrate that, even though the simulation results are clearly beyond the power-counting regime, the description of the lattice results by a low-order expansion can be significantly improved by allowing the regularisation scale of the effective field theory to be determined by the lattice data itself. The model dependence of our analysis is demonstrated to be small compared with the present statistical precision. In addition to the extrapolation of the absolute values of the baryon masses, this analysis provides a method to solve the difficult problem of fine-tuning the strange-quark mass. We also report a determination of the sigma terms for all of the octet baryons, including an accurate value of the pion-nucleon sigma term and the first determination of the strangeness sigma term based on 2+1-flavour l
211 citations
TL;DR: In this paper, a revised NuTeV result of sin 2 θ W = 0.2221 ± 0.0013 ( stat ) ±0.0020 ( syst ) was given, which is in excellent agreement with the running of the sin 2 ǫ W predicted by the Standard Model.
71 citations
TL;DR: In this article, the authors used the Cloudy Bag model to extract the flavour-singlet axial charge from deep inelastic data and showed that the value of g A(8 ) is reduced by as much as 20% below the usual phenomenological value.
54 citations
TL;DR: In this article, the experimental and theoretical status of partonic charge symmetry violation in parton distribution functions is discussed, and a series of new experiments are proposed, which might reveal partonic symmetry violation or alternatively might lower the current upper limits on parton charge-symmetry violation.
Abstract: 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. Various theoretical and phenomenological models for charge-symmetry violation in parton distribution functions are reviewed. After summarizing the current experimental upper limits on charge-symmetry violation in parton distributions, a series of new experiments are proposed, which might reveal partonic charge-symmetry violation or alternatively might lower the current upper limits on parton charge-symmetry violation.
53 citations
TL;DR: In this article, dispersion relations are used to evaluate the contribution of parity-violating electron scattering in the forward limit arising from the axial-vector coupling at the electron vertex, which gives a correction of 0.0047{sub -0.0004}{sup +0.0011} to the standard model value 0.0713(8) of the proton weak charge.
Abstract: 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 Q{sub weak} experiment, this gives a correction of 0.0047{sub -0.0004}{sup +0.0011} 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.
49 citations
TL;DR: In particular, the strangeness component is now being resolved to a precision that far surpasses best phenomenological estimates as mentioned in this paper, which opens the window for lattice QCD simulations to lead the field in resolving this aspect of nucleon structure.
42 citations
TL;DR: In this article, the authors discuss the tremendous progress that has been made toward an understanding of how the spin of the proton is distributed on its quark and gluon constituents.
Abstract: We discuss the tremendous progress that has been made toward 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 20 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.
30 citations
TL;DR: In this article, the first moments of the generalized parton distributions using heavy baryon chiral perturbation theory were derived from lattice data and the extrapolation was shown to be consistent with the results obtained with dimensional regularization.
Abstract: 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.
18 citations
TL;DR: In this article, the chiral extrapolation of the nucleon mass, M n, was investigated using data coming from 2-flavour partially-quenched lattice simulations.
16 citations
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TL;DR: The latest observation of a Shapiro delay of the binary millisecond pulsar J1614-2230 by Demorest et al. as discussed by the authors yielded the pulsar gravitational mass to be 1.97 +/- 0.04 solar mass, the heaviest observed pulsar to-date.
Abstract: The latest observation of a Shapiro delay of the binary millisecond pulsar J1614-2230 by Demorest et al. Nature 467 1081 (2010) yielded the pulsar gravitational mass to be 1.97 +/- 0.04 solar mass, the heaviest observed pulsar to-date. This result produces a stringent constraint on Equation(s) of State (EoS) of high density neutron star matter. One of the main conclusions of Demorest et al. was that their result makes the presence of non-nucleonic components in the neutron star matter unlikely. We compare the result with our recent work and conclude that hyperons in high-density matter are fully consistent with the observation and that their presence is a necessary consequence of general physical laws.
9 citations
TL;DR: The origin of the spin of the proton is one of the most fundamental questions in modern hadron physics as mentioned in this paper, and much remains to be understood, especially in the case of lattice QCD, what is known and what is not known.
Abstract: The origin of the spin of the proton is one of the most fundamental questions in modern hadron physics. Although tremendous progress has been made since the discovery of the "spin crisis" brought the issue to the fore, much remains to be understood. We carefully review what is known and, especially in the case of lattice QCD, what is not known. We also explain the importance of QCD inspired models in providing a physical picture of proton structure and the connection between those models and what is measured experimentally and on the lattice. We specifically apply these ideas to the issue of quark orbital angular momentum in the proton. We show that the Myhrer–Thomas resolution of the proton spin crisis is remarkably consistent with modern information from lattice QCD.
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TL;DR: In this paper, the authors show that mixing plays an important role in understanding the value of the eta-nucleon scattering length, and they find a factor of two enhancement from mixing relative to the prediction with a pure octet eta.
Abstract: Eta bound states in nuclei are sensitive to the flavour-singlet component in the eta. The bigger the singlet component, the more attraction and the greater the binding. eta-etaprime mixing plays an important role in understanding the value of the eta-nucleon scattering length a_{\eta N}. Working with the Quark Meson Coupling model, we find a factor of two enhancement from mixing relative to the prediction with a pure octet eta.
TL;DR: In this article, the binding of hypernuclei based on the latest version of quark-meson coupling model was studied, and the photoproduction cross sections for the reaction were estimated using the bound Λ spinors obtained in the model.
Abstract: We study the binding of hypernuclei based on the latest version of quark-meson coupling model, and estimate the photoproduction cross sections for the reaction using the bound Λ spinors obtained in the model.
TL;DR: In this paper, the authors investigated some QCD corrections that contribute to the Gross-Llewellyn Smith sum rule but have not been included in previous analyses of it, and they found that additional corrections from strange quarks and parton charge symmetry violation are likely to have a small but potentially significant role in decreasing the current discrepancy between the experimental and theoretical estimates of the Gross--LLEWELLNSmith sum rule.
Abstract: We investigate some QCD corrections that contribute to the Gross--Llewellyn Smith sum rule but have not been included in previous analyses of it. We first review the techniques by which the $x{F}_{3}$ structure function is extracted from combinations of neutrino and antineutrino cross sections. Next we investigate corrections to the Gross--Llewellyn Smith sum rule, with particular attention to contributions arising from strange quark distributions and from charge symmetry violating parton distributions. We find that additional corrections from strange quarks and parton charge symmetry violation are likely to have a small but potentially significant role in decreasing the current discrepancy between the experimental and theoretical estimates of the Gross--Llewellyn Smith sum rule.
TL;DR: In this paper, the authors review the status of the knowledge of the strangeness content of the nucleon, both experimental and theoretical, and recall how the success of the corresponding experimental program at JLab led to an unanticipated improvement in the precision with which they constrain the possible existence of parity violating lepton-quark interactions beyond the Standard Model.
12 Jul 2010
TL;DR: In this paper, a description of fragmentation functions which satisfy the momentum and isospin sum rules is presented in an effective chiral quark theory of QCD, taking into account cascade-like processes in a generalized jet-model approach.
Abstract: A description of fragmentation functions which satisfy the momentum and isospin sum rules is presented in an effective chiral quark theory of QCD. We concentrate on the pion fragmentation function, taking into account cascade-like processes in a generalized jet-model approach. Numerical results obtained in this NJL- jet model are presented and compared to empirical parametrizations.
TL;DR: In this paper, the origin of the spin of the nucleon in terms of spin and orbital angular momentum of the quarks and gluons was studied and the importance of the corrections associated with chiral extrapolation and finite volume corrections, especially for the term B(0) extracted from the appropriate low moment of the deeply virtual Compton scattering amplitude.
Abstract: One of the most fascinating challenges facing modern strong interaction physics is to understand the origin of the spin of the nucleon in terms of the spin and orbital angular momentum of the quarks and gluons. We review recent progress on this problem as well as some of the uncertainties associated with state of the art lattice QCD simulations. In particular, we explain the importance of the corrections associated with chiral extrapolation and finite volume corrections, especially for the term B(0) extracted from the appropriate low moment of the deeply virtual Compton scattering amplitude.
TL;DR: In this article, a broad range of topics are discussed and in just a few pages it is impossible to even touch on all the areas which will be discussed, but we have chosen to summarise just one area where there has recently been impressive progress, namely our quantitative understanding of strangeness in the nucleon.
Abstract: This conference covers an extremely broad range of topics and in just a few pages it is impossible to even touch on all the areas which will be discussed. We have chosen to summarise just one area where there has recently been impressive progress, namely our quantitative understanding of strangeness in the nucleon. We also discuss a couple of examples where theoretical progress is of direct importance for future experiments as well as for the interpretation of hitherto anomalous results. Finally we make some remarks on meson and baryon spectroscopy and the exciting array of new facilities that are coming on-line in the near future.
TL;DR: Recently, there has been remarkable progress in the understanding of the role of quarks in the structure of the nucleon as discussed by the authors, which has profound consequences for the searches for dark matter which are currently underway.
Abstract: We review recent progress in our understanding of the role of strange quarks in the structure of the nucleon. For the contribution to its mass the result is remarkably small, an order of magnitude smaller than commonly assumed. This has profound consequences for the searches for dark matter which are currently underway. There has also been remarkable progress in the understanding of hypernuclei. In particular, there is a very natural explanation at the quark level of why Λ-hypernuclei are bound whereas Σ-hypernuclei are not. The consequences for dense matter, for example in neutron stars, are not yet fully understood but we know they are significant.