Showing papers by "Anthony W. Thomas published in 2014"
University of Edinburgh1, University of Mainz2, University of Glasgow3, Kent State University4, University of Bonn5, Petersburg Nuclear Physics Institute6, University of Giessen7, George Washington University8, Lebedev Physical Institute9, University of Basel10, Mount Allison University11, University of Indonesia12, The Catholic University of America13
TL;DR: In this paper, the fine center-of-mass energy (W) resolution and statistical accuracy of the new data results in a significant impact on partial wave analyses aiming to better establish the excitation spectrum of the nucleon.
63 citations
TL;DR: In this article, the Nambu-Jona-Lasinio model was used to obtain the bound-state amplitude of the nucleon in the spacelike region.
Abstract: Electromagnetic form factors of the nucleon in the spacelike region are investigated within the framework of a covariant and confining Nambu--Jona-Lasinio model. The bound-state amplitude of the nucleon is obtained as the solution of a relativistic Faddeev equation, where diquark correlations appear naturally as a consequence of the strong coupling in the color $\overline{3} qq$ channel. Pion degrees of freedom are included as a perturbation to the ``quark-core'' contribution obtained using the Poincar\'e covariant Faddeev amplitude. While no model parameters are fit to form-factor data, excellent agreement is obtained with the empirical nucleon form factors (including the magnetic moments and radii) where pion loop corrections play a critical role for ${Q}^{2}\ensuremath{\lesssim}1\phantom{\rule{0.28em}{0ex}}{\mathrm{GeV}}^{2}$. Using charge symmetry, the nucleon form factors can be expressed as proton quark sector form factors. The latter are studied in detail, leading, for example, to the conclusion that the $d$-quark sector of the Dirac form factor is much softer than the $u$-quark sector, a consequence of the dominance of scalar diquark correlations in the proton wave function. On the other hand, for the proton quark sector Pauli form factors we find that the effect of the pion cloud and axial-vector diquark correlations overcomes the effect of scalar diquark dominance, leading to a larger $d$-quark anomalous magnetic moment and a form factor in the $u$-quark sector that is slightly softer than in the $d$-quark sector.
62 citations
TL;DR: In this paper, the equation of state for nuclear matter in the quark-meson coupling model, including full Fock terms, was explored, and the comparison with phenomenological constraints can be used to restrict the few additional parameters appearing in the FOCK terms which are not present at the Hartree level.
Abstract: We explore the equation of state for nuclear matter in the quark-meson coupling model, including full Fock terms. The comparison with phenomenological constraints can be used to restrict the few additional parameters appearing in the Fock terms which are not present at the Hartree level. Because the model is based upon the in-medium modification of the quark structure of the bound hadrons, it can be readily extended to include hyperons and to calculate the equation of state of dense matter in $\ensuremath{\beta}$ equilibrium. This leads naturally to a study of the properties of neutron stars, including their maximum mass, radii, and density profiles.
58 citations
University of Basel1, University of Mainz2, University of Glasgow3, Kent State University4, University of Bonn5, Petersburg Nuclear Physics Institute6, Istituto Nazionale di Fisica Nucleare7, University of Edinburgh8, George Washington University9, University of California, Los Angeles10, Russian Academy of Sciences11, University of Giessen12, Tomsk Polytechnic University13, Mount Allison University14, University of Regina15, Joint Institute for Nuclear Research16, The Catholic University of America17
TL;DR: In this paper, the effects from nuclear Fermi motion were removed by a kinematic reconstruction of the final-state invariant mass and possible nuclear effects on the quasifree-proton cross section were investigated by a comparison of free and quasIFreeproton data.
Abstract: Differential and total cross sections for the quasifree reactions $\ensuremath{\gamma}p\ensuremath{\rightarrow}\ensuremath{\eta}p$ and $\ensuremath{\gamma}n\ensuremath{\rightarrow}\ensuremath{\eta}n$ have been determined at the MAMI-C electron accelerator by using a liquid deuterium target. Photons were produced via bremsstrahlung from the 1.5 GeV incident electron beam and energy tagged with the Glasgow photon tagger. Decay photons of the neutral decay modes $\ensuremath{\eta}\ensuremath{\rightarrow}2\ensuremath{\gamma}$ and $\ensuremath{\eta}\ensuremath{\rightarrow}3{\ensuremath{\pi}}^{0}\ensuremath{\rightarrow}6\ensuremath{\gamma}$ and coincident recoil nucleons were detected in a combined setup of the Crystal Ball and the TAPS calorimeters. The $\ensuremath{\eta}$-production cross sections were measured in coincidence with recoil protons, recoil neutrons, and in an inclusive mode without a condition on recoil nucleons, which allowed a check of the internal consistency of the data. The effects from nuclear Fermi motion were removed by a kinematic reconstruction of the final-state invariant mass and possible nuclear effects on the quasifree cross section were investigated by a comparison of free and quasifree-proton data. The results, which represent a significant improvement in statistical quality compared to previous measurements, agree with the known neutron-to-proton cross-section ratio in the peak of the ${S}_{11}$(1535) resonance and confirm a peak in the neutron cross section, which is absent for the proton, at a center-of-mass energy $W=(1670\ifmmode\pm\else\textpm\fi{}5)$ MeV with an intrinsic width of $\ensuremath{\Gamma}\ensuremath{\approx}30$ MeV.
51 citations
TL;DR: In this paper, the use of the finite-volume Hamiltonian approach to resolve scattering observables from lattice QCD spectra was investigated, with only a minor degree of model dependence.
Abstract: Within a multichannel formulation of $\ensuremath{\pi}\ensuremath{\pi}$ scattering, we investigate the use of the finite-volume Hamiltonian approach to resolve scattering observables from lattice QCD spectra. The asymptotic matching of the well-known L\"uscher formalism encodes a unique finite-volume spectrum. Nevertheless, in many practical situations, such as coupled-channels systems, it is advantageous to interpolate isolated lattice spectra in order to extract physical scattering parameters. Here we study the use of the Hamiltonian framework as a parametrization that can be fit directly to lattice spectra. We find that, with a modest amount of lattice data, the scattering parameters can be reproduced rather well, with only a minor degree of model dependence.
48 citations
TL;DR: In this paper, a formalism inspired by heavy-baryon chiral perturbation theory with finite-range regularization was applied to dynamical $2+1$-flavor CSSM/QCDSF/UKQCD Collaboration lattice QCD simulation results for the electric form factors of the octet baryons.
Abstract: We apply a formalism inspired by heavy-baryon chiral perturbation theory with finite-range regularization to dynamical $2+1$-flavor CSSM/QCDSF/UKQCD Collaboration lattice QCD simulation results for the electric form factors of the octet baryons. The electric form factor of each octet baryon is extrapolated to the physical pseudoscalar masses, after finite-volume corrections have been applied, at six fixed values of ${Q}^{2}$ in the range $0.2--1.3\text{ }\text{ }{\mathrm{GeV}}^{2}$. The extrapolated lattice results accurately reproduce the experimental form factors of the nucleon at the physical point, indicating that omitted disconnected quark loop contributions are small relative to the uncertainties of the calculation. Furthermore, using the results of a recent lattice study of the magnetic form factors, we determine the ratio ${\ensuremath{\mu}}_{p}{G}_{E}^{p}/{G}_{M}^{p}$. This quantity decreases with ${Q}^{2}$ in a way qualitatively consistent with recent experimental results.
45 citations
Kent State University1, University of Glasgow2, University of Mainz3, University of Bonn4, Massachusetts Institute of Technology5, Joint Institute for Nuclear Research6, Istituto Nazionale di Fisica Nucleare7, George Washington University8, Russian Academy of Sciences9, Saint Mary's University10, University of Basel11, Tomsk Polytechnic University12, University of Edinburgh13, Mount Allison University14, University of Regina15, Ruhr University Bochum16, University of Massachusetts Amherst17, University of California, Los Angeles18
TL;DR: New data are presented for the transverse target asymmetry T and the very first data for the beam-target asymmetry F in the γ[over →]p[ over →]→ηp reaction up to a center-of-mass energy of W=1.9 GeV indicating a significant impact on the understanding of the underlying dynamics of η meson photoproduction.
Abstract: We present new data for the transverse target asymmetry T and the very first data for the beam-target asymmetry F in the γ[over →]p[over →]→ηp reaction up to a center-of-mass energy of W=1.9 GeV. The data were obtained with the Crystal-Ball/TAPS detector setup at the Glasgow tagged photon facility of the Mainz Microtron MAMI. All existing model predictions fail to reproduce the new data indicating a significant impact on our understanding of the underlying dynamics of η meson photoproduction. The peculiar nodal structure observed in existing T data close to threshold is not confirmed.
43 citations
TL;DR: In this article, the electromagnetic contribution to the charge symmetry breaking in the octet baryon masses using a subtracted dispersion relation based on the Cottingham formula was explored.
Abstract: We explore the electromagnetic contribution to the charge symmetry breaking in the octet baryon masses using a subtracted dispersion relation based on the Cottingham formula. For the proton-neutron mass splitting we report a minor revision of the recent analysis of Walker-Loud, Carlson and Miller. For the electromagnetic structure of the hyperons we constrain our analysis, where possible, by a combination of lattice QCD and SU(3) symmetry breaking estimates. The results for the baryon mass splittings are found to be compatible with recent lattice QCD+QED determinations. The uncertainties in the dispersive analysis are dominated by the lack of knowledge of the hyperon inelastic structure.
32 citations
26 citations
TL;DR: In this paper, a chiral effective model motivated by the heavy baryon formalism at next-to-leading order was proposed to predict the strange magnetic form factor of the nucleon.
Abstract: The strange magnetic form factor of the nucleon is studied in a chiral effective model motivated by the heavy baryon formalism at next-to-leading order. The one-loop contributions from kaon and intermediate octet and decuplet hyperons are included, using finite-range regularization to deal with the ultraviolet divergences. Drawing on an established connection between quenched and full QCD, this model makes it possible to predict the strange magnetic form factor under the hypothesis that for a dipole regulator mass. around 0.8 GeV, strangeness in the core is negligible. The strange magnetic form factor is found to be small and negative over a range of momentum transfer, while the strange magnetic moment is consistent with the best lattice QCD determinations.
25 citations
TL;DR: In this article, the azimuthal modulations of dihadron fragmentation functions (DiFFs) of a transversely polarised quark using an NJL-jet based model that incorporates the Collins effect for single hadron emission were studied.
TL;DR: In this paper, the lightest baryon octet was studied within a covariant and confining Nambu-Jona-Lasinio model, and the axial charges were determined by solving the Poincar\'e covariant Faddeev equations.
Abstract: The lightest baryon octet is studied within a covariant and confining Nambu--Jona-Lasinio model. By solving the Poincar\'e covariant Faddeev equations---including scalar and axialvector diquarks---we determine the baryon octet masses and axial charges for strangeness conserving transitions. For the axial charges the degree of violation of SU(3) flavor symmetry, arising because of the strange spectator quark(s), is found to be no more than 10%.
TL;DR: In this paper, it was shown that the measured value of the cosmological constant can originate from supergravity (SUGRA) models with degenerate vacua, which leads to the theory that the Higgs boson is formed by small to vanishing values of the quartic Higgs self-coupling and the corresponding beta function at the Planck scale.
TL;DR: In this paper, the fractional Kolmogorov-Feller equations for the probabilities at time t can be represented by an infinite linear system of ordinary differential equations of first order in a transformed time variable.
TL;DR: In this paper, the parity-violating asymmetry in Moller scattering with sufficient accuracy to determine sin 2 θ W to 0.1% offers a complementary path to the discovery of new physics to that followed at high energy colliders.
TL;DR: It is shown that sin(φ(R)- φ(S) modulations can be significant, especially if the authors impose asymmetric cuts on the momenta of the hadrons in the pairs, and a modified version of the lepto Monte Carlo event generator is employed to estimate the size of single spin asymmetries corresponding to these modulations.
Abstract: The Sivers effect in single hadron semi-inclusive deep inelastic scattering on a transversely polarized nucleon describes the modulation of the cross section with the sine of the azimuthal angle between the produced hadron's transverse momentum and the nucleon spin (${\mathbit{P}}_{h}$ and ${\ensuremath{\varphi}}_{S}$, respectively). This effect is attributed to the so-called Sivers parton distribution function of the nucleon. We employ a simple phenomenological parton model to derive the relevant cross section for two-hadron production in semi-inclusive deep inelastic scattering including the Sivers effect. We show that the Sivers effect can be observed in such a process as sine modulations involving the azimuthal angles ${\ensuremath{\varphi}}_{T}$ and ${\ensuremath{\varphi}}_{R}$ of both the total and the relative transverse momenta of the hadron pair. The existence of the modulation with respect to ${\ensuremath{\varphi}}_{R}$ is new. Finally, we employ a modified version of the lepto Monte Carlo event generator that includes the Sivers effect to estimate the size of single spin asymmetries corresponding to these modulations. We show that $\mathrm{sin}({\ensuremath{\varphi}}_{R}\ensuremath{-}{\ensuremath{\varphi}}_{S})$ modulations can be significant, especially if we impose asymmetric cuts on the momenta of the hadrons in the pairs.
01 Mar 2014
TL;DR: In this article, the authors analyzed the properties of unpo- larised DFFs using their probabilistic interpretation and used both the NJL-jet hadroniza- tion model and PYTHIA 8.1 event generator to explore the e ect of the strong decays of the vector mesons produced in the quark hadronization process on the pseudoscalar DFF.
Abstract: Dihadron Fragmentation Functions (DFF) provide a vast amount of informa- tion on the intricate details of the parton hadronization process. Moreover, they provide a unique access to the "clean" extraction of nucleon transversity parton distribution func- tions in semi inclusive deep inelastic two hadron production process with a transversely polarised target. On the example of the u ! + , we analyse the properties of unpo- larised DFFs using their probabilistic interpretation. We use both the NJL-jet hadroniza- tion model and PYTHIA 8.1 event generator to explore the e ect of the strong decays of the vector mesons produced in the quark hadronization process on the pseudoscalar DFFs. Our study shows that, even though it is less probable to produce vector mesons in the hadronization process than pseudo scalar mesons of the same charge, the products of their strong decays drastically a ect the DFFs for pions because of the large combinato- rial factors. Thus, an accurate description of both vector meson production and decays are crucial for theoretical understanding of DFFs.
TL;DR: In this article, the dependence of the sum of the electric and magnetic polarizabilities of the proton was calculated over the range $0, √ √ q √ Q 2 = 0.06 using the generalized Baldin sum rule using photoproduction data.
Abstract: The ${Q}^{2}$-dependence of the sum of the electric and magnetic polarizabilities of the proton is calculated over the range $0\ensuremath{\le}{Q}^{2}\ensuremath{\le}6\text{ }{\mathrm{GeV}}^{2}$ using the generalized Baldin sum rule. Employing a parametrization of the ${F}_{1}$ structure function valid down to ${Q}^{2}=0.06\text{ }\text{ }{\mathrm{GeV}}^{2}$, the polarizabilities at the real photon point are found by extrapolating the results of finite ${Q}^{2}$ to ${Q}^{2}=0\text{ }\text{ }{\mathrm{GeV}}^{2}$. We determine the evolution over four-momentum transfer to be consistent with the Baldin sum rule using photoproduction data, obtaining $\ensuremath{\alpha}+\ensuremath{\beta}=13.7\ifmmode\pm\else\textpm\fi{}0.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}\text{ }\text{ }{\mathrm{fm}}^{3}$.
TL;DR: In this article, the Sivers effect is measured in a two-hadron SIDIS process as sine modulations involving the azimuthal angles of both the total and the relative transverse momenta of the hadron pair.
Abstract: The Sivers effect describes the correlation of the unpolarized parton's transverse momentum with the transverse spin of the nucleon. It manifests as a sine modulation of the cross section for single-hadron semi-inclusive deep inelastic scattering (SIDIS) on a transversely polarized nucleon with the azimuthal angle between the produced hadron's transverse momentum and the nucleon spin (${\ensuremath{\varphi}}_{h}$ and ${\ensuremath{\varphi}}_{S}$, respectively). It has been recently suggested that the Sivers effect can also be measured in a two-hadron SIDIS process as sine modulations involving the azimuthal angles ${\ensuremath{\varphi}}_{T}$ and ${\ensuremath{\varphi}}_{R}$ of both the total and the relative transverse momenta of the hadron pair. Here we present the detailed derivation of the two-hadron SIDIS cross section using simple parton-model inspired functional forms for both the parton distribution and the fragmentation functions. We show explicitly that the terms corresponding to the $\mathrm{sin}({\ensuremath{\varphi}}_{R}\ensuremath{-}{\ensuremath{\varphi}}_{S})$ and $\mathrm{sin}({\ensuremath{\varphi}}_{T}\ensuremath{-}{\ensuremath{\varphi}}_{S})$ modulations are nonzero. Further, we derive the cross section expressions for single-hadron production in the two-hadron sample. Finally, we employ a modified version of the lepto Monte Carlo event generator that includes the Sivers effect to estimate the size of single spin asymmetries corresponding to these modulations independent of the formalism developed.
21 Feb 2014
TL;DR: In this paper, the authors focus on what one expects to happen to the structure of a hadron when it is immersed in a nuclear medium and argue that the necessary changes in the quark structure are intimately related to nuclear binding and saturation.
Abstract: In the quest to understand QCD there are a number of outstanding challenges. Here we focus on one of these, namely what one expects to happen to the structure of a hadron when it is immersed in a nuclear medium. We argue that the necessary changes in the quark structure are intimately related to nuclear binding and saturation. Some of the potential experimental implications of these ideas are discussed.
26 Mar 2014
TL;DR: In this paper, a chiral extrapolation of these lattice results to the physical point suggested that the state, identified with the famed H dibaryon, is most likely slightly unbound (by 13 ± 14 MeV) with respect to the Λ − Λ threshold.
Abstract: Recent lattice QCD calculations from the HAL and NPLQCD Collaborations have reported evidence for the existence of a bound state with strangeness −2 and baryon number 2 at quark masses somewhat higher than the physical values. A controlled chiral extrapolation of these lattice results to the physical point suggested that the state, identified with the famed H dibaryon, is most likely slightly unbound (by 13 ± 14 MeV) with respect to the Λ − Λ threshold. We report the results of an updated analysis which finds the H unbound by 26± 11 MeV. Apart from the insight it would give us into how QCD is realized in Nature, the H is of great interest because of its potential implications for the equation of state of dense matter and studies of neutron stars. It may also explain the enhancement above the Λ − Λ threshold already reported experimentally. It is clearly of great importance that the latter be pursued in experiments at the new J-PARC facility.
University of Bonn1, University of Glasgow2, Petersburg Nuclear Physics Institute3, University of Basel4, University of Edinburgh5, George Washington University6, University of California, Los Angeles7, Lebedev Physical Institute8, Mount Allison University9, Kent State University10, The Catholic University of America11
TL;DR: In this paper, a new precise determination of the meson mass is presented based on a measurement of the threshold for the $ \gamma p\rightarrow p\eta$ reaction using the tagger focal-plane microscope detector at the MAMI-B facility in Mainz.
Abstract: A new precise determination of the $ \eta$
meson mass is presented. It is based on a measurement of the threshold for the $ \gamma p\rightarrow p\eta$
reaction using the tagger focal-plane microscope detector at the MAMI-B facility in Mainz. The tagger microscope has a higher energy resolution than the standard tagging spectrometer and, hence, allowed an improvement in the accuracy compared to the previous $ \eta$
mass measurement at MAMI-B. Special emphasis was put on a very careful energy calibration of the electron beam and the tagging device, which reduced considerably the systematic uncertainty compared to the previous MAMI experiment. The result $ m_{\eta}=(547.865 \pm 0.031_{stat.}\pm 0.062_{syst.})$
MeV agrees very well with the precise values of the NA48, KLOE, CLEO and COSY-ANKE Collaborations and deviates by $ 6\sigma$
from the smaller value obtained by the GEM Collaboration at COSY.
TL;DR: In this paper, the authors present a vision for the next decade of hadron physics in which the central question being addressed is how one might win new physical insight into the way hadronic systems work.
Abstract: We present a vision for the next decade of hadron physics in which the central question being addressed is how one might win new physical insight into the way hadronic systems work. The topics addressed include the relevance of model building, the role of spontaneously broken chiral symmetry, spectroscopy, form factors and physics in the deep inelastic regime.
TL;DR: In this paper, the authors present a vision for the next decade of hadron physics in which the central question being addressed is how one might win new physical insight into the way hadronic systems work.
Abstract: We present a vision for the next decade of hadron physics in which the central question being addressed is how one might win new physical insight into the way hadronic systems work. The topics addressed include the relevance of model building, the role of spontaneously broken chiral symmetry, spectroscopy, form factors and physics in the deep inelastic regime.
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TL;DR: In this paper, a new lattice QCD simulation showing that its strange magnetic form factor vanishes, together with a comprehensive Hamiltonian analysis of the lattice energy levels, has unambiguously established that the structure is dominated by a bound anti-kaon-nucleon component.
Abstract: For almost 50 years the structure of the Lambda 1405 resonance has been a mystery. Recently, a new lattice QCD simulation showing that its strange magnetic form factor vanishes, together with a comprehensive Hamiltonian analysis of the lattice QCD energy levels, has unambiguously established that the structure is dominated by a bound anti-kaon--nucleon component [1]. Here we present supplementary information for Ref. [1] including a presentation of the relevant Hamiltonian effective field theory and an illustration of the volume dependence of the results and their connection to the infinite volume limit of Nature.
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TL;DR: In this article, the authors focus on what one expects to happen to the structure of a hadron when it is immersed in a nuclear medium and argue that the necessary changes in the quark structure are intimately related to nuclear binding and saturation.
Abstract: In the quest to understand QCD there are a number of outstanding challenges. Here we focus on one of these, namely what one expects to happen to the structure of a hadron when it is immersed in a nuclear medium. We argue that the necessary changes in the quark structure are intimately related to nuclear binding and saturation. Some of the potential experimental implications of these ideas are discussed.
Posted Content•
TL;DR: In this paper, it was shown that the observed value of the dark energy density can be reproduced if in the second vacuum local SUSY breaking is induced by gaugino condensation at a scale which is just slightly lower than \Lambda_{QCD} in the physical vacuum.
Abstract: In N=1 supergravity the scalar potential of the hidden sector may have degenerate supersymmetric (SUSY) and non-supersymmetric Minkowski vacua. In this case local SUSY in the second supersymmetric Minkowski phase can be broken dynamically. Assuming that such a second phase and the phase associated with the physical vacuum are exactly degenerate, we estimate the value of the cosmological constant. We argue that the observed value of the dark energy density can be reproduced if in the second vacuum local SUSY breaking is induced by gaugino condensation at a scale which is just slightly lower than \Lambda_{QCD} in the physical vacuum. The presence of a third degenerate vacuum, in which local SUSY and electroweak (EW) symmetry are broken near the Planck scale, may lead to small values of the quartic Higgs self--coupling and the corresponding beta function at the Planck scale in the phase in which we live.
TL;DR: In this paper, the level of charge symmetry violation in moments of parton distributions using (2 + 1)-flavor lattice QCD is resolved by applying the methods used for that analysis by applying them to determine the strong contribution to the proton-neutron mass difference.
Abstract: Recent work unambiguously resolves the level of charge symmetry violation in moments of parton distributions using (2 + 1)-flavor lattice QCD. We introduce the methods used for that analysis by applying them to determine the strong contribution to the proton–neutron mass difference. We also summarize related work which reveals that the fraction of baryon spin which is carried by the quarks is in fact structure-dependent rather than universal across the baryon octet.