Showing papers by "Anthony W. Thomas published in 2005"
TL;DR: In this article, the effect of hadron structure changes in a nuclear medium using the quark-meson coupling (QMC) model is reviewed, which is based on a mean field description of non-overlapping nucleon (or baryon) bags bound by the self-consistent exchange of scalar and vector mesons.
Abstract: We review the effect of hadron structure changes in a nuclear medium using the quark-meson coupling (QMC) model, which is based on a mean field description of non-overlapping nucleon (or baryon) bags bound by the self-consistent exchange of scalar and vector mesons. This approach leads to simple scaling relations for the changes of hadron masses in a nuclear medium. It can also be extended to describe finite nuclei, as well as the properties of hypernuclei and meson-nucleus deeply bound states. It is of great interest that the model predicts a variation of the nucleon form factors in nuclear matter. We also study the empirically observed, Bloom-Gilman (quark-hadron) duality.
Other applications of the model include subthreshold kaon production in heavy ion collisions, D and D-bar meson production in antiproton-nucleus collisions, and J/Psi suppression. In particular, the modification of the D and D-bar meson properties in nuclear medium can lead to a large J/Psi absorption cross section, which explains the observed J/Psi suppression in relativistic heavy ion collisions.
142 citations
TL;DR: By combining the constraints of charge symmetry with new chiral extrapolation techniques and recent low mass quenched lattice-QCD simulations of the individual quark contributions to the magnetic moments of the nucleon octet, the authors obtained a precise determination of the strange magnetic moment of the proton.
Abstract: By combining the constraints of charge symmetry with new chiral extrapolation techniques and recent low mass quenched lattice-QCD simulations of the individual quark contributions to the magnetic moments of the nucleon octet, we obtain a precise determination of the strange magnetic moment of the proton. The result, namely, ${G}_{M}^{s}=(\ensuremath{-}0.046\ifmmode\pm\else\textpm\fi{}0.019){\ensuremath{\mu}}_{N}$ is consistent with the latest experimental measurements but an order of magnitude more precise. This poses a tremendous challenge for future experiments.
134 citations
28 Jan 2005
131 citations
TL;DR: In this article, the authors investigated the nature of the near-threshold enhancement in the pp invariant-mass spectrum of the reaction J/{psi}{yields}{gamma}pp reported recently by the BES Collaboration.
Abstract: We investigate the nature of the near-threshold enhancement in the pp invariant-mass spectrum of the reaction J/{psi}{yields}{gamma}pp reported recently by the BES Collaboration. Using the Juelich NN model we show that the mass dependence of the pp spectrum close to the threshold can be reproduced by the S-wave pp final state interaction in the isospin I=1 state within the Watson-Migdal approach. However, because of our poor knowledge of the NN interaction near-threshold and of the J/{psi}{yields}{gamma}pp reaction mechanism and in view of the controversal situation in the decay J/{psi}{yields}{pi}{sup 0}pp, where no obvious signs of a pp final state interaction are seen, explanations other than final state interactions cannot be ruled out at the present stage.
113 citations
TL;DR: In this paper, an experiment on the radiative π{+}-meson photoproduction from the proton was carried out at the Mainz Microtron MAMI in the kinematic region 537MeV < Eγ < 817MeV, 140°≤ 170° ≤ 180°.
Abstract: An experiment on the radiative π{+}-meson photoproduction from the proton ( γp → γπ{+}n) was carried out at the Mainz Microtron MAMI in the kinematic region 537MeV < Eγ < 817MeV, 140°≤
$ \theta_{{\gamma \gamma ^{\prime }}}^{{{{\rm cm}}}}$
≤180°. The π{+}-meson polarizabilities have been determined from a comparison of the data with the predictions of two different theoretical models, the first one being based on an effective pole model with pseudoscalar coupling while the second one is based on diagrams describing both resonant and nonresonant contributions. The validity of the models has been verified by comparing the predictions with the present experimental data in the kinematic region where the pion polarizability contribution is negligible ( s1 < 5mπ2) and where the difference between the predictions of the two models does not exceed 3%. In the region, where the pion polarizability contribution is substantial ( 5 < s1/mπ2 < 15, -12 < t/mπ2 < - 2), the difference $\ensuremath{(\alpha -\beta )_{\pi^{+}}}$
of the electric (α) and the magnetic (β) polarizabilities has been determined. As a result we find $\ensuremath{(\alpha -\beta )_{\pi^{+}}=(11.6\pm 1.5_{{\rm stat}}\pm 3.0_{{\rm syst}}\pm 0.5_{{\rm mod}})\times 10^{-4}{}{\rm fm^{3}}}$
. This result is at variance with recent calculations in the framework of chiral perturbation theory.
76 citations
TL;DR: An excellent description of both spin-independent and spin-dependent quark distributions and structure functions has been obtained with a modified Nambu--Jona-Lasinio model, and it is found that the ratio g1p(A)/g1p differs significantly from unity, with the quenching caused by the nuclear medium being about twice that of the spin- independent case.
Abstract: An excellent description of both spin-independent and spin-dependent quark distributions and structure functions has been obtained with a modified Nambu-Jona-Lasinio model, which is free of unphysical thresholds for nucleon decay into quarks--hence incorporating an important aspect of confinement. We utilize this model to investigate nuclear medium modifications to structure functions and find that we are readily able to reproduce both nuclear matter saturation and the experimental F{sub 2N}{sup A}/F{sub 2N} ratio, that is, the European Muon Collaboration (EMC) effect. Applying this framework to determine g{sub 1p}{sup A}, we find that the ratio g{sub 1p}{sup A}/g{sub 1p} differs significantly from unity, with the quenching caused by the nuclear medium being about twice that of the spin-independent case. This represents an exciting result, which, if confirmed experimentally, will reveal much about the quark structure of nuclear matter.
68 citations
TL;DR: In this article, the authors investigated the extrapolation of quenched nucleon magnetic moments in chiral effective field theory and used established techniques in finite-range regularization and compare with standard dimensional regularization methods.
Abstract: We present the first investigation of the extrapolation of quenched nucleon magnetic moments in quenched chiral effective field theory. We utilize established techniques in finite-range regularization and compare with standard dimensional regularization methods. Finite-volume corrections to the relevant loop integrals are also addressed. Finally, the contributions of dynamical sea quarks to the proton moment are estimated using a recently discovered phenomenological link between quenched and physical QCD.
62 citations
TL;DR: In this paper, the vector meson mass is extracted from a large sample of partially quenched, two-flavor lattice QCD simulations, and discretisation, finite-volume and partial quenching artefacts are treated in a unified framework which is consistent with the low-energy behaviour of QCD.
56 citations
TL;DR: In this paper, the authors analyzed the daily closures of four of the most important indices worldwide since 2000: the DAX for Germany and the NASDAQ-100, the S&P 500 and the Dow Jones for the United States.
Abstract: The presence of log-periodic structures before and after stock market crashes is considered to be an imprint of an intrinsic discrete scale invariance (DSI) in this complex system. The fractal framework of the theory leaves open the possibility of observing self-similar log-periodic structures at different time scales. In the present work, we analyze the daily closures of four of the most important indices worldwide since 2000: the DAX for Germany and the NASDAQ-100, the S&P 500 and the Dow Jones for the United States. The qualitative behavior of these different markets is similar during the temporal frame studied. Evidence is found for decelerating log-periodic oscillations of duration about two years and starting in September 2000. Moreover, a nested sub-structure starting in May 2002 is revealed, bringing more evidence to support the hypothesis of self-similar, log-periodic behavior. Ongoing log-periodic oscillations are also revealed. A Lomb analysis over the aforementioned periods indicates a preferential scaling factor λ~2. Higher order harmonics are also present. The spectral pattern of the data has been found to be similar to that of a Weierstrass-type function, used as a prototype of a log-periodic fractal function.
52 citations
TL;DR: The dynamics of opinion formation in large groups of people is mimicked as a stochastic response of each agent to the opinion of his/her neighbors in the social network and to feedback from the average opinion of the whole.
Abstract: The dynamics of opinion formation in large groups of people is a complex nonlinear phenomenon whose investigation is just beginning. Both collective behavior and personal views play an important role in this mechanism. In the present work we mimic the dynamics of opinion formation of a group of agents, represented by two states $\ifmmode\pm\else\textpm\fi{}1$, as a stochastic response of each agent to the opinion of his/her neighbors in the social network and to feedback from the average opinion of the whole. In the light of recent studies, a scale-free Barab\'asi-Albert network has been selected to simulate the topology of the interactions. A turbulentlike dynamics, characterized by an intermittent behavior, is observed for a certain range of the model parameters. The problem of uncertainty in decision taking is also addressed both from a topological point of view, using random and targeted removal of agents from the network, and by implementing a three-state model, where the third state, zero, is related to the information available to each agent. Finally, the results of the model are tested against the best known network of social interactions: the stock market. A time series of daily closures of the Dow-Jones index has been used as an indicator of the possible applicability of our model in the financial context. Good qualitative agreement is found.
52 citations
TL;DR: In this article, a modified Nambu-Jona-Lasinio model was used to simulate the decay of a nucleon into quarks, where confinement was achieved by eliminating unphysical thresholds for nucleon decay.
TL;DR: In this article, a wavelet transform method is used to separate high activity periods, related to the avalanches found in sandpile models, from quiescent periods, and a statistical analysis of the filtered data shows a power law behaviour in the avalanche size, duration and laminar times.
Abstract: The stock market is a complex self-interacting system, characterized by intermittent behaviour. Periods of high activity alternate with periods of relative calm. In the present work we investigate empirically the possibility that the market is in a self-organized critical state (SOC). A wavelet transform method is used in order to separate high activity periods, related to the avalanches found in sandpile models, from quiescent. A statistical analysis of the filtered data shows a power law behaviour in the avalanche size, duration and laminar times. The memory process, implied by the power law distribution of the laminar times, is not consistent with classical conservative models for self-organized criticality. We argue that a “near-SOC” state or a time dependence in the driver, which may be chaotic, can explain this behaviour.
TL;DR: Helicity-dependent total photoabsorption cross sections on the deuteron have been measured for the first time at ELSA (Bonn) in the photon energy range from 815 to 1825 MeV.
Abstract: Helicity-dependent total photoabsorption cross sections on the deuteron have been measured for the first time at ELSA (Bonn) in the photon energy range from 815 to 1825 MeV. Circularly polarized tagged photons impinging on a longitudinally polarized LiD target have been used together with a highly efficient 4 pi detector system. The data around 1 GeV are not compatible with predictions from existing multipole analyses. From the measured energy range an experimental contribution to the GDH integral on the neutron of [33.9 +/- 5.5(stat)+/- 4.5(syst)] mu b is extracted.
TL;DR: In this article, it was shown that there is no evidence supporting the existence of a spin-(1/2) pentaquark resonance in quenched lattice QCD simulations, and that the standard lattice resonance signal of binding at quark masses near the physical regime is not observed for established baryon resonances.
Abstract: Claims concerning the possible discovery of the {theta}{sup +} pentaquark, with minimal quark content uudds, have motivated our comprehensive study into possible pentaquark states using lattice QCD. We review various spin-(1/2) pentaquark interpolating fields in the literature and create a new candidate ideal for lattice QCD simulations. Using these interpolating fields we attempt to isolate a signal for a five-quark resonance. Calculations are performed using improved actions on a large 20{sup 3}x40 lattice in the quenched approximation. The standard lattice resonance signal of binding at quark masses near the physical regime, observed for established baryon resonances, is not observed for spin-(1/2) pentaquark states. Thus we find no evidence supporting the existence of a spin-(1/2) pentaquark resonance in quenched QCD.
28 Jan 2005
TL;DR: In this article, the power-counting regime (PCR) of chiral perturbation theory has been investigated and the chiral extrapolation problem has been solved using finite-range regularization.
TL;DR: The G asymmetry of the γp↦Nπ reaction has been measured for the first time for Eγ = 340±14 MeV as discussed by the authors, which plays an important role to disentangle the contributions of the various nucleon resonances.
Abstract: The G asymmetry of the γp↦Nπ reaction has been measured for the first time for Eγ = 340±14 MeV. This observable, for which very scarce published data exist, plays an important role to disentangle the contributions of the various nucleon resonances. The experiment, performed at the Mainz microtron MAMI, used a 4π-detector system, a linearly polarized, tagged photon beam, and a longitudinally polarized proton target.
TL;DR: In this paper, the masses and radii of pure neutron stars and hadronic stars were obtained, and it was shown that for a pure neutron star, the maximum mass is about 1.8 M, while for a hadronic star, it is around 1.45M.
Abstract: We investigate the equations of state for pure neutron matter and strange hadronic matter in {beta}-equilibrium, including {Lambda}, {Sigma} and {Xi} hyperons. The masses and radii of pure neutron stars and strange hadronic stars are obtained. For a pure neutron star, the maximum mass is about 1.8 M{sub sun}, while for a strange hadronic star, the maximum mass is around 1.45M{sub sun}. The typical radii of pure neutron stars and strange hadronic stars are about 11.0-12.3 km and 10.7-11.7 km, respectively.
TL;DR: Systematic uncertainties in the recent precise determination of the strangeness magnetic moment of the nucleon are identified and quantified.
Abstract: Systematic uncertainties in the recent precise determination of the strangeness magnetic moment of the nucleon are identified and quantified. In summary, G M s = −0.046 ± 0.019 μ N.
TL;DR: In this paper, the first exploratory study in quenched lattice QCD of pentaquarks with spin $N$ and $K$ operators was performed. But the results of this study were limited to the case of quark masses.
Abstract: The possible discovery of the ${\ensuremath{\Theta}}^{+}$ pentaquark has motivated a number of studies of its nature using lattice QCD. While all the analyses thus far have focused on spin-$\frac{1}{2}$ states, here we report the results of the first exploratory study in quenched lattice QCD of pentaquarks with spin $\frac{3}{2}$. For the spin-$\frac{3}{2}$ interpolating field we use a product of the standard $N$ and ${K}^{*}$ operators. We do not find any evidence for the standard lattice resonance signature of attraction (i.e., binding at quark masses near the physical regime) in the ${J}^{P}={\frac{3}{2}}^{\ensuremath{-}}$ channel. Some evidence of binding is inferred in the isoscalar ${\frac{3}{2}}^{+}$ channel at several quark masses, in accord with the standard lattice resonance signature. This suggests that this is a good candidate for the further study of pentaquarks on the lattice.
10 Jan 2005
TL;DR: In this article, the authors present the compelling scientific case for upgrading the Continuous Electron Beam Accelerator Facility (CEBAF) at Jefferson Lab to 12 GeV, which will make profound contributions to the study of hadronic matter.
Abstract: This Conceptual Design Report (CDR) presents the compelling scientific case for upgrading the Continuous Electron Beam Accelerator Facility (CEBAF) at Jefferson Lab to 12 GeV. Such a facility will make profound contributions to the study of hadronic matter.
TL;DR: In this article, the suitability of a modern vector meson dominance (VMD) model as a method for chiral extrapolation of nucleon electromagnetic form factor simulations in lattice QCD was explored.
Abstract: We explore the suitability of a modern vector meson dominance (VMD) model as a method for chiral extrapolation of nucleon electromagnetic form factor simulations in lattice QCD. It is found that the VMD fits to experimental data can be readily generalized to describe the lattice simulations. However, the converse is not true. That is, the VMD form is unsuitable as a method of extrapolation of lattice simulations at large quark mass to the physical regime.
TL;DR: In this paper, the authors explore the possibility of extrapolating state-of-the-art lattice QCD calculations of nucleon form factors to the physical regime by using the light front cloudy bag model by letting its parameters be analytic functions of the quark mass.
Abstract: We explore the possibility of extrapolating state of the art lattice QCD calculations of nucleon form factors to the physical regime. We find that the lattice results can be reproduced using the light front cloudy bag model by letting its parameters be analytic functions of the quark mass. We then use the model to extend the lattice calculations to large values of ${Q}^{2}$ of interest to current and planned experiments. These functions are also used to define extrapolations to the physical value of the pion mass, thereby allowing us to study how the predicted zero in ${G}_{E}({Q}^{2})/{G}_{M}({Q}^{2})$ varies as a function of quark mass.
TL;DR: In this paper, the B? and B? K transitions involved in hadronic B decays are investigated in a phenomenological way through the framework of QCD factorization, and the authors propose boundaries for the transition form factors B?? and b? K depending on the CKM matrix element parameters? and?.
Abstract: The B ? ? and B ? K transitions involved in hadronic B decays are investigated in a phenomenological way through the framework of QCD factorization. By comparing our results with experimental branching ratios from the BELLE, BABAR and CLEO collaborations for all the B decays including either a pion or a kaon, we propose boundaries for the transition form factors B ? ? and B ? K depending on the CKM matrix element parameters ? and ?. From this analysis, the form factors required to reproduce the experimental data for branching ratios are FB?? = 0.31 ? 0.12 and FB?K = 0.37 ? 0.13. We calculate the direct CP violating asymmetry parameter, aCP, for B ? ?+??? and B ? ?+??K decays, in the case where ??? mixing effects are taken into account. Based on these results, we find that the direct CP asymmetry for B? ? ?+????, and , reaches its maximum when the invariant mass ?+?? is in the vicinity of the ? meson mass. The inclusion of ??? mixing provides an opportunity to erase, without ambiguity, the phase uncertainty mod(?) in the determination of the CKM angles ? in the case of b ? u and ? in the case of b ? s.
TL;DR: In this paper, the authors investigated the possibility of kaon condensation in high-density symmetric nuclear matter including both s - and p -wave kaon-baryon interactions within the relativistic mean field (RMF) theory.
TL;DR: In this article, a systematic error in the extraction of sin2θW from nuclear deep inelastic scattering of neutrinos and antineutrinos arises from higher-twist effects arising from nuclear shadowing.
Abstract: A systematic error in the extraction of sin2θW from nuclear deep inelastic scattering of neutrinos and antineutrinos arises from higher-twist effects arising from nuclear shadowing. We explain that these effects cause a correction to the results of the recently reported significant deviation from the Standard Model that is potentially as large as the deviation claimed, and of a sign that cannot be determined without an extremely careful study of the data set used to model the input parton distribution functions.
TL;DR: In this paper, the authors investigated OZI rule violation in meson photoproduction off nucleons, showing that the perturbative QCD regime is not approached at the maximal available photon energy.
Abstract: We investigate OZI rule violation in $\ensuremath{\omega}$ and $\ensuremath{\phi}$-meson photoproduction off nucleons. Data on the total cross sections indicate a large $\ensuremath{\phi}/\ensuremath{\omega}$ ratio of about 0.8 at the maximal available photon energy that is in good agreement with expectations from QCD. On the other hand, data at large four-momentum transfer exhibit a ratio of about 0.07, showing that the perturbative QCD regime is not approached at $|t|g2\text{ }\text{ }{\mathrm{GeV}}^{2}$ and photon energies ${E}_{\ensuremath{\gamma}}l4\text{ }\text{ }\mathrm{GeV}$. The anomanously large $\ensuremath{\phi}/\ensuremath{\omega}$ ratio at low energies, that is close to the reaction threshold, remains to be explained within nonperturbative QCD.
TL;DR: In this paper, the authors used a chiral SU (3 ) quark mean field model to study the properties of nuclear systems at finite temperature and found that the critical temperature, T c, for symmetric nuclear matter is 15.8 MeV and 17.9 MeV.
01 Apr 2005
TL;DR: In this paper, the authors use the model to investigate the equation of state of asymmetric nuclear matter and then use it to compute the properties of neutron stars, which leads in mean field approximation to a natural mechanism for the saturation of nuclear matter.
Abstract: The NJL model has recently been extended with a method to simulate confinement. This leads in mean field approximation to a natural mechanism for the saturation of nuclear matter. We use the model to investigate the equation of state of asymmetric nuclear matter and then use it to compute the properties of neutron stars.