Showing papers by "Anthony W. Thomas published in 2015"
TL;DR: A new lattice QCD simulation is presented showing that the strange magnetic form factor of the Λ(1405) vanishes, signaling the formation of an antikaon-nucleon molecule, which clarifies that not all states occurring in nature can be described within a simple quark model framework and points to the existence of exotic molecular meson- nucleon bound states.
Abstract: For almost 50 years the structure of the Λ(1405) resonance has been a mystery. Even though it contains a heavy strange quark and has odd parity, its mass is lower than any other excited spin-1/2 baryon. Dalitz and co-workers speculated that it might be a molecular state of an antikaon bound to a nucleon. However, a standard quark-model structure is also admissible. Although the intervening years have seen considerable effort, there has been no convincing resolution. Here we present a new lattice QCD simulation showing that the strange magnetic form factor of the Λ(1405) vanishes, signaling the formation of an antikaon-nucleon molecule. Together with a Hamiltonian effective-field-theory model analysis of the lattice QCD energy levels, this strongly suggests that the structure is dominated by a bound antikaon-nucleon component. This result clarifies that not all states occurring in nature can be described within a simple quark model framework and points to the existence of exotic molecular meson-nucleon bound states.
148 citations
University of Massachusetts Amherst1, University of Mainz2, Kent State University3, University of Glasgow4, University of Bonn5, Massachusetts Institute of Technology6, Joint Institute for Nuclear Research7, George Washington University8, Lebedev Physical Institute9, Dalhousie University10, University of Basel11, University of Edinburgh12, Mount Allison University13, University of Regina14, Ruhr University Bochum15, University of California, Los Angeles16, Saint Mary's University17
TL;DR: Fits to asymmetry data were performed using a dispersion model calculation and a baryon chiral perturbation theory calculation, and a separation of all four proton spin polarizabilities in the multipole basis was achieved.
Abstract: The spin polarizabilities of the nucleon describe how the spin of the nucleon responds to an incident polarized photon. The most model-independent way to extract the nucleon spin polarizabilities is through polarized Compton scattering. Double-polarized Compton scattering asymmetries on the proton were measured in the Delta(1232) region using circularly polarized incident photons and a transversely polarized proton target at the Mainz Microtron. Fits to asymmetry data were performed using a dispersion model calculation and a baryon chiral perturbation theory calculation, and a separation of all four proton spin polarizabilities in the multipole basis was achieved. The analysis based on a dispersion model calculation yields gamma(E1E1) = -3.5 +/- 1.2, gamma(M1M1) = 3.16 +/- 0.85, gamma(E1M2) = -0.7 +/- 1.2, and gamma(M1E2) = 1.99 +/- 0.29, in units of 10(-4) fm(4).
47 citations
TL;DR: Differential cross sections for the γp→π0p reaction have been measured with the A2 tagged-photon facilities at the Mainz Microtron, MAMI C, up to the center-of-mass energy W=1.9GeV as mentioned in this paper.
Abstract: Differential cross sections for the γp→π0p reaction have been measured with the A2 tagged-photon facilities at the Mainz Microtron, MAMI C, up to the center-of-mass energy W=1.9GeV. The new results, obtained with a fine energy and angular binning, increase the existing quantity of π0 photoproduction data by ∼47%. Owing to the unprecedented statistical accuracy and the full angular coverage, the results are sensitive to high partial-wave amplitudes. This is demonstrated by the decomposition of the differential cross sections in terms of Legendre polynomials and by further comparison to model predictions. A new solution of the SAID partial-wave analysis obtained after adding the new data into the fit is presented.
38 citations
TL;DR: In this article, the electromagnetic contribution to the proton-neutron mass splitting was studied by combining lattice simulations and the modified Cottingham sum rule of Walker-Loud, Carlson and Miller.
Abstract: We study the electromagnetic contribution to the proton-neutron mass splitting by combining lattice simulations and the modified Cottingham sum rule of Walker-Loud, Carlson and Miller. This analysis yields an estimate of the isovector nucleon magnetic polarizability as a function of pion mass. The physical value, obtained by chiral extrapolation to the physical pion mass, is $\beta_{p-n}=(-1.12 \pm 0.40)\times 10^{-4}\ \mathrm{fm}^3$, which is in agreement with the empirical result, albeit with a somewhat smaller error. As a result, we find $\delta M^{\gamma}_{p-n}=1.04 \pm 0.11\ \mathrm{MeV}$, which represents a significant improvement in precision.
35 citations
TL;DR: In this article, the authors considered a composite Higgs model embedded into a grand unified theory (GUT) based on the gauge group and presented a six-dimensional orbifold GUT model in which the approximate gauge coupling unification can be attained if the right-handed top quark is a composite state and the elementary sector involves extra exotic matter beyond the SM which ensures anomaly cancellation.
Abstract: We consider a composite Higgs model embedded into a grand unified theory (GUT) based on the ${E}_{6}$ gauge group. The phenomenological viability of this ${E}_{6}\text{inspired}$ composite Higgs model (${\mathrm{E}}_{6}\mathrm{CHM}$) implies that standard model (SM) elementary fermions with different baryon or lepton number should stem from 27 different representations of ${E}_{6}$. We present a six-dimensional orbifold GUT model in which the ${E}_{6}$ gauge symmetry is broken to the SM gauge group so that the appropriate splitting of the bulk 27-plets takes place. In this model the strongly coupled sector is localized on one of the branes and possesses an $SU(6)$ global symmetry that contains the $SU(3{)}_{C}\ifmmode\times\else\texttimes\fi{}SU(2{)}_{W}\ifmmode\times\else\texttimes\fi{}U(1{)}_{Y}$ subgroup. In this case the approximate gauge coupling unification can be attained if the right-handed top quark is a composite state and the elementary sector involves extra exotic matter beyond the SM which ensures anomaly cancellation. The breakdown of the approximate $SU(6)$ symmetry at low energies in this model results in a set of the pseudo-Nambu-Goldstone states which include a Higgs doublet and scalar color triplet. We discuss the generation of the masses of the SM fermions in the ${\mathrm{E}}_{6}\mathrm{CHM}$. The presence of the TeV scale vectorlike exotic quarks and scalar color triplet may provide spectacular new physics signals that can be observed at the LHC.
25 citations
TL;DR: The strange contribution to the electric and magnetic form factors of the nucleon is determined at a range of discrete values of Q^{2} up to 1.4 GeV 2 by combining a recent analysis of lattice QCD results for the electromagnetic form factor of the octet baryons with experimental determinations of those quantities.
Abstract: The strange contribution to the electric and magnetic form factors of the nucleon is determined at a range of discrete values of ${Q}^{2}$ up to $1.4\text{ }\text{ }{\mathrm{GeV}}^{2}$. This is done by combining a recent analysis of lattice QCD results for the electromagnetic form factors of the octet baryons with experimental determinations of those quantities. The most precise result is a small negative value for the strange magnetic moment: ${G}_{M}^{s}({Q}^{2}=0)=\ensuremath{-}0.07\ifmmode\pm\else\textpm\fi{}0.03{\ensuremath{\mu}}_{N}$. At larger values of ${Q}^{2}$ both the electric and magnetic form factors are consistent with zero to within 2 standard deviations.
22 citations
TL;DR: In this article, a corrected leading-order expression for the pion valence-quark distribution function is proposed to ensure that the valence quarks carry all of the light-front momentum at the initial scale.
22 citations
University of Basel1, University of Mainz2, University of Glasgow3, Kent State University4, University of Bonn5, Petersburg Nuclear Physics Institute6, University of Giessen7, University of Edinburgh8, George Washington University9, University of California, Los Angeles10, Lebedev Physical Institute11, Tomsk Polytechnic University12, Mount Allison University13, University of Regina14, The Catholic University of America15
TL;DR: In this article, the authors measured total cross sections, angular distributions, and invariant-mass distributions for the photoproduction of pairs off free protons and off nucleons bound in the deuteron.
Abstract: Total cross sections, angular distributions, and invariant-mass distributions have been measured for the photoproduction of pairs off free protons and off nucleons bound in the deuteron. The experiments were performed at the MAMI accelerator facility in Mainz using the Glasgow photon tagging spectrometer and the Crystal Ball/TAPS detector. The accelerator delivered electron beams of 1508 and 1557MeV, which produced bremsstrahlung in thin radiator foils. The tagged photon beam covered energies up to 1400MeV. The data from the free proton target are in good agreement with previous measurements and were only used to test the analysis procedures. The results for differential cross sections (angular distributions and invariant-mass distributions) for free and quasi-free protons are almost identical in shape, but differ in absolute magnitude up to 15%. Thus, moderate final-state interaction effects are present. The data for quasi-free neutrons are similar to the proton data in the second resonance region (final-state invariant masses up to MeV), where both reactions are dominated by the decay. At higher energies, angular and invariant-mass distributions are different. A simple analysis of the shapes of the invariant-mass distributions in the third resonance region is consistent with strong contributions of an decay for the proton, while the reaction is dominated by a sequential decay via a intermediate state for the neutron. The data are compared to predictions from the Two-Pion-MAID model and the Bonn-Gatchina coupled-channel analysis.
18 citations
TL;DR: A brief review of the low-lying even and odd-parity excitations of the nucleon obtained in recent lattice QCD calculations can be found in this paper, where a consensus on the lowlying spectrum has emerged among many different lattice groups.
Abstract: This brief review focuses on the low-lying even- and odd-parity excitations of the nucleon obtained in recent lattice QCD calculations. Commencing with a survey of the 2014-15 literature we'll see that results for the first even-parity excitation energy can differ by as much as 1 GeV, a rather unsatisfactory situation. Following a brief review of the methods used to isolate excitations of the nucleon in lattice QCD, and drawing on recent advances, we'll see how a consensus on the low-lying spectrum has emerged among many different lattice groups. To provide insight into the nature of these states we'll review the wave functions and electromagnetic form factors that are available for a few of these states. Consistent with the Luscher formalism for extracting phase shifts from finite volume spectra, the Hamiltonian approach to effective field theory in finite volume can provide guidance on the manner in which physical quantities manifest themselves in the finite volume of the lattice. With this insight, we will address the question; Have we seen the Roper in lattice QCD?
16 citations
TL;DR: In this article, the sensitivity of the Large Hadron Collider (LHC) to top quark chromomagnetic (CMDM) and chromoelectric (CEDM) dipole moments and W tb effective couplings in single-top production in association with a W − boson was studied.
Abstract: We study the sensitivity of the Large Hadron Collider (LHC) to top quark chromomagnetic (CMDM) and chromoelectric (CEDM) dipole moments and W tb effective couplings in single-top production in association with a W − boson, followed by semileptonic decay of the top. The W t single-top production mode helps to isolate the anomalous ttg and W tb couplings, in contrast to top-pair production and other single-top production modes, where other new-physics effects can also contribute. We calculate the top polarization and the effects of these anomalous couplings on it at two centre-of-mass (cm) energies, 8 TeV and 14 TeV. As a measure of top polarization, we look at decay-lepton angular distributions in the laboratory frame, without requiring reconstruction of the rest frame of the top, and study the effect of the anomalous couplings on these distributions. We construct certain asymmetries to study the sensitivity of these distributions to top-quark couplings. We determine individual limits on the dominant couplings, viz., the real part of the CMDM Reρ
2, the imaginary part of the CEDM Imρ
3, and the real part of the tensor W tb coupling Ref2R, which may be obtained by utilizing these asymmetries at the LHC. We also obtain simultaneous limits on pairs of these couplings taking two couplings to be non-zero at a time.
16 citations
TL;DR: In this paper, the Nambu-Jona-Lasinio model was used to calculate the elastic electromagnetic form factor for the meson in a proper-time regularization scheme.
Abstract: Elastic electromagnetic form factors for the ${\ensuremath{\rho}}^{+}$ meson are calculated in a Nambu--Jona-Lasinio model which incorporates quark confinement through the use of the proper-time regularization scheme. A comparison is made with recent lattice QCD results and previous quark model calculations for static quantities and the Sachs form factors. The results are qualitatively in good agreement with the lattice QCD calculations, with the exception of the quadrupole moment and corresponding form factor, which may be related to a lack of spherical symmetry on the lattice.
TL;DR: In this paper, the sensitivity of the Large Hadron Collider (LHC) to top quark chromomagnetic (CMDM) and chromoelectric (CEDM) dipole moments and effective couplings in single-top production in association with a $W^-$ boson, followed by semileptonic decay of the top.
Abstract: We study the sensitivity of the Large Hadron Collider (LHC) to top quark chromomagnetic (CMDM) and chromoelectric (CEDM) dipole moments and $Wtb$ effective couplings in single-top production in association with a $W^-$ boson, followed by semileptonic decay of the top. We calculate the top polarization and the effects of these anomalous couplings on it at two centre-of-mass (cm) energies, 8 TeV and 14 TeV. As a measure of top polarization, we look at decay-lepton angular distributions in the laboratory frame, without requiring reconstruction of the rest frame of the top, and study the effect of the anomalous couplings on these distributions. We construct certain asymmetries to study the sensitivity of these distributions to top-quark couplings. The Wt single-top production mode helps to isolate the anomalous $ttg$ and $Wtb$ couplings, in contrast to top-pair production and other single-top production modes, where other new-physics effects can also contribute. We determine individual limits on the dominant couplings, viz., the real part of the CMDM $Re\rho_2$, the imaginary part of the CEDM $Im\rho_3$, and the real part of the tensor Wtb coupling $Ref_{2r}$, which may be obtained by utilizing these asymmetries at the LHC. We also obtain simultaneous limits on pairs of these couplings taking two couplings to be non-zero at a time.
TL;DR: In this paper, a modified version of the pythia6 Monte Carlo event generator that includes the Sivers effect is presented, which is used to estimate the size of these SSAs, in the kinematics of these experiments, for both one and two hadron final states of pions and kaons.
Abstract: The study of the Sivers effect, describing correlations between the transverse polarization of the nucleon and its constituent (unpolarized) parton's transverse momentum, has been the topic of a great deal of experimental, phenomenological and theoretical effort in recent years. Semi-inclusive deep inelastic scattering measurements of the corresponding single spin asymmetries (SSA) at the upcoming CLAS12 experiment at JLab and the proposed Electron-Ion Collider will help to pinpoint the flavor structure and the momentum dependence of the Sivers parton distribution function describing this effect. Here we describe a modified version of the pythia6 Monte Carlo event generator that includes the Sivers effect. Then we use it to estimate the size of these SSAs, in the kinematics of these experiments, for both one and two hadron final states of pions and kaons. For this purpose we utilize the existing Sivers parton distribution function (PDF) parametrization extracted from HERMES and COMPASS experiments. Using this modified version of pythia6, we also show that the leading order approximation commonly used in such extractions may provide significantly underestimated values of Sivers PDFs, as in our Monte Carlo simulations the omitted parton showers and non-DIS processes play an important role in these SSAs, for example in the COMPASS kinematics.
TL;DR: In this paper, the pure sea-quark contributions to the magnetic form factors of Sigma baryons, G(Sigma-u) and G (Sigma+)(d), were investigated for lattice QCD simulations and possible future experimental measurement.
Abstract: We propose the pure sea-quark contributions to the magnetic form factors of Sigma baryons, G(Sigma-)(u) and G(Sigma+)(d), as priority observables for the examination of sea-quark contributions to baryon structure, both in present lattice QCD simulations and possible future experimental measurement. G(Sigma-)(u), the u-quark contribution to the magnetic form factor of Sigma(-), and G(Sigma+)(d), the d-quark contribution to the magnetic form factor of Sigma(+), are similar to the strange-quark contribution to the magnetic form factor of the nucleon, but promise to be larger by an order of magnitude. We explore the size of this quantity within chiral effective field theory, including both octet and decuplet intermediate states. The finite range regularization approach is applied to deal with ultraviolet divergences. Drawing on an established connection between quenched and full QCD, this approach makes it possible to predict the sea-quark contribution to the magnetic form factor purely from the meson loop. In the familiar convention where the quark charge is set to unity G(Sigma-)(u) = G(Sigma+)(d). We find a value of -0.38(-0.17)(+0.16) mu N, which is about seven times larger than the strange magnetic moment of the nucleon found in the same approach. Including quark charge factors, the u-quark contribution to the Sigma(-) magnetic moment exceeds the strange-quark contribution to the nucleon magnetic moment by a factor of 14.
TL;DR: In this article, the SU(3)-breaking corrections to the hyperon transition vector form factors were calculated in heavy-baryon chiral perturbation theory with finite-range regularization.
Abstract: We present a calculation of the SU(3)-breaking corrections to the hyperon transition vector form factors to $\mathcal{O}({p}^{4})$ in heavy-baryon chiral perturbation theory with finite-range regularization. Both octet and decuplet degrees of freedom are included. We formulate a chiral expansion at the kinematic point ${Q}^{2}=\ensuremath{-}({M}_{{B}_{1}}\ensuremath{-}{M}_{{B}_{2}}{)}^{2}$, which can be conveniently accessed in lattice QCD. The two unknown low-energy constants at this point are constrained by lattice QCD simulation results for the ${\mathrm{\ensuremath{\Sigma}}}^{\ensuremath{-}}\ensuremath{\rightarrow}n$ and ${\mathrm{\ensuremath{\Xi}}}^{0}\ensuremath{\rightarrow}{\mathrm{\ensuremath{\Sigma}}}^{+}$ transition form factors. Hence, we determine lattice-informed values of ${f}_{1}$ at the physical point. This work constitutes progress toward the precise determination of $|{V}_{us}|$ from hyperon semileptonic decays.
TL;DR: In this article, the authors present a survey of the state-of-the-art technologies in the field of computer vision and artificial intelligence, including: E.P. Shanahan, R. Horsley, Y. Nakamura, D. Pleiter, P. Schierholz, H. Stuben, A.Rakow, G.Schmidt, W.Thomas, R., D. Young, and J. Zanotti.
Abstract: P. E. Shanahan, R. Horsley, Y. Nakamura, D. Pleiter, P. E. L. Rakow, G. Schierholz, H. Stuben, A. W. Thomas, R. D. Young, and J. M. Zanotti
TL;DR: In this paper, left-truncated Weibull distributions are used to model not only the single claim severity, but also the inter-arrival times between two consecutive claims.
Abstract: In risk theory with application to insurance, the identification of the relevant distributions for both the counting and the claim size processes from given observations is of major importance. In some situations left-truncated distributions can be used to model, not only the single claim severity, but also the inter-arrival times between two consecutive claims. We show that left-truncated Weibull distributions are particularly relevant, especially for the claim severity distribution. For that, we first demonstrate how the parameters can be estimated consistently from the data, and then show how a Kolmogorov-Smirnov goodness-of-fit test can be set up using modified critical values. These critical values are universal to all left-truncated Weibull distributions, independent of the actual Weibull parameters. To illustrate our findings we analyse three applications using real insurance data, one from a Swiss excess of loss treaty over automobile insurance, another from an American private passenger automobile insurance and a third from earthquake inter-arrival times in California.
Journal Article•
Kurchatov Institute1, George Washington University2, Kent State University3, University of Glasgow4, University of Mainz5, University of Bonn6, Joint Institute for Nuclear Research7, Lebedev Physical Institute8, Saint Mary's University9, University of Pavia10, University of Basel11, University of Edinburgh12, Mount Allison University13, University of Regina14, University of Massachusetts Amherst15, Hebrew University of Jerusalem16
TL;DR: In this paper, a measurement of the neutron detection efficiency in NaI crystals in the Crystal Ball detector obtained from a study of single π0 photoproduction on deuterium using the tagged photon beam at the Mainz Microtron.
Abstract: We report on a measurement of the neutron detection efficiency in NaI crystals in the Crystal Ball detector obtained from a study of single π0 photoproduction on deuterium using the tagged photon beam at the Mainz Microtron. The results were obtained up to a neutron energy of 400 MeV . They are compared to previous measurements made more than 15 years ago at the pion beam at the BNL AGS.
Kurchatov Institute1, George Washington University2, Kent State University3, University of Glasgow4, University of Mainz5, University of Bonn6, Joint Institute for Nuclear Research7, Lebedev Physical Institute8, Saint Mary's University9, University of Pavia10, University of Basel11, University of Edinburgh12, Mount Allison University13, University of Regina14, University of Massachusetts Amherst15, Hebrew University of Jerusalem16
TL;DR: In this paper, a measurement of the neutron detection efficiency in NaI crystals in the Crystal Ball detector was obtained from a study of single p0 photoproduction on deuterium using the tagged photon beam at the Mainz Microtron.
Abstract: We report on a measurement of the neutron detection efficiency in NaI crystals in the Crystal Ball detector obtained from a study of single p0 photoproduction on deuterium using the tagged photon beam at the Mainz Microtron. The results were obtained up to a neutron energy of 400 MeV. They are compared to previous measurements made more than 15 years ago at the pion beam at the BNL AGS.
Posted Content•
TL;DR: In this article, the exact degeneracy of vacua in N = 1 supergravity can shed light on the smallness of the cosmological constant, and the presence of such vacua, which are degenerate to very high accuracy, may also result in small values of the quartic Higgs coupling and its beta function at the Planck scale.
Abstract: We argue that the exact degeneracy of vacua in N=1 supergravity can shed light on the smallness of the cosmological constant. The presence of such vacua, which are degenerate to very high accuracy, may also result in small values of the quartic Higgs coupling and its beta function at the Planck scale in the phase in which we live.
Posted Content•
TL;DR: In this paper, Hamiltonian effective field theory (HEFT) is used to predict the positions of the finite-volume energy levels to be observed in lattice QCD simulations of the lowest-lying $J^P=1/2^-$ nucleon excitation.
Abstract: Drawing on experimental data for baryon resonances, Hamiltonian effective field theory (HEFT) is used to predict the positions of the finite-volume energy levels to be observed in lattice QCD simulations of the lowest-lying $J^P=1/2^-$ nucleon excitation. In the initial analysis, the phenomenological parameters of the Hamiltonian model are constrained by experiment and the finite-volume eigenstate energies are a prediction of the model. The agreement between HEFT predictions and lattice QCD results obtained on volumes with spatial lengths of 2 and 3 fm is excellent. These lattice results also admit a more conventional analysis where the low-energy coefficients are constrained by lattice QCD results, enabling a determination of resonance properties from lattice QCD itself. Finally, the role and importance of various components of the Hamiltonian model are examined.
TL;DR: In this paper, the decay amplitude arising from an intermediate state meson and the direct decay amplitude were calculated using the standard effective weak Hamiltonian, and the effect of final state interactions was also included.
Abstract: The weak decay of the kaon to two pions is studied within the model of Nambu and Jona-Lasinio. Using the standard effective weak Hamiltonian, both the decay amplitude arising from an intermediate state $\ensuremath{\sigma}$ meson and the direct decay amplitude are calculated. The effect of final state interactions is also included. When the matching scale is chosen such that the decay amplitude with isospin $I=2$ is close to its experimental value, our model including the $\ensuremath{\sigma}$ meson contributes up to 80% of the total $I=0$ amplitude. This supports recent suggestions that the $\ensuremath{\sigma}$ meson should play a vital role in explaining the $\mathrm{\ensuremath{\Delta}}I=1/2$ rule in this system.
TL;DR: In this article, the use of the finite-volume Hamiltonian approach to resolving scattering observables from lattice QCD spectra was investigated for both S- and P-wave cases.
Abstract: Within a formulation of $\pi\pi$ scattering, we investigate the use of the finite-volume Hamiltonian approach to resolving scattering observables from lattice QCD spectra. We consider spectra in the centre-of-mass and moving frames for both S- and P-wave cases. Furthermore, we investigate the multi-channel case. Here we study the use of the Hamiltonian framework as a parametrization that can be fit directly to lattice spectra. Through this method, the hadron properties, such as mass, width and coupling, can be directly extracted from the lattice spectra.
21 May 2015
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 Λ(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.