Showing papers by "Anthony W. Thomas published in 2013"

Sigma terms from an SU(3) chiral extrapolation

Abstract: We report a new analysis of lattice simulation results for octet baryon masses in $2+1$-flavor QCD, with an emphasis on a precise determination of the pion-nucleon and strangeness nucleon sigma terms. A controlled chiral extrapolation of a recent PACS-CS Collaboration data set yields baryon masses which exhibit remarkable agreement both with experimental values at the physical point and with the results of independent lattice QCD simulations at unphysical meson masses. Using the Feynman-Hellmann relation, we evaluate sigma commutators for all octet baryons. The small statistical uncertainty and considerably smaller model dependence allows a significantly more precise determination of the pion-nucleon sigma commutator and the strangeness sigma term than hitherto possible, subject to an unresolved issue concerning the lattice scale setting.

Revealing dressed quarks via the proton's charge distribution.

Abstract: The proton is arguably the most fundamental of nature's readily detectable building blocks. It is at the heart of every nucleus and has never been observed to decay. It is nevertheless a composite object, defined by its valence-quark content: u+u+d--i.e., two up (u) quarks and one down (d) quark; and the manner by which they influence, inter alia, the distribution of charge and magnetization within this bound state. Much of novelty has recently been learned about these distributions; and it now appears possible that the proton's momentum-space charge distribution possesses a zero. Experiments in the coming decade should answer critical questions posed by this and related advances; we explain how such new information may assist in charting the origin and impact of key emergent phenomena within the strong interaction. Specifically, we show that the possible existence and location of a zero in the proton's electric form factor are a measure of nonperturbative features of the quark-quark interaction in the standard model, with particular sensitivity to the running of the dressed-quark mass.

Finite-volume matrix Hamiltonian model for a $\Delta \to N\pi$ system

Abstract: A matrix Hamiltonian model is developed to address the finite-volume effects appearing in studies of baryon resonances in lattice QCD. The Hamiltonian model includes interaction terms in a transparent way and can be readily generalized to address multichannel problems. The eigenvalue equation of the model is exactly solvable and can be matched onto chiral effective field theory. The model is investigated in the case of $\ensuremath{\Delta}\ensuremath{\rightarrow}N\ensuremath{\pi}$ scattering. A robust method for determining the resonance parameters from lattice QCD is developed. It involves constraining the free parameters of the model based on the lattice spectrum in question. The method is tested in the context of a set of pseudodata, and a picture of the model dependence is obtained by examining a variety of regularization schemes in the model. A comparison is made with the L\"uscher method, and it is found that the matrix Hamiltonian method is equally robust. Both methods are tested in a more realistic scenario, where a background interaction corresponding to direct $N\ensuremath{\pi}\ensuremath{\leftrightarrow}N\ensuremath{\pi}$ scattering is incorporated into the pseudodata. The resulting extraction of the resonance parameters associated with the $\ensuremath{\Delta}$ baryon resonance provides evidence that an effective field theory style of approach yields a successful realization of finite-volume effects in the context of baryon resonances.

Octet baryon electromagnetic form factors in nuclear medium

Abstract: We study the octet baryon electromagnetic form factors in nuclear matter using the covariant spectator quark model extended to the nuclear matter regime. The parameters of the model in vacuum are fixed by the study of the octet baryon electromagnetic form factors. In nuclear matter the changes in hadron properties are calculated by including the relevant hadron masses and the modification of the pion–baryon coupling constants calculated in the quark–meson coupling model. In nuclear matter the magnetic form factors of the octet baryons are enhanced in the low Q2 region, while the electric form factors show a more rapid variation with Q2. The results are compared with the modification of the bound proton electromagnetic form factors observed at Jefferson Lab. In addition, the corresponding changes for the bound neutron are predicted.

Pion momentum distributions in the nucleon in chiral effective theory

Abstract: We compute the light-cone momentum distributions of pions in the nucleon in chiral effective theory using both pseudovector and pseudoscalar pion-nucleon couplings. For the pseudovector coupling we identify $\ensuremath{\delta}$-function contributions associated with end-point singularities arising from the pion-nucleon rainbow diagrams, as well as from pion bubble and tadpole diagrams which are not present in the pseudoscalar model. Gauge invariance is demonstrated, to all orders in the pion mass, with the inclusion of Kroll-Ruderman couplings involving operator insertions at the $\ensuremath{\pi}NN$ vertex. The results pave the way for phenomenological applications of pion cloud models that are manifestly consistent with the chiral symmetry properties of QCD.

QCD symmetries in eta and etaprime mesic nuclei

Abstract: We discuss the role of QCD symmetries and confinement in understanding eta and etaprime mesic nuclei. Eta and etaprime bound states in nuclei are sensitive to the flavour-singlet component in the meson. The bigger the singlet component, the more attraction and the greater the binding. Recent results on the etaprime mass in nuclei from the CBELSA/TAPS collaboration are very similar to the prediction of the Quark Meson Coupling model. In the model eta-etaprime mixing induces a factor of two enhancement of the eta-nucleon scattering length relative to the prediction with a pure octet eta, with real part about 0.8 fm.

Constrained $\gamma Z$ interference corrections to parity-violating electron scattering

Abstract: We present a comprehensive analysis of $\ensuremath{\gamma}Z$ interference corrections to the weak charge of the proton measured in parity-violating electron scattering, including a survey of existing models and a critical analysis of their uncertainties. Constraints from parton distributions in the deep-inelastic region, together with new data on parity-violating electron scattering in the resonance region, result in significantly smaller uncertainties on the corrections compared to previous estimates. At the kinematics of the ${Q}_{\mathrm{weak}}$ experiment, we determine the $\ensuremath{\gamma}Z$ box correction to be $\ensuremath{\mathfrak{R}}e{\ensuremath{\square}}_{\ensuremath{\gamma}Z}^{V}=(5.57\ifmmode\pm\else\textpm\fi{}0.36)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$. The new constraints also allow precise predictions to be made for parity-violating deep-inelastic asymmetries on the deuteron.

Anatomy of relativistic pion loop corrections to the electromagnetic nucleon coupling

Abstract: We present a relativistic formulation of pion loop corrections to the coupling of photons with nucleons on the light front. Vertex and wave function renormalization constants are computed to lowest order in the pion field, including their nonanalytic behavior in the chiral limit, and studied numerically as a function of the ultraviolet cutoff. Particular care is taken to explicitly verify gauge invariance and Ward-Takahashi identity constraints to all orders in the ${m}_{\ensuremath{\pi}}$ expansion. The results are used to compute the chiral corrections to matrix elements of local operators, related to moments of deep-inelastic structure functions. Finally, comparison of results for pseudovector and pseudoscalar coupling allows the resolution of a longstanding puzzle in the computation of pion cloud corrections to structure function moments.

Equation of State of Dense Matter and Consequences for Neutron Stars

Abstract: There is currently tremendous interest in the role of hyperons and other exotic forms of matter in neutron stars. This is particularly so following the measurement by Demorest et al. of a star with a mass almost 2 solar masses. Given that we know of no physical mechanism to stop the occurrence of hyperons at matter in beta–equilibrium above roughly 3 times nuclear matter density, we discuss the constraints on the possible maximum mass when hyperons are included in the equation of state. The discussion includes a careful assessment of the constraints from low energy nuclear properties as well as the properties of hypernuclei. The model within which these calculations are carried out is the quark-meson coupling (QMC) model, which is derived starting at the quark level.

Dihadron fragmentation functions within the Nambu-Jona-Lasinio-jet model

Abstract: Dihadron Fragmentation Functions (DFF) provide a vast amount of information on the intricate details of the parton hadronization process. Moreover, they provide a unique access to the "clean" extraction of nucleon transversity parton distribution functions in semi inclusive deep inelastic two hadron production process with a transversely polarised target. The NJL-jet model has been extended for calculations of light and strange quark unpolarised DFFs to pions, kaons and several vector mesons. This is accomplished by using the probabilistic interpretation of the DFFs, and employing the NJL-jet hadronization model in the Monte Carlo simulations that includes the transverse momentum of the produced hadrons. The strong decays of the vector mesons and the subsequent modification of the pseudoscalar meson DFFs are also considered. The resulting pseudoscalar meson DFFs are strongly influenced by the decays of the relevant vector mesons. This is because of the large combinatorial factors involved in counting the number of the hadron pairs that include the decay products. The evolution of the DFFs from the model scale to a typical experimental scale has also been performed.

A Study of Structure Functions for the Bag Beyond Leading Order

Abstract: There has recently been surprising progress in understanding the spin and flavor dependence of deep inelastic structure functions in terms of the same physics needed in the simple quark models used for hadronic spectroscopy. However, the corresponding scale is usually very low, casting doubt on the use of leading order $QCD$ evolution. We show that the conclusions are not significantly altered if one goes to next-to-leading order. In particular, the excellent agreement with unpolarized and polarized valence quark distributions is retained.

Octet spin fractions and the proton spin problem.

Abstract: The relatively small fraction of the spin of the proton carried by its quarks presents a major challenge to our understanding of the strong interaction. Traditional efforts to explore this problem have involved new and imaginative experiments and QCD based studies of the nucleon. We propose a new approach to the problem that exploits recent advances in lattice QCD. In particular, we extract values for the spin carried by the quarks in other members of the baryon octet in order to see whether the suppression observed for the proton is a general property or depends significantly on the baryon structure. We compare these results with the values for the spin fractions calculated within a model that includes the effects of confinement, relativity, gluon exchange currents, and the meson cloud required by chiral symmetry, finding a very satisfactory level of agreement given the precision currently attainable.

Charge symmetry breaking from a chiral extrapolation of moments of quark distribution functions

Abstract: [...] We present a determination, from lattice QCD, of charge symmetry violation in the spin-independent and spin-dependent parton distribution functions of the nucleon. This is done by chirally extrapolating recent QCDSF/UKQCD Collaboration lattice simulations of the first several Mellin moments of the parton distribution functions of octet baryons to the physical point. We find small chiral corrections for the polarized moments, while the corrections are quantitatively significant in the unpolarized case.

Variations of nuclear binding with quark masses

Abstract: We investigate the variation with light quark mass of the mass of the nucleon as well as the masses of the mesons commonly used in a one-boson-exchange model of the nucleon-nucleon force. Care is taken to evaluate the meson mass shifts at the kinematic point relevant to that problem. Using these results, the corresponding changes in the energy of the 1 S0 anti-bound state, the binding energies of the deuteron, triton and selected finite nuclei are evaluated using a one-boson exchange model. The results are discussed in the context of possible corrections to the standard scenario for big bang nucleosynthesis in the case where, as suggested by recent observations of quasar absorption spectra, the quark masses may have changed over the age of the Universe.

The Crystal Ball at MAMI and A2 Collaborations

Abstract: The 12 C(� pp) reaction has been studied in the photon energy range 200-450 MeV at the Mainz microtron MAMI-C, where linearly polarised photons were energy-tagged using the Glasgow-Mainz Tagged Photon Spectrometer and protons were detected in the Crystal Ball detector. The photon asymmetry Σ has been measured over a wider Eγ range than previous measurements. The strongest asymmetries were found at low missing energies where direct emission of nucleon pairs is expected. Cuts on the difference in azimuthal angles of the two ejected protons increased the magnitude of the observed asymmetries. At low missing energies the Σ data exhibit a strong angular dependence, similar to deuteron photodisintegration.

Comment on “Taming the Pion Cloud of the Nucleon”

Abstract: A Comment on the Letter by M. Alberg and G.A. Miller, Phys. Rev. Lett. 108 172001 (2012) The authors of the Letter offer a Reply.

The calorimeter project for the Mu2e experiment

Abstract: The Mu2e experiment at Fermilab aims to measure the charged lepton flavor violating neutrinoless conversion of a negative muon into an electron. The conversion results in a monochromatic electron with an energy slightly below the rest mass of the muon (104.97 MeV). We expect to set a limit of ∼ 6×10^(−17) at 90% CL in three years of running, using an intense and clean pulsed μ^− beam providing ∼10^(18) stopped muons on target in three years of running. The experiment performs a strong suppression of potential background by gating off the prompts and performing precise momentum determination in conjunction with an highly efficient cosmic veto. The calorimeter should confirm that the candidates reconstructed by the tracker system are indeed conversion electrons and provide an independent trigger (or event reduction filter) for the experiment. It should also provide standalone muon to electron rejection. Moreover, it must be able to keep functionality in a high radiation dose environment inside a 10^(−4)_(torr) vacuum enclosure and in a presence of 1 T axial magnetic field. In order to accomplish all these tasks, a LYSO crystals calorimeter has been chosen. We show the proposed design and the experimental results obtained by exposing a small size calorimeter prototype to a tagged photon beam from 40 to 300 MeV at the A2 photon facility of the Mainz Microton (MAMI), Germany.

SU(3) chiral perturbation theory expansion of moments of quark distributions

Abstract: We present formulas for the chiral extrapolation of spin-dependent and spin-independent moments of quark distributions of octet baryons, including loop corrections and counterterms, to leading nonanalytic order. This analysis allows for isospin breaking, and may be used for the chiral extrapolation of both ($2+1$)- and ($1+1+1$)-flavor lattice QCD results. An example of such an application is given, with the extrapolation formulas applied, using the finite-range regularization scheme, to recent ($2+1$)-flavor QCDSF/UKQCD Collaboration lattice results for the first spin-independent and first two spin-dependent Mellin moments.

Scale setting, sigma terms and the Feynman-Hellman theorem

Abstract: The authors recently presented new values for the octet baryon sigma terms. These were extracted using the Feynman-Hellman theorem from a chiral perturbation theory fit to octet baryon mass data from the PACS-CS collaboration. Of particular interest is the precise determination of the strangeness sigma term $\sigma_s = 21 \pm 6$ MeV. In this work, we elaborate on the critical effect which the choice of scale setting has on this value. We discuss the prospect that the comparison of direct and 'spectrum' determinations of the sigma terms from the lattice can provide insight not only into scale setting on the lattice, but into QCD itself.

Strange Quarks and Lattice QCD

Abstract: The last few years have seen a dramatic improvement in our knowledge of the strange form factors of the nucleon. With regard to the vector from factors the level of agreement between theory and experiment gives us considerable confidence in our ability to calculate with non-perturbative QCD. The calculation of the strange scalar form factor has moved significantly in the last 2 years, with the application of new techniques which yield values considerably smaller than believed for the past 20 years. These new values turn out to have important consequences for the detection of neutralinos, a favourite dark matter candidate. Finally, very recent lattice studies have resurrected interest in the famed H-dibaryon, with modern chiral extrapolation of lattice data suggesting that it may be only slightly unbound. We review some of the major sources of uncertainty in that chiral extrapolation.

Production of the H dibaryon via the (K-, K+) reaction on a C-12 target

Abstract: We study the production of the stable six-quark $H$ dibaryon via the $({K}^{\ensuremath{-}},{K}^{+})$ reaction on a ${}^{12}$C target within a covariant effective Lagrangian model. The calculations are performed within a factorization approximation, in which the full production amplitude is written as a product of the amplitudes for the ${K}^{\ensuremath{-}}+p\ensuremath{\rightarrow}{K}^{+}+{\ensuremath{\Xi}}^{\ensuremath{-}}$ and ${\ensuremath{\Xi}}^{\ensuremath{-}}+p\ensuremath{\rightarrow}H$ processes. The ${K}^{+}{\ensuremath{\Xi}}^{\ensuremath{-}}$ production vertex is described by excitation, propagation, and decay of $\ensuremath{\Lambda}$ and $\ensuremath{\Sigma}$ resonance states in the initial collision of a ${K}^{\ensuremath{-}}$ meson with a target proton in the incident channel. The parameters of the resonance vertices are taken to be the same as those determined previously by describing the available data on total and differential cross sections for the $p({K}^{\ensuremath{-}},{K}^{+}){\ensuremath{\Xi}}^{\ensuremath{-}}$ reaction within a similar model. The ${\ensuremath{\Xi}}^{\ensuremath{-}}+p\ensuremath{\rightarrow}H$ fusion process is treated within a quark model where the $H$ dibaryon is considered as a stable particle. For the ${K}^{+}$ meson angle fixed at 0${}^{\ensuremath{\circ}}$, the $H$ production cross section is found to be about 2.9 $\ensuremath{\mu}b/sr$ for $H$ mass just below the $\ensuremath{\Lambda}\ensuremath{\Lambda}$ threshold at a ${K}^{\ensuremath{-}}$ beam momentum of 1.67 GeV/$c$. This is an order of magnitude larger than the value for this quantity reported earlier in calculations performed on a ${}^{3}$He target using a different model for the cascade hyperon production. We have also calculated the beam momentum dependence of the $H$ production cross section and the energy spectrum of the emitted ${K}^{+}$ meson.

Updated Analysis of the Mass of the H Dibaryon from Lattice QCD

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 $\pm$ 14 MeV) with respect to the $\Lambda--\Lambda$ threshold. We report the results of an updated analysis which finds the H unbound by 26 $\pm$ 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 $\Lambda--\Lambda$ threshold already reported experimentally. It is clearly of great importance that the latter be pursued in experiments at the new J-PARC facility.

Production of Ξ − -Hypernuclei via the ( K − , K + ) Reaction in a Quark-Meson Coupling Model

Abstract: We study the production of Ξ−-hypernuclei, \({^{12}{_{\Xi^-}}}\) Be and \({^{28}{_{\Xi^-}}}\) Mg, via the (K−, K+) reaction within a covariant effective Lagrangian model, employing the bound Ξ− and proton spinors calculated by the latest quark-meson coupling model. The present treatment yields the 0° differential cross sections for the formation of simple s-state Ξ− particle-hole states peak at a beam momentum around 1.0 GeV/c with a value in excess of 1 μb.

Constrained γZ correction to parity-violating electron scattering

Abstract: We update the calculation of γZ interference corrections to the weak charge of the proton. We show how constraints from parton distributions, together with new data on parity-violating electron scattering in the resonance region, significantly reduce the uncertainties on the corrections compared to previous estimates.

Quark mass variation and its effect on nuclear binding

Abstract: We consider the variations of the nucleon mass and the masses of the mesons typically used in a one-boson exchange model arising from possible changes in light quark masses. These results are used to calculate the corresponding changes of the energy of the S0 anti-bound state and the binding energies of 2H, 3H, and other nuclei. The possible consequences for the standard scenario for big bang nucleosynthesis are discussed [9]. In particular the possible solution of the 7Li problem through a variation of quark masses is not supported.

Progress in resolving charge symmetry violation in nucleon structure

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.