198 research outputs found
Comparison of Nucleon Form Factors from Lattice QCD Against the Light Front Cloudy Bag Model and Extrapolation to the Physical Mass Regime
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.Comment: 31 pages, 22 figure
Nucleon Magnetic Moments Beyond the Perturbative Chiral Regime
The quark mass dependence of nucleon magnetic moments is explored over a wide
range. Quark masses currently accessible to lattice QCD, which lie beyond the
regime of chiral perturbation theory (chiPT), are accessed via the cloudy bag
model (CBM). The latter reproduces the leading nonanalytic behavior of chiPT,
while modeling the internal structure of the hadron under investigation. We
find that the predictions of the CBM are succinctly described by the simple
formula, \mu_N(m_\pi) = \mu^{(0)}_N / (1 + \alpha m_\pi + \beta m_\pi^2), which
reproduces the lattice data, as well as the leading nonanalytic behavior of
chiPT. As this form also incorporates the anticipated Dirac moment behavior in
the limit m_\pi \to \infty, it constitutes a powerful method for extrapolating
lattice results to the physical mass regime.Comment: Revised version accepted for publication includes a new section
demonstrating extrapolations of lattice QCD result
Nucleon Charge Symmetry Breaking and Parity Violating Electron-Proton Scattering
The consequences of the charge symmetry breaking effects of the mass
difference between the up and down quarks and electromagnetic effects for
searches for strangeness form factors in parity violating electron scattering
from the proton are investigated. The formalism necessary to identify and
compute the relevant observables is developed by separating the Hamiltonian
into charge symmetry conserving and breaking terms. Using a set of SU(6)
non-relativistic quark models, the effects of the charge symmetry breaking
Hamiltonian are considered for experimentally relevant alues of the momentum
transfer and found to be less than about 1 percent. The charge symmetry
breaking corrections to the Bjorken sum rule are also studied and shown to
vanish in first-order perturbation theory.Comment: 35 pages, 9 figure
The Flavor Asymmetry of the Nucleon Sea
We re-examine the effects of anti-symmetry on the anti-quarks in the nucleon
sea arising from gluon exchange and pion exchange between confined quarks.
While the effect is primarily to suppress anti-down relative to anti-up quarks,
this is numerically insignificant for the pion terms.Comment: To appear in Phys. Rev.
Electromagnetic nucleon-delta transition in the perturbative chiral quark model
We apply the perturbative chiral quark model to the gamma N -> Delta
transition. The four momentum dependence of the respective transverse helicity
amplitudes A(1/2) and A(3/2) is determined at one loop in the pseudoscalar
Goldstone boson fluctuations. Inclusion of excited states in the quark
propagator is shown to result in a reasonable description of the experimental
values for the helicity amplitudes at the real photon point.Comment: 25 page
Non-perturbative Gluons and Pseudoscalar Mesons in Baryon Spectroscopy
We study baryon spectroscopy including the effects of pseudoscalar meson
exchange and one gluon exchange potentials between quarks, governed by
. The non-perturbative, hyperspherical method calculations show that
one can obtain a good description of the data by using a quark-meson coupling
constant that is compatible with the measured pion-nucleon coupling constant,
and a reasonably small value of .Comment: 12 pages; Submitted to Phys. Rev. C. Rapid Communication
Revealing Nuclear Pions Using Electron Scattering
A model for the pionic components of nuclear wave functions is obtained from
light front dynamical calculations of binding energies and densities. The
pionic effects are small enough to be consistent with measured nuclear di-muon
production data and with the nucleon sea. But the pion effects are large enough
to predict substantial nuclear enhancement of the cross section for
longitudinally polarized virtual photons for the kinematics accessible at
Jefferson Laboratory.Comment: 9 pages, 4 figure
A global optimisation approach to range-restricted survey calibration
Survey calibration methods modify minimally unit-level sample weights to fit domain-level benchmark constraints (BC). This allows exploitation of auxiliary information, e.g. census totals, to improve the representativeness of sample data (addressing coverage limitations, non-response) and the quality of estimates of population parameters. Calibration methods may fail with samples presenting small/zero counts for some benchmark groups or when range restrictions (RR), such as positivity, are imposed to avoid unrealistic or extreme weights. User-defined modifications of BC/RR performed after encountering non-convergence allow little control on the solution, and penalization approaches modelling infeasibility may not guarantee convergence. Paradoxically, this has led to underuse in calibration of highly disaggregated information, when available. We present an always-convergent flexible two-step Global Optimisation (GO) survey calibration approach. The feasibility of the calibration problem is assessed, and automatically controlled minimum errors in BC or changes in RR are allowed to guarantee convergence in advance, while preserving the good properties of calibration estimators. Modelling alternatives under different scenarios, using various error/change and distance measures are formulated and discussed. The GO approach is validated by calibrating the weights of the 2012 Health Survey for England to a fine age-gender-region cross-tabulation (378 counts) from the 2011 Census in England and Wales
Chiral extrapolation of lattice moments of proton quark distributions
We present the resolution of a long-standing discrepancy between the moments
of parton distributions calculated from lattice QCD and their experimental
values. We propose a simple extrapolation formula for the moments of the
nonsinglet quark distribution u-d, as a function of quark mass, which embodies
the general constraints imposed by the chiral symmetry of QCD. The inclusion of
the leading nonanalytic behavior leads to an excellent description of both the
lattice data and the experimental values of the moments.Comment: 9 pages, 1 figure, to appear in Physical Review Letter
A global optimisation approach to range-restricted survey calibration
Survey calibration methods modify minimally unit-level sample weights to fit domain-level benchmark constraints (BC). This allows exploitation of auxiliary information, e.g. census totals, to improve the representativeness of sample data (addressing coverage limitations, non-response) and the quality of estimates of population parameters. Calibration methods may fail with samples presenting small/zero counts for some benchmark groups or when range restrictions (RR), such as positivity, are imposed to avoid unrealistic or extreme weights. User-defined modifications of BC/RR performed after encountering non-convergence allow little control on the solution, and penalization approaches modelling infeasibility may not guarantee convergence. Paradoxically, this has led to underuse in calibration of highly disaggregated information, when available. We present an always-convergent flexible two-step Global Optimisation (GO) survey calibration approach. The feasibility of the calibration problem is assessed, and automatically controlled minimum errors in BC or changes in RR are allowed to guarantee convergence in advance, while preserving the good properties of calibration estimators. Modelling alternatives under different scenarios, using various error/change and distance measures are formulated and discussed. The GO approach is validated by calibrating the weights of the 2012 Health Survey for England to a fine age-gender-region cross-tabulation (378 counts) from the 2011 Census in England and Wales
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