308 research outputs found
Analysis of full-QCD and quenched-QCD lattice propagators
Recent lattice-QCD results for the dressed-gluon propagator are used within
the quark Dyson-Schwinger equation to determine the gluon-quark vertex dressing
necessary to reproduce the lattice-QCD results for the dressed-quark
propagator. Both quenched and full QCD lattice simulations, for a range of low
quark current masses, are analyzed. The chiral extrapolation is made through
this continuum DSE form. Resulting chiral and physical pion observables are
investigated.Comment: Talk given at PANIC 2005 Conference, Santa Fe, October 2005. To be
published in the Proceeding
Pion electromagnetic form factor at spacelike momenta
A novel method is employed to compute the pion electromagnetic form factor,
F_\pi(Q^2), on the entire domain of spacelike momentum transfer using the
Dyson-Schwinger equation (DSE) framework in quantum chromodynamics (QCD). The
DSE architecture unifies this prediction with that of the pion's valence-quark
parton distribution amplitude (PDA). Using this PDA, the leading-order,
leading-twist perturbative QCD result for Q^2 F_\pi(Q^2) underestimates the
full computation by just 15% on Q^2>~8GeV^2, in stark contrast with the result
obtained using the asymptotic PDA. The analysis shows that hard contributions
to the pion form factor dominate for Q^2>~8GeV^2 but, even so, the magnitude of
Q^2 F_\pi(Q^2) reflects the scale of dynamical chiral symmetry breaking, a
pivotal emergent phenomenon in the Standard Model.Comment: 5 pages, 2 figures. To appear in Phys. Rev. Let
Pion distribution amplitude from lattice-QCD
A method is explained through which a pointwise accurate approximation to the
pion's valence-quark distribution amplitude (PDA) may be obtained from a
limited number of moments. In connection with the single nontrivial moment
accessible in contemporary simulations of lattice-regularised quantum
chromodynamics (QCD), the method yields a PDA that is a broad concave function
whose pointwise form agrees with that predicted by Dyson-Schwinger equation
analyses of the pion. Under leading-order evolution, the PDA remains broad to
energy scales in excess of 100 GeV, a feature which signals persistence of the
influence of dynamical chiral symmetry breaking. Consequently, the asymptotic
distribution, \phi_\pi^asy(x), is a poor approximation to the pion's PDA at all
such scales that are either currently accessible or foreseeable in experiments
on pion elastic and transition form factors. Thus, related expectations based
on \phi_\pi^asy(x) should be revised.Comment: 5 pages, 2 figure
Quark-gluon vertex dressing and meson masses beyond ladder-rainbow truncation
We include a generalized infinite class of quark-gluon vertex dressing
diagrams in a study of how dynamics beyond the ladder-rainbow truncation
influences the Bethe-Salpeter description of light quark pseudoscalar and
vector mesons. The diagrammatic specification of the vertex is mapped into a
corresponding specification of the Bethe-Salpeter kernel, which preserves
chiral symmetry. This study adopts the algebraic format afforded by the simple
interaction kernel used in previous work on this topic. The new feature of the
present work is that in every diagram summed for the vertex and the
corresponding Bethe-Salpeter kernel, each quark-gluon vertex is required to be
the self-consistent vertex solution. We also adopt from previous work the
effective accounting for the role of the explicitly non-Abelian three gluon
coupling in a global manner through one parameter determined from recent
lattice-QCD data for the vertex. With the more consistent vertex used here, the
error in ladder-rainbow truncation for vector mesons is never more than 10% as
the current quark mass is varied from the u/d region to the b region.Comment: 15 pages, 12 figure
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