70 research outputs found
The quark-photon vertex and meson electromagnetic form factors
The ladder Bethe-Salpeter solution for the dressed photon-quark vertex is
used to study the low-momentum behavior of the pion electromagnetic and the
transition form factors. With model parameters
previously fixed by light meson masses and decay constants, the low-momentum
slope of both form factors is in excellent agreement with the data. In
comparison, the often-used Ball-Chiu Ansatz for the vertex is found to be
deficient; less than half of the obtained is generated by that Ansatz
while the remainder of the charge radius could be attributed to the tail of the
resonance.Comment: 4 pages, 2 figures, uses espcrc1.sty, talk presented at PANIC99,
Uppsala, Swede
Covariant QCD Modeling of Light Meson Physics
We summarize recent progress in soft QCD modeling based on the set of
Dyson--Schwinger equations truncated to ladder-rainbow level. This covariant
approach to hadron physics accommodates quark confinement and implements the
QCD one-loop renormalization group behavior. We compare the dressed quark
propagator, pseudoscalar and vector meson masses as a function of quark mass,
and the rho -> pi pi coupling to recent lattice-QCD data. The error in the
Gell-Mann--Oakes--Renner relation with increasing quark mass is quantified by
comparison to the exact pseudoscalar mass relation as evaluated within the
ladder-rainbow Dyson-Schwinger model.Comment: Presented at the International School on Nuclear Physics, 24th
course: Quarks in Nuclei, Erice, Sicily, September 2002; to be published in
Prog. Part. Nucl. Phys.; 6 pages, 6 fig
Nonperturbative QCD Phenomenology and Light Quark Physics
Recent progress in modeling QCD for hadron physics through truncated
Dyson-Schwinger equations is reviewed. Special emphasis is put upon comparison
of dressed quark propagators and the dressed quark-gluon vertex with
lattice-QCD results.Comment: 6 pages, 7 figures. Invited talk at the QCD Down Under workshop at
the CSSM/University of Adelaide, March 200
QCD modeling of hadron physics
We review recent developments in the understanding of meson properties as
solutions of the Bethe-Salpeter equation in rainbow-ladder truncation. Included
are recent results for the pseudoscalar and vector meson masses and leptonic
decay constants, ranging from pions up to c\bar{c} bound states; extrapolation
to b\bar{b} states is explored. We also present a new and improved calculation
of F_\pi(Q^2) and an analysis of the \pi\gamma\gamma transition form factor for
both \pi(140) and \pi(1330). Lattice-QCD results for propagators and the
quark-gluon vertex are analyzed, and the effects of quark-gluon vertex dressing
and the three-gluon coupling upon meson masses are considered.Comment: 17 pages, 19 postscript figures, contribution to the proceedings of
LC05, Cairns, Australia, July 200
The Quark-Photon Vertex and the Pion Charge Radius
The rainbow truncation of the quark Dyson-Schwinger equation is combined with
the ladder Bethe-Salpeter equation for the dressed quark-photon vertex to study
the low-momentum behavior of the pion electromagnetic form factor. With model
gluon parameters previously fixed by the pion mass and decay constant, the pion
charge radius is found to be in excellent agreement with the data. When
the often-used Ball-Chiu Ansatz is used to construct the quark-photon vertex
directly from the quark propagator, less than half of is generated.
The remainder of is seen to be attributable to the presence of the
-pole in the solution of the ladder Bethe-Salpeter equation.Comment: 21 pages, 9 figure
Tomato marchitez virus, a new plant picorna-like virus from tomato related to tomato torrado virus
A new virus was isolated from a tomato plant from the state of Sinaloa in Mexico. This plant showed symptoms locally known as `marchitez diseaseÂż: severe leaf necrosis, beginning at the base of the leaflets, and necrotic rings on the fruits. A virus was isolated from the infected plant consisting of isometric particles with a diameter of approximately 28Âżnm. The viral genome consists of two (+)ssRNA molecules of 7221 (RNA1) and 4898Âżnts (RNA2). The viral capsid contains three coat proteins of 35, 26 and 24ÂżkDa, respectively. The abovementioned characteristics: symptoms, morphology, number and size of coat proteins, and number of RNAs are similar to those of the previously described tomato torrado virus (ToTV). Sequence analysis of the entire viral genome shows that this new virus is related to, but distinct from, ToTV and that these members of two obviously new virus species belong to the recently proposed plant virus genus Torradovirus. For this new virus, the name tomato marchitez virus (ToMarV) is proposed
Selected nucleon form factors and a composite scalar diquark
A covariant, composite scalar diquark, Fadde'ev amplitude model for the
nucleon is used to calculate pseudoscalar, isoscalar- and isovector-vector,
axial-vector and scalar nucleon form factors. The last yields the nucleon
sigma-term and on-shell sigma-nucleon coupling. The calculated form factors are
soft, and the couplings are generally in good agreement with experiment and
other determinations. Elements in the dressed-quark-axial-vector vertex that
are not constrained by the Ward-Takahashi identity contribute ~20% to the
magnitude of g_A. The calculation of the nucleon sigma-term elucidates the only
unambiguous means of extrapolating meson-nucleon couplings off the meson
mass-shell.Comment: 12 pages, REVTEX, 5 figures, epsfi
The , , and electromagnetic form factors
The rainbow truncation of the quark Dyson-Schwinger equation is combined with
the ladder Bethe-Salpeter equation for the meson amplitudes and the dressed
quark-photon vertex in a self-consistent Poincar\'e-invariant study of the pion
and kaon electromagnetic form factors in impulse approximation. We demonstrate
explicitly that the current is conserved in this approach and that the obtained
results are independent of the momentum partitioning in the Bethe-Salpeter
amplitudes. With model gluon parameters previously fixed by the condensate, the
pion mass and decay constant, and the kaon mass, the charge radii and spacelike
form factors are found to be in good agreement with the experimental data.Comment: 8 pages, 6 figures, Revte
Valence-quark distributions in the pion
We calculate the pion's valence-quark momentum-fraction probability
distribution using a Dyson-Schwinger equation model. Valence-quarks with an
active mass of 0.30 GeV carry 71% of the pion's momentum at a resolving scale
q_0=0.54 GeV = 1/(0.37 fm). The shape of the calculated distribution is
characteristic of a strongly bound system and, evolved from q_0 to q=2 GeV, it
yields first, second and third moments in agreement with lattice and
phenomenological estimates, and valence-quarks carrying 49% of the pion's
momentum. However, pointwise there is a discrepancy between our calculated
distribution and that hitherto inferred from parametrisations of extant
pion-nucleon Drell-Yan data.Comment: 8 pages, 3 figures, REVTEX, aps.sty, epsfig.sty, minor corrections,
version to appear in PR
Het genus Torradovirus, een nieuw geslacht van plantenvirussen
Het genus Torradovirus is een nieuw geslacht van plantenvirussen. Hiertoe behoren het tomatentorradovirus (ToTV), het tomatenmarchitezvirus (ToMarV) en het tomatenchocolĂ tevirus (ToChV)
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