100 research outputs found
Electromagnetic properties of ground and excited state pseudoscalar mesons
The axial-vector Ward-Takahashi identity places constraints on particular
properties of every pseudoscalar meson. For example, in the chiral limit all
pseudoscalar mesons, except the Goldstone mode, decouple from the axial-vector
current. Nevertheless, all neutral pseudoscalar mesons couple to two photons.
The strength of the \pi_n^0 \gamma \gamma coupling, where n=0 denotes the
Goldstone mode, is affected by the Abelian anomaly's continuum contribution.
The effect is material for n \neq 0. The \gamma* \pi_n \gamma* transition form
factor, T_{\pi_n}(Q^2), is nonzero for all n, and T_{\pi_n}(Q^2) \approx
(4\pi^2/3) (f_{\pi_n}/Q^2) at large Q^2. For all pseudoscalars but the
Goldstone mode, this leading contribution vanishes in the chiral limit. In this
instance the ultraviolet power-law behaviour is 1/Q^4 for n \neq 0, and we find
numerically T_{\pi_1}(Q^2) \simeq (4\pi^2/3) (-/Q^4). This subleading
power-law behaviour is always present. In general its coefficient is not simply
related to f_{\pi_n}. The properties of n \neq 0 pseudoscalar mesons are
sensitive to the pointwise behaviour of the long-range piece of the interaction
between light-quarks.Comment: 13 pages, 5 figure
Schwinger functions and light-quark bound states
We examine the applicability and viability of methods to obtain knowledge
about bound-states from information provided solely in Euclidean space.
Rudimentary methods can be adequate if one only requires information about the
ground and first excited state and assumptions made about analytic properties
are valid. However, to obtain information from Schwinger functions about higher
mass states, something more sophisticated is necessary. A method based on the
correlator matrix can be dependable when operators are carefully tuned and
errors are small. This method is nevertheless not competitive when an
unambiguous analytic continuation of even a single Schwinger function to
complex momenta is available.Comment: 27 pages, 14 figure
The relevance of particulate organic carbon (POC) for carbon composition in the pore water of drained and rewetted fens of the "Donauried" (South-Germany)
International audienceNumerous studies have dealt with carbon (C) concentrations in Histosols, but there are no studies quantifying the relative importance of all individual C components in pore waters. For this study, measurements were made of all the carbon components (i.e., particulate organic carbon, POC; dissolved organic carbon, DOC; dissolved inorganic carbon, DIC; dissolved methane, CH4) in the soil pore water of a calcareous fen under three different water management regimes (re-wetted, deeply and moderately drained). Pore water was collected weekly or biweekly (April 2004 to April 2006) at depths between 10 and 150 cm. The main results obtained were: (1) DIC (94?280 mg C l?1) was the main C-component. (2) POC and DOC concentrations in the pore water (14?125 mg C l?1 vs. 41?95 mg C l?1) were pari passu. (3) Dissolved CH4 was the smallest C component (0.005?0.9 mg C l?1). Interestingly, about 30% of the POM particles were colonized by microbes indicating that they are active in the internal C transfer in the soil profile ("C-Shuttles"). Consequently, it was concluded that POC is at least as important as DOC for internal soil C turnover. There is no reason to assume significant biochemical differences between POC and DOC as they only differ in size. Therefore, both POC and DOC fractions are essential components of C budgets of peatlands. Furthermore dissolved CO2 in all forms of DIC apparently is an important part of peatland C-balances
Frontiers of the physics of dense plasmas and planetary interiors: experiments, theory, applications
Recent developments of dynamic x-ray characterization experiments of dense
matter are reviewed, with particular emphasis on conditions relevant to
interiors of terrestrial and gas giant planets. These studies include
characterization of compressed states of matter in light elements by x-ray
scattering and imaging of shocked iron by radiography. Several applications of
this work are examined. These include the structure of massive "Super Earth"
terrestrial planets around other stars, the 40 known extrasolar gas giants with
measured masses and radii, and Jupiter itself, which serves as the benchmark
for giant planets.Comment: Accepted to Physics of Plasmas special issue. Review from
HEDP/HEDLA-08, April 12-15, 200
Nucleon electromagnetic form factors
Elastic electromagnetic nucleon form factors have long provided vital
information about the structure and composition of these most basic elements of
nuclear physics. The form factors are a measurable and physical manifestation
of the nature of the nucleons' constituents and the dynamics that binds them
together. Accurate form factor data obtained in recent years using modern
experimental facilities has spurred a significant reevaluation of the nucleon
and pictures of its structure; e.g., the role of quark orbital angular
momentum, the scale at which perturbative QCD effects should become evident,
the strangeness content, and meson-cloud effects. We provide a succinct survey
of the experimental studies and theoretical interpretation of nucleon
electromagnetic form factors.Comment: Topical review invited by Journal of Physics G: Nuclear and Particle
Physics; 34 pages (contents listed on page 34), 11 figure
Sigma Terms of Light-Quark Hadrons
A calculation of the current-quark mass dependence of hadron masses can help
in using observational data to place constraints on the variation of nature's
fundamental parameters. A hadron's sigma-term is a measure of this dependence.
The connection between a hadron's sigma-term and the Feynman-Hellmann theorem
is illustrated with an explicit calculation for the pion using a rainbow-ladder
truncation of the Dyson-Schwinger equations: in the vicinity of the chiral
limit sigma_pi = m_pi/2. This truncation also provides a decent estimate of
sigma_rho because the two dominant self-energy corrections to the rho-meson's
mass largely cancel in their contribution to sigma_rho. The truncation is less
accurate for the omega, however, because there is little to compete with an
omega->rho+pi self-energy contribution that magnifies the value of sigma_omega
by ~25%. A Poincare' covariant Faddeev equation, which describes baryons as
composites of confined-quarks and -nonpointlike-diquarks, is solved to obtain
the current-quark mass dependence of the masses of the nucleon and Delta, and
thereby sigma_N and sigma_Delta. This "quark-core" piece is augmented by the
"pion cloud" contribution, which is positive. The analysis yields sigma_N~60MeV
and sigma_Delta~50MeV.Comment: 22 pages, reference list expande
Current quark mass dependence of nucleon magnetic moments and radii
A calculation of the current-quark-mass-dependence of nucleon static
electromagnetic properties is necessary in order to use observational data as a
means to place constraints on the variation of Nature's fundamental parameters.
A Poincare' covariant Faddeev equation, which describes baryons as composites
of confined-quarks and -nonpointlike-diquarks, is used to calculate this
dependence The results indicate that, like observables dependent on the
nucleons' magnetic moments, quantities sensitive to their magnetic and charge
radii, such as the energy levels and transition frequencies in Hydrogen and
Deuterium, might also provide a tool with which to place limits on the allowed
variation in Nature's constants.Comment: 23 pages, 2 figures, 4 tables, 4 appendice
Mean field exponents and small quark masses
We demonstrate that the restoration of chiral symmetry at finite-T in a class
of confining Dyson-Schwinger equation (DSE) models of QCD is a mean field
transition, and that an accurate determination of the critical exponents using
the chiral and thermal susceptibilities requires very small values of the
current-quark mass: log_{10}(m/m_u) < -5. Other classes of DSE models
characterised by qualitatively different interactions also exhibit a mean field
transition. Incipient in this observation is the suggestion that mean field
exponents are a result of the gap equation's fermion substructure and not of
the interaction.Comment: 13 pages, 3 figures, REVTEX, epsfi
Current quark mass effects on chiral phase transition of QCD in the improved ladder approximation
Current quark mass effects on the chiral phase transition of QCD is studied
in the improved ladder approximation. An infrared behavior of the gluon
propagator is modified in terms of an effective running coupling. The analysis
is based on a composite operator formalism and a variational approach. We use
the Schwinger-Dyson equation to give a ``normalization condition'' for the
Cornwall-Jackiw-Tomboulis effective potential and to isolate the ultraviolet
divergence which appears in an expression for the quark-antiquark condensate.
We study the current quark mass effects on the order parameter at zero
temperature and density. We then calculate the effective potential at finite
temperature and density and investigate the current quark mass effects on the
chiral phase transition. We find a smooth crossover for , and a
first-order phase transition for , T=0. Critical exponents are also
studied and our model gives the classical mean-field values. We also study the
temperature dependence of masses of scalar and pseudoscalar bosons. A critical
end point in the - plane is found at MeV,
MeV.Comment: 19 pages, 13 figure
Bethe-Salpeter equation and a nonperturbative quark-gluon vertex
A Ward-Takahashi identity preserving Bethe-Salpeter kernel can always be
calculated explicitly from a dressed-quark-gluon vertex whose diagrammatic
content is enumerable. We illustrate that fact using a vertex obtained via the
complete resummation of dressed-gluon ladders. While this vertex is planar, the
vertex-consistent kernel is nonplanar and that is true for any dressed vertex.
In an exemplifying model the rainbow-ladder truncation of the gap and
Bethe-Salpeter equations yields many results; e.g., pi- and rho-meson masses,
that are changed little by including higher-order corrections. Repulsion
generated by nonplanar diagrams in the vertex-consistent Bethe-Salpeter kernel
for quark-quark scattering is sufficient to guarantee that diquark bound states
do not exist.Comment: 16 pages, 12 figures, REVTEX
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