143 research outputs found
QCD Green functions in a gluon field
We formulate a dressed perturbative expansion of QCD, where the standard
diagrams are evaluated in the presence of a constant external gluon field whose
magnitude is gaussian distributed. The approach is Poincar{\'e} and gauge
invariant, and modifies the usual results for hard processes only by power
suppressed contributions. Long distance propagation of quarks and gluons turns
out to be inhibited due to a branch point singularity instead of a pole at
in the quark and gluon propagators. The dressing keeps the (massless)
quarks in q qbar fluctuations of the photon at a finite distance from each
other.Comment: 21 pages, 7 figures. Minor modifications in text. Version to be
published in JHE
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
Monte Carlo Calculations for Liquid He at Negative Pressure
A Quadratic Diffusion Monte Carlo method has been used to obtain the equation
of state of liquid He including the negative pressure region down to the
spinodal point. The atomic interaction used is a renewed version (HFD-B(HE)) of
the Aziz potential, which reproduces quite accurately the features of the
experimental equation of state. The spinodal pressure has been calculated and
the behavior of the sound velociy around the spinodal density has been
analyzed.Comment: 10 pages, RevTex 3.0, with 4 PostScript figures include
Heavy pseudoscalar mesons in a Schwinger-Dyson--Bethe-Salpeter approach
The mass spectrum of heavy pseudoscalar mesons, described as quark-antiquark
bound systems, is considered within the Bethe-Salpeter formalism with
momentum-dependent masses of the constituents. This dependence is found by
solving the Schwinger-Dyson equation for quark propagators in rainbow-ladder
approximation. Such an approximation is known to provide both a fast
convergence of numerical methods and accurate results for lightest mesons.
However, as the meson mass increases, the method becomes less stable and
special attention must be devoted to details of numerical means of solving the
corresponding equations. We focus on the pseudoscalar sector and show that our
numerical scheme describes fairly accurately the , , , and
ground states. Excited states are considered as well. Our calculations
are directly related to the future physics programme at FAIR.Comment: 9 pages, 3 figures; Based on materials of the contribution
"Relativistic Description of Two- and Three-Body Systems in Nuclear Physics",
ECT*, October 19-23, 200
Critical temperature of the superfluid transition in bose liquids
A phenomenological criterion for the superfluid transition is proposed, which
is similar to the Lindemann criterion for the crystal melting. Then we derive a
new formula for the critical temperature, relating to the mean
kinetic energy per particle above the transition. The suppression of the
critical temperature in a sufficiently dense liquid is described as a result of
the quantum decoherence phenomenon. The theory can account for the observed
dependence of on density in liquid helium and results in an
estimate K for molecular hydrogen.Comment: 4 pages, 1 fi
Quantum cavitation in liquid helium
Using a functional-integral approach, we have determined the temperature
below which cavitation in liquid helium is driven by thermally assisted quantum
tunneling. For both helium isotopes, we have obtained the crossover temperature
in the whole range of allowed negative p essures. Our results are compatible
with recent experimental results on 4He.Comment: Typeset using Revtex, 10 pages and 2 figures, Phys. Rev B (1996
Nonperturbative Aspect of Axial Vector Vertex in the Global Color Symmetry Model
It is shown how the axial vector current of current quarks is related to that
of constituent quarks within the framework of the global color symmetry model.
Gluon dressing of the axial vector vertex and the quark self-energy functions
is described by the inhomogeneous Bethe-Salpeter equation in the ladder
approximation and the Schwinger-Dyson equation in the rainbow approximation,
respectively.Comment: 10 page
Survey of nucleon electromagnetic form factors
A dressed-quark core contribution to nucleon electromagnetic form factors is
calculated. It is defined by the solution of a Poincare' covariant Faddeev
equation in which dressed-quarks provide the elementary degree of freedom and
correlations between them are expressed via diquarks. The nucleon-photon vertex
involves a single parameter; i.e., a diquark charge radius. It is argued to be
commensurate with the pion's charge radius. A comprehensive analysis and
explanation of the form factors is built upon this foundation. A particular
feature of the study is a separation of form factor contributions into those
from different diagram types and correlation sectors, and subsequently a
flavour separation for each of these. Amongst the extensive body of results
that one could highlight are: r_1^{n,u}>r_1^{n,d}, owing to the presence of
axial-vector quark-quark correlations; and for both the neutron and proton the
ratio of Sachs electric and magnetic form factors possesses a zero.Comment: 43 pages, 17 figures, 12 tables, 5 appendice
Removal of superfluid helium films from surfaces below 0.1 K
We have constructed an apparatus that is able to maintain a helium‐free surface at low temperature (T≤0.1 K) in a cell containing superfluid helium. We discuss the considerations involved in the design of this device, and describe tests that we have made to confirm that a film‐free surface has been produced
Analysis of a quenched lattice-QCD dressed-quark propagator
Quenched lattice-QCD data on the dressed-quark Schwinger function can be
correlated with dressed-gluon data via a rainbow gap equation so long as that
equation's kernel possesses enhancement at infrared momenta above that
exhibited by the gluon alone. The required enhancement can be ascribed to a
dressing of the quark-gluon vertex. The solutions of the rainbow gap equation
exhibit dynamical chiral symmetry breaking and are consistent with confinement.
The gap equation and related, symmetry-preserving ladder Bethe-Salpeter
equation yield estimates for chiral and physical pion observables that suggest
these quantities are materially underestimated in the quenched theory: |<bar-q
q>| by a factor of two and f_pi by 30%.Comment: 9 pages, LaTeX2e, REVTEX4, 6 figure
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